Blooming Science-9-2077 final final for press

12. The factors, which influence on the speed of sound in gases are temperature, density,
humidity and direction of wind.

13. The range of frequency of audible sound is in between 20Hz to 20000 Hz.

14. The sound having frequency above 20,000 Hz is ultrasound and that below 20Hz is
infrasound.

15. Pitch and intensity are the major components of the note of sound.

16. Pitch is a non measurable quantity, responsible for the sharpness of sound. It depends on
the frequency of sound wave.

17. Intensity is a measurable quantity, responsible for the loudness of sound. It depends on the
amplitude of sound wave.

18. Reflection of sound causes echo and reverberation.

19. Echo is that phenomenon, in which the reflected sound does not mix with original sound
and the sound is repeated.

20. Reverberation is the phenomenon, in which the reflected sound mixes with original sound
and the sound is prolonged.

21. Bending of sound wave, when it passes from one medium to other is called refraction of
sound.

22. Unpleasant sound is called noise.

23. Pleasant sound, which has regularity and rhythm is called music.

24. Noise caused pollution called noise-pollution.

PRO J ECTWORK

Throw a store in a pond with water. Observe the nature of waves produce in water and write
about the waves with diagram.

Exercise

A. Choose the correct answer from the given alternatives:

1. Unit of intensity is :

a. Decibel (dB) b. lambda (λ) c. Hertz (Hz) d. m/s

2. Complete wave is

a. 1 crest and 1 trough b. 1 rarefaction and 1 compression

c. Both a and b d. None of the above

3. Time period is calculated by

a. f×λ b= 1 c. 1 d. E
f density t×A

Blooming Science Book 9 101

4. Which of the following is the frequency of the ultra sonic sound?

a. above 20 KHz b. 20Hz to 2000Hz c. less than 20Hz d. 20Hz to 200Hz
d. detration
5. Which of the following is required to make music melodious?

a. reverberation b. refraction c. echo

B. Answer the following questions.
1. a. What is sound? How is it produced?
b. What is a wave? Name its types with examples of each.
c. Define: frequency, wave length, amplitude, pitch, intensity of sound and sound
pollution.
d. What is longitudinal wave?
e. Write any two ways of controlling sound pollution. Mention also its two effects
of it.
f. Give any two uses of ultra sound.
g. What is transverse wave?
h. What is mechanical wave? What will be the frequency of audible sound?
i. Give conditions for occurring echo and reverberation.
j. Explain the factors that affect the velocity of sound in gas.
k. What is threshold of hearing? Write its value for the sound of frequency 1000
Hz.
l. What is persistence of hearing?
m. What is echo? Show that the minimum distance between the listener and the
reflecting surface for the echo to be heard is 17m.

n. Define intensity of sound. Write the factor which affect the intensity of sound.
1
Show that I ∝
r2

2. Write differences between:

a. Transverse wave and longitudinal wave.

b. Echo and reverberation.

c. Mechanical wave and electromagnetic wave.

d. Infrasonic sound and ultrasound

e. Pitch and intensity of sound

f. Compression and rarefaction

g. Music and noise

3. Draw a labeled diagram to show: (a) (b)

a. Two waves having same frequency but different amplitude

b. Sound wave (spectrum of sound wave).

c. Amplitude and wave length.

d. Echo

102 Blooming Science Book 9

4. Give reason:
a. Sound is heard more distinct at night than at day.
b. The velocity of sound is not affected by change of its wave length or frequency in

the same medium.
c. The speed of sound increases with increase in temperature in the air.
d. The speed of sound is inversely proportional to the density of air.
e. Sound does not travel through vacuum.
f. The speed of sound produced by a boy or a girl is the same but the pitch of their

voice is different.
g. Mosquito produces sound while flying.
h. It is not suitable for habitat near airports.
i. Sound is weaker and weaker when it travels far from its origin.
j. The speed of sound is slow in gas compared to in solid or liquid.
k. The back of the auditorium is made curved.
m. Sound is heard louder in empty room than in furnished room.
n. Cinema halls are well furnished by carpets and seat covers.

5. Answer the following with the help of given figure.

a. Which one wave (a or b) has high frequency?

b. Which one wave will give high amplitude and why?

c. Which one has higher velocity in the same medium and why?

6. Write the relation between:
a. Frequency and wave length
b. Intensity of sound and amplitude
c. Speed of sound and density of air
d. Pitch and frequency
e. Speed of sound and temperature in air.

7. The speed of sound in different gases (P, Q and R) are given in a table.

Answer the following questions:

Gases Speed of sound (m/s)
P 300
Q 800
R 600

a. Which one gas will have the highest density and the least density?

b. If sound wave having the same frequency is passed through each gas, in which one
gas will have the highest wave length and least wave-length?

Blooming Science Book 9 103

8. The speed of sound in solid, liquid and gaseous medium are give below. Study and
answer the following questions:

Medium Speed (m/s)
X 332
Y 6000
Z 1450

a) Identify the mediums, X, Y and Z.

b) Calculate the frequency of sound wave in Y medium if the wave length is 10m.
[600Hz]

9. The diagram shows A and B sound waves. Answer the questions after study the

diagram. B
(i) Is wave ‘A’ or ‘B’ sharper? Why? A

(ii) Is wave ‘A’ or ‘B’ louder? Why?

Numerical problems

10. Solve the following problems:

a. The frequency and wave length of a wave is 30Hz and 2m respectively, find out its
wave-velocity. (60m/s)

b. Find out the frequency of a radio wave if it wave length is 50m. (6×106Hz)

c. The speed of sound in air is 332m/s and its frequency is 10KHz, find out its wave

length. (33.2m)

d. The wave length of sound having 15KHz frequency is 0.022m, find out its wave
velocity. (330m/s)

e. If the speed of sound in air is 332m/s

i. What will be the maximum and minimum frequencies of audible sound?
(20KHz and 20Hz)

ii. What will be the maximum and minimum wave-lengths of audible sound

wave. (16.6m and 0.166m)

f. A fathometer send an ultra sound towards the bed of a sea and echo is received

after 3 seconds, find out the depth of sea. (speed of ultrasound in the water is

1450m/s). [2175m]

g. A sound is produced in front of cliff which is 200m far from it. Find out the

time required to reach the cliff from its origin. (1.66 second)

104 Blooming Science Book 9

Chapter

7 ELECTRICITY AND MAGNETISM

Learning Outcomes Estimated Periods: 10+2

On the completion of this unit, the students will be able to:
• express units of electricity like ampere, volts, watt and ohm and to use these in

measurement.
• demonstrate Ohm’s law and define laws of resistance.
• compare conductivity of different substances.
• define and demonstrate magnetic field and magnetic lines of force.
• define terrestrial magnetism.

Introduction

Electricity is a general term encompassing a variety of phenomenon resulting from the presence
and flow of electric charge. Some common phenomena related to electricity are lightning, static
electricity and the flow of electrical current in an electrical wire, whereas electromagnetic field
and electromagnetic induction are less familiar concepts.

The word electricity is from the New Latin “electricus”, which means “amber-like” coined from
the Greek word electron meaning “amber” because electrical effects were produced classically
by rubbing amber.

Current Electricity

The present age is called the age of electricity because at every step of our life we take the
help of electricity. It is this electricity lights up our houses, industries, offices, etc. and operates
communication media like telephone, radio, computer etc. Electric energy can be easily
converted into other forms of energy. Since electricity can be generated by using chemical,
heat and magnetic energy, the use of electricity is wide spread in the world. In order to use the
electricity it is necessary to make a circuit. Thus the electric circuit makes the continuous flow
of electricity for the use of different electrical appliances. The electricity produces by continous
flow of electron through circuit is called current electricity.

Current and its Measurement

We have known that a body can be charged by friction. When these charges are made to flow in a
circuit, the motion of them becomes an electric current. The flow of charge per second is defined
called as electric current. If Q is the charge that flows in time t through a wire,

then, Current (I) = Charge (Q)
time (t)

Unit of Current: When a charge is measured in coulomb and time in second, the unit of current

is coulomb per second called an Ampere.

Blooming Science Book 9 105

∴ 1 Ampere = 1 coulomb
1 second

The SI unit of current is ampere.

The small amount of currents are measured in milliampere and microampere.

1 Milliampere (m A) = 10-3 Ampere (A)

1 Microampere (µ A) = 10-6 Ampere (A)

Since an electron has a charge of 1.6 × 10-19 C, 1 coulomb charge carries 1.6 1 = 6.25 × 1018
electrons. × 10-19

Charge of 1 electron = 1.6 × 10-19 C

or, 1.6 × 10-19 = 1e

or, 1C = 1.6 1 e
or 1 Coulomb × 10-19

= 6.25 × 1018 electrons

Thus, when 1 ampere of current flows in a circuit, 6.25 × 1018 number of electrons flow in 1
second across the wire.

Electric Circuit

Activity

Take a cell, connecting wires, a switch and a bulb, and join them as shown in the figure.

When the switch is on, the bulb glows and we say that the current is
flowing through the circuit. Of course, in the circuit, the electrons are
flowing from one end of the cell to the other end of the cell.

The flow of electron is called current. key

When the end of a cell is joined with connecting wires, a conduction + Cell -
path is formed which allow the electron to pass from one end of
the cell to the other end. The conducting path so formed is called a A simple electric
circuit. The path through which an electric current flows is called a circuit
circuit.

The closed and continuous conducting path consisting of three main components such as
electric source, conducting wire and electric appliances (loads) is called the electric circuit.

The source provides the current. Electric loads are connected with the source by the conducting
wire. The current from the electric source passes through the load and the load works. If the
switch is used in the circuit, the circuit can be of two types (i) closed circuit (ii) Open circuit.

(i) Open Circuit: When the switch is off, the bulb does not glow, because there is a break or
disconnection in the circuit. Such a circuit, in which there is a break or disconnection is
called open circuit. Current cannot flow through the such circuit.

106 Blooming Science Book 9

Bulb glows Bulb does not glow
Switch on Switch off

off on off on

+ Cell - + Cell -

(a) Closed circuit (b) Open circuit

(ii) Closed Circuit: When the switch is on, the bulb glows. There is no gap or discontinuity
in the circuit. Such a circuit in which there is no gap or discontinuity is called a closed
circuit. Current can easily flow through such circuit and bulb can glow.

Ammeter: The instrument which is used to measure the magnitude of current flowing in a circuit
is called an Ammeter. It is arranged in series as shown in the circuit. The whole current of the
circuit is allowed to pass through it. Due to its small resistance, it does not affect the total current
flowing in the circuit. The ammeter has a red coloured terminal which is positive and a black
coloured terminal which is negative.

To connect an ammeter in the circuit, the red terminal is connected to the positive side of the
circuit and the black to the negative side.

+– A

A

01 2 34 3A A

A

+ A
Ammeter 3A
Three ammeters in a circuit

In figure three ammeters are connected in series with the circuit. Although these ammeters are
connected at different places in the circuit, all three ammeters show the same amount of current.
So we may conclude that the amount of current flowing through a circuit at any place is the same,
and it does not depend upon the number of appliances connected in the circuit.

Potential Difference

The term potential is very important in electricity.

Fig. shows two vessels, A and B, connected by a pipe which isA B
provide with a tap T. Closing the tap, water is poured in both the
T
vessels so that the level of water in A is higher than that in B. If

the tap T is now opened, water will flow from vessel A to vessel B.

Blooming Science Book 9 107

The flow of water continues till the level of water in the vessels are equal. So, the level of water
gives us an indication of the direction of flow of water.

When a body is charged, it acquires a potential. The body having an excess of electrons is said to
be at a negative or lower potential while the body having an excess of positive charges is said to
be at a positive or higher potential.

Analogous to the water level which determines the condition for the flow of water, the flow of
charge from one point to another depends upon a property which is called electric potential.

When two bodies at different potential are connected, current flows between them as long as their
potential remain different.

The earth is at zero potential

A +++++ B
++++++

+++++++++
++++++++

++++++

+++++

current Conducting wire current
Flow of electrons

(a) When body is at positive potential (b) When body is at negative potential .

In figure (a) body A has a positive charge and is at a positive potential. When the body A is
connected to the ground by a conducing wire, the electrons flow from ground to the body. The
body A then becomes neutral. In fact when the electron flow stops, the body A is also at zero
potential. Thus, electron flow is possible due to the potential difference.

In figure (b) the body B has a negative charge and it is at a negative potential. When it is connected
to ground, the electrons flow from it to the ground after a few second when all the electrons flow
from the body B to the ground, no charge is left on it and has a zero potential.

The unit of potential difference is a volt (V).

The amount of work done in moving a unit positive charge from one point to another in an
electrical circuit is called potential difference (p.d.).

The potential difference between two points is one volt if one joule work is done while carrying
one coulomb of charge from one point to the other.

According to definition,

Potential difference (V) = Work (W)
Charge (Q)

or, V = W
Q

[Work (W) = Charge (Q) × Potential difference (V)]

According to this, when 5 coulombs of charge flows in an electric circuit having potential
difference of 4V, the amount of work done will be 20 Joule.

108 Blooming Science Book 9

Voltmeter: The instrument which measures the potential difference across any two points in a
circuit is called a voltmeter.

The voltmeter is connected in parallel to the cell or any other part of the circuit across which
potential difference has to be measured as shown in fig. It has a red positive terminal and a black
negative terminal. Similarly to the ammeter, the red terminal is connected to the positive side of
the circuit and the black terminal is connected to the negative side.

V

01 2 3 4 5 6 7 89

V

(a) Voltmeter VV
(b) Three voltmeters in a circuit

Ammeters are connected in series but voltmeters are connected in parallel

Ammeters are the devices which are used to measure the current and are necessary to make the
flow of current through them. In order to make the flow of current through the ammeters, they
are always connected in series combination, electricity divides up among the resistances (Load).

But the voltmeters are the devices used to measure only the electrical pressure (pd) between
any two points. Hence, they are necessary to join two points in which the pressure or potential
differences are to be measured, then the connection will be parallel.

That’s why ammeters are always connected in series whereas the voltmeters are connected in
parallel.

Electromotive Force (e.m.f.)

When electrons flow in a circuit, an electric current will result, the continuous flow of electrons
in the circuit is maintained by a cell. The cell has to do the work in the circuit. In doing so, the
energy is liberated from the cell by the chemical reaction which takes place inside it.

Electromotive force of a cell is defined as the energy supplied by the cell to drive a unit charge
round the whole circuit.

The SI unit of e.m.f. is volt.

“The e.m.f. of a cell is 1.5 V” means that 1.5 joule of work is to be done when a charge of 1
coulomb is transferred from the positive pole of a cell to the negative pole.”

Comparative study between e.m.f. and p.d

Let us consider a circuit as shown in fig. Take the reading on voltmeter when the switch is open
i.e. when the circuit is open circuit. This reading gives the e.m.f. of the cell. Thus the potential
difference between the terminals of a cell when no current flows through it (or when it is an open
circuit) is known as the electromotive force (e.m.f.) of the cell.

Blooming Science Book 9 109

Now, close the switch in the circuit so that the current V
flows in the circuit. Take the reading on voltmeter in K
this stage. This reading gives the value of the p.d. of
the cell. Thus the potential difference between the
terminals of a cell when a current flows through the
cell is known as the p.d. of the cell.

It can be said that the potential difference created R

between the terminals of a cell as a result of a

chemical reaction that takes place in the cell is Difference between e.m.f. and p.d.

called the e.m.f. of the cell. But as soon as the cell

is connected in a circuit, it sends current from the positive to negative terminal in the external

circuit and some work has to be done against internal resistance of the cell. Hence it is clear that

the p.d. of a cell is slightly less than its e.m.f.

Thus, the e.m.f. of a cell causes the flow of current in a circuit. As consequence of the e.m.f. of a
cell, the p.d. is developed across any two points in the circuit. In short, e.m.f. is regarded as the
cause and the p.d. is then its effect.

Differences between P.d and E.m.f

E.m.f. P.d.

1. It is a measure of the energy supplied by 1. It is a measure of the work done in taking

source to carry a unit charge through a a unit charge from one point to the other

circuit. through a circuit.

2. It is a cause. 2. It is an effect.

3. It is measured in open circuit. 3. It is measured in closed circuit.

4. It is greater than p.d. 4. It is less than e.m.f.

Resistance

A conductor opposes the flow of electric current through it. This property of the conductor is called
its resistance. The resistance of a conductor is its property that restricts the movement of free
electrons through it. It is measured in ohm (Ω) in SI system and is denoted by the symbol R.

If a current of 1A through a conductor maintains a p.d. of 1V across its ends, then its resistance
is 1Ω.

We know that metals contain large number of free electrons and thus electric current can pass
through them easily. For example, metals offer a little resistance to the flow of free electrons
through them, i.e., they have low resistance. Substances like manganin, constantan, nichrome,
tungsten, etc., offer high resistance to the flow of electric current through them. Hence, they
have high resistance. Constantan (or ureka) is an alloy of copper (55%) and nickel (45%). It
is used in the construction of pyrometer and thermocouple. Manganin is an alloy of copper
(83%), manganese (13%) and nickel (4%). It is used in the construction of rheostats and resistor.
Nichrome is an alloy of nickel (60%) and chromium (40%). It is used in the construction of
heating element.

110 Blooming Science Book 9

Insulators contain practically no free electrons of heaters in them. So, normally current does not
flow through them. Thus, insulators have very high resistance.

Semi conductors contains a few free electrons and so the flow of current through them is small.
However, if there temperature rises, more electrons are ejected from the valance shell and become
free electrons. So, they are better conductors at higher temperatures. Semiconductors have high
resistance than conductors but have low resistance than insulators.

Factors affecting Resistance

Do the following activities for the understanding about the factors of resistance.

1. Place two nicrome wires having lengths 10cm and 100 cm one by one in the gap PQ of on
electric circuit, as shown in the figure. Note the current indicated by the ammeter for each
wire. We find that higher current flows for the wire of length 10cm than that of the wire
of length 100cm. Thus, a long wire has high resistance than the shorter one. That is, the
resistance of a conductor is directly proportional to its length,

2. Now repeat the activity 1 by 2 nichrome wires of same size and thickness. First connect
the gaps with single wire and then by using the both the wires. Find the difference.

Thus, thin wire has high resistance than thicker one (Two wires) even though they have
same length. That is, the resistance af a conductor is inversely proportional to its cross-
sectional area.

3. Now repeat the activity 1 with copper and nichrome wires of same thickness and same
length. Note the current in the ammeter for each wire. We find that copper wire flows
high current than nichrome wire. Thus, different materials have different resistance even
though they have same length and thickness. That is, the resistance of a conductor depends
on nature of material used.

4. Note the current in the ammeter when a nicrome wire is

connecting the gap PQ. Then, heat the wire with the help of a A
spirit lamp or burner. We find that current decreases as the wire

becomes hotter and hotter. Thus, the resistance of a conductor PQ
increases with increase in temperature. Furthermore, a coiled

wire has more resistance than a straight wire, length and thickness of each wire being

same.

The wire in the filament bulb is coiled. why?

From the above activities the relation of resistance with its factors can be summed up as
following:

1. Resistance is directly proportional to the length of the conducting wire.

2. Resistance is inversely proportional to the thickness, (cross sectional area) of the conducting wire.

3. Conducting wires of different materials have different values of resistance.

4. Resistance is directly proportional to the temperature of the conducting wire.

5. Resistance of coiled wire is more than that of a straight wire.

Blooming Science Book 9 111

Resistivity and Conductivity

We know that the resistance of a conductor is directly proportional to its length and inversely
proportional to its cross- sectional area. If ‘l’ is the length of a wire, ‘a’ is its cross-sectional area
and ‘R’ is its resistance, then

R ∝ l ………………… (i)

R ∝ 1 ………………… (ii)
a

From eq (i) and (ii) we have

R ∝ l
a
l
∴ R = ρ a ………………… (i)

Where ρ is the resistivity or specific resistance (ρ) of the material. The SI unit of resistivity (ρ) is

ohm-metre (Ωm).

If 1 = 1m, A = 1m2, then R = ρ

ρ = Rla
The resistivity of a material is numerically equal to the resistance of a conductor of unit area per
unit length.

The reciprocal of the resistivity (ρ) of a material is called its conductivity (σ).

1
∴ Conductivity (σ) = resistivity (ρ)
The SI unit of conductivity (σ) is Ω-1m-1.

The reciprocal of resistance is called the conductance. i.e. conductance (c) = 1 . Its SI unit is
per Ohm or mho or Siemens. R

Differences between Resistance and Resistivity

Resistance Resistivity (Ω m)

1. It is the property of a conductor to oppose 1. It is the resistance of a conductor of 1m

the flow of electrons. length and 1 m2 cross section area.

2. Its value changes with the change of its 2. Its value does not change with the change

length and area. of length and area.

3. Its unit is ohm (Ω). 3. Its unit is ohm meter (Ωm).

Ohm’s Law

The relation between current through a metallic conductor and potential difference across its
ends was studied systematically by a German physicist, George Simon Ohm in 1826 A.D.
This relation is now known as Ohm’s Law. It states that the electric current passing through a
conductor is directly proportional to the potential difference across its two ends at a constant
physical conditions. [Temperature, cross-sectional area, length, shape, nature of material etc.]

112 Blooming Science Book 9

If ‘I’ is the electric current through a wire and ‘V’ is the p.d. across its ends,

I ∝V

or, V ∝ I

∴ V = RI …………….. (i)

Here, R is electrical resistance of the wire (conductor), which is used as constant.

From equation (i), we get

R = V
I

Resistance = Potential difference
Current

According to the above relationship, resistance of a conductor is the ratio of the p.d. across its
two ends to the electric current following through it.

Experimental Verification of Ohm’s law V

The experimental set up for the verification of Ohm’s law is PQ
as shown in the figure. A voltmeter (V) is connected across AK
a nichrome wire PQ. The wire PQ is then connected with an
ammeter (A), a key K and cells. The ammeter measures the N OR
current (I) through the circuit and the voltmeter measures the I
p.d. (V) across the wire PQ.
M
At first, take only one cell (join M and N). When the key K is
closed, electric current flows in the circuit. Record the readings Circuit to verify Ohm’s law
of the ammeter and the voltmeter. Repeat the experiment with
two cells by joining M and O. Now join M and R and record V
the readings of ammeter and voltmeter. If a graph is plotted
between p.d. (V) and current (I), we get a straight line through
origin as shown in the figure. This shows that the current is
directly proportional to the p.d. which verifies Ohm’s law.

I

Result of Experimental Verification of Ohm’s Law

No. of cells used p.d Readings in Ammeter Resistance = p.d.

1. current
2.
3. 1.5 0.3 5
4. 3.0 0.6 5
4.5 0.9 5
6.0 1.2 5

Blooming Science Book 9 113

Solved Numerical Problems

1. Calculate the current flowing through a 4Ω resistor when a potential difference of 8V
is applied across it.

Solution:

Here, Resistance (R) = 4Ω

Potential difference (V) = 8V

Current (I) =?

From Ohm’s law, we have

V = IR

8=I×4

∴ I = 2A

Hence, the current in the circuit is 2A

2. What is the length of a manganing wire of diameter 0.44 mm required to construct a
10Ω coil? Resistivity of manganin is 44.3 × 10-8Ωm.

Solution:

Here,

Length (l) = ?

Diameter (d) = 0.44 mm = 0.44 × 10-3m

Resistance (R) = 10Ω

Resistivity (ρ) = 44.3 × 10-8Ωm

We know,

R= ρ1
A

or, R = ρ 1 × 4
πd2 πd2
[ cross-secotional area (a) of wire having diameter ‘d’ = 4 ]
1
or, 10 = 44.3 × 10-8 (0.44 × 10-2) × 4

∴ l = 3.432 m.

Hence, the length of the manganin wire is 3.432m.

3. Cell of e.m.f. 2V drives a current through a resistor of resistance 20W. What is the
current?

Solution:

Voltage (V) = 2V

Resistance (R) = 20Ω

Current (I) = ?

114 Blooming Science Book 9

According to the formula,

V = RI

or, I = V = 2
R 20

= 110 = 0.1A

∴ The 0.1 A of electric current flows through it.

4. What potential difference is needed to make an electric current of 0.2 A flows through
a torch bulb of resistance 22.5Ω?

Given,
Current (I) = 0.2A
Resistance (R) = 22.5 Ω
Voltage (V) = ?
According to the formula

V = RI

or, V = 22.5 × 0.2

= 4.5 V

∴ The required potential difference is 4.5 V.

5. A potential difference of 16V is needed to make a current of 4A to flow through a car
head light. What is the resistance of the bulb in car head light?

Solution:

Voltage (V) = 16V

Current (I) = 4A

Resistance (R) = ?

According to the formula,

V = RI

V
R = I
16
= 4 = 4Ω

∴ The resistance of the bulb in the car head light is 4 Ω.

Magnetism

Magnet is a substance that attracts magnetic substances and the opposite pole of the other magnet.
If a magnet is brought near iron filings, it will be found that some iron filings cling together at
the two ends but almost no iron filings collect at the middle part of it. Thus, near the ends there
appears to be a point at which there is maximum attractive property. These points of the magnet
with maximum attractive property are called the poles of a magnet.

Blooming Science Book 9 115

If a magnet is suspended freely a particular end of the magnet always points towards the north
and the other towards the south. The end which points towards the north is called the north pole
and the end which points towards the south is called the south pole.

If the north pole of two magnets are brought close, the two magnets repel each other. The force
between two poles is also repulsive. But if the north pole of the magnet is brought closer to the
south pole of another magnet, they attract each other and a kind of force is experienced. The force of
attraction between two unlike poles becomes weak when they are moved apart. Further moving the
magnets will have no effect on one another. This shows that a magnet can exert forces at a distance.

Properties of Magnet
a) A magnet attracts magnetic materials around it.
b) A freely suspended magnet always points north and south at rest.
c) Like poles repel and unlike poles attract each other.
d) Magnetic poles are stronger than middle part.
e) The poles of a magnet are not separated.

Magnetic Field

Magnetic substances are attracted strongly by a magnet near to it. If it is kept gradually away
from such substances, attractive magnetic force also decreases in the same way. Beyond a certain
region of magnetic field around the magnet, it has no influence on magnetic substances. The
space around a magnet, where Magnetic force can be felt by a magnetic body is called Magnetic
field of the magnet. The field can be represented by lines called magnetic lines of force.

Activity

To draw magnetic lines of force a bar magnet, is put in N-S directions onto a sheet of paper
over a drawing board. A small compass is placed near to the north pole of the magnet. When
the compass needle has settled, the ends N, S, of the needle are marked with a sharp pencil
as dots A and B. The compass is now moved so that S-pole rests on the dot B which was
previously occupied by N-pole. Now put a dot C against N-pole. This process is continued
until the compass reaches the south pole of the magnet. The series of dots obtained thus gives
the magnetic lines of force. An arrow is put on the line to indicate direction of magnetic lines
of force from north to south.

C B
A
N
S

(a) (b)

Magnetic Lines of Force

The imaginary curved path that runs N pole to S externally the magnet in a magnetic field is
called magnetic line of force.

116 Blooming Science Book 9

A magnetic line of force in a magnetic field is a line, straight or curved, the tangent to which
at any point gives the direction of the magnetic field at that point. It is the path in which an
imaginary free N-pole travels if it is free to do so. However, free north pole does not exist.

The pattern of lines of force shown in figure above are due to two similar magnetic poles. The
effect of earth’s magnetism was not considered in those cases. As a matter of fact, the pattern of
lines of force changes due to earth’s magnetic field. The resultant pattern of lines of force of a
magnet and that of the earth depends on the direction in which the magnet is placed.

Properties of magnetic lines of force

i. There are imaginary lines which form closed loop.

ii. They never intersect with each other.

iii. They started from North to South pole externally and South to North pole internally of a
magnet.

iv. The tangent at any point on the loop gives the magnetic field at that point.

v. T hey are concentrated more at poles and less at middle of magnet.

A neutral point is that point where the resultant of the magnetic field due to the magnet and
the earth’s magnetic field is zero.

At neutral points, the magnetic field of bar magnet and earth’s magnetic field are equal and
opposite, hence the resultant field becomes zero.

Magnet with its North pole pointing towards Geographical North

Place a bar magnet with its north pole N
pointing north on a sheet of paper on a
drawing board. Trace the lines of force with
the help of a compass needle. The lines
of force due to the combined fields of the
magnet and the earth’s magnet is shown in
figure.

The above figure shows that the lines of

force near the magnet are mainly due to the

magnetic field of the magnet which is much

stronger than the earth’s magnetic field. As Fig: North pole pointing towards Geographical North
the distance from the magnet increases, the

magnetic field due to the magnet becomes weak. At very distant point, the field of the magnet

becomes much weaker than the earth’s magnetic field. So the lines of force at the distant points

from the magnet are mainly due to the earth’s magnetic field. These lines of force are nearly

straight and parallel running from South to North (geographical). Further two points marked

cross sign are seen. These two points are at equal distances from the center of the magnet. Lines

of forces do not pass through such points. If a compass needle is placed at such points, no force

acts on it and it may come to rest pointing in any direction. Such points are known as neutral

points.

Blooming Science Book 9 117

Magnet with its South pole pointing towards N
Geographical North wE

A bar magnet is kept on a sheet of paper on a drawing board S
with its south pole pointing north. The resultant field pattern
is obtained as shown in figure. In this case, the neutral points
are situated on the axis of the magnet and they are placed at
equal distances from the respective poles. In the figure the
neutral points is marked with X signs.

Terrestrial Magnetism Fig: South pole pointing towards
Geographical North

The earth acts as a huge magnet. The magnetism related to earth is called terrestrial magnetism.

The earth’s magnetic south pole is situated in the northern part of Canada and north pole is
situated in the corner of Antarctica, which are 1600km away from geographical poles.

Following facts prove the magnetic property of earth (or Evidences of terrestrial magnetism)

a) Freely suspended bar magnet always points north and south direction at rest.

b) Presence of magnetic property in iron ore.

c) Angle of dip and angle of declination.

d) Formation of neutral points in the magnetic field of a bar magnet.

Why does a freely suspended magnet always points north and south at rest? ?Do

The earth’s magnetic south pole lies under You
geographical north pole and its north pole lies If a iron nail is dipped in a
under the geographical south and hence the pole earth and left it there for 3-4 Know

of a suspended magnet will be attracted by unlike days it turns into magnet.

poles of the earth’s magnet. The nail must be kept

showing N-S direction.

Angle of Declination

The vertical line passing through any place on the Magnetic south pole Geographical
surface of the earth which contains geographic axis north pole
of the earth is called geographic meridian and the
vertical plane passing through any place on the Angle of declination
surface of earth which contains magnetic axis of
earth is called magnetic meridian.

The angle between magnetic meridian and geographical meridian at any place on the surface
of the earth is called angle of declination. It varies from place to place. To find the geographical
direction at a place by compass, one must know the angle of declination at that place. This is very
important in finding the direction when traveling in oceans by ship and flying in aeroplanes at
night or in the cloudy weather. Angle of declination 15o means magnetic meridian is 15o west of
geographic meridian i.e. compass needle points 15o west of the true north.

118 Blooming Science Book 9

Angle of Dip

The angle made by the dip needle with the horizontal

axis is called angle of dip or it is the angle made by 0 0
freely suspended magnet with horizon. Dip needle is

a magnetic needle, mounted on a horizontal support so Angle of dip 30 Dip needle 30

that it can rotate freely in a vertical plane. Instrument

that contains dip needle along with circular axis is 60 60
called dip circle. 90

Angle of dip

If the dip circle is brought to north pole of earth, its

north pole points vertically downwards and angle of dip is 90o. The effect of north and south pole

of the earth’s magnet is equal throughout magnetic equator and hence dip needle remains at rest

being with horizontal so angle of dip at equator is 0o. So the angle of dip is 90o at poles and 0o

at the equator. At magnetic poles the bar magnet rest vertically, while at magnetic equator it rest

horizontally.

Let’s Learn

1. It is quite safe for birds to perch on overhead power cables, but it is very dangerous for
a person standing on the ground touching a cable. The bird perching on a cable has both
claws on the cable, therefore, they are at the same potential. Thus no charge flows through
the bird. If a person, somehow, comes in contact with a cable, he/she instantly may be
killed. This is because human body is good conductor for electricity so, current flows
through the body.

2. An ammeter should be connected in series in an electric circuit. This is because it has low
resistance and hence it measures current in the circuit without any appreciable potential
drop across it.

3. A voltmeter should be connected in parallel across a device whose p.d. is to measure. This
is because it has high resistance and hence it measures the potential difference across the
device without causing any appreciable decrease in current through the device.

4. A freely suspended bar magnet on the earth’s surface always comes to rest in north-south
direction. The N-pole of the magnet shows geographical north and its S-pole shows
geographical south. Since the earth behaves as a huge magnet, the N-pole points towards
earth’s magnetic south pole and the S-pole points towards earth’s magnetic north pole.
This is because unlike poles always attract.

5. The angle of dip of Kathmandu is 42°. It means, a freely suspended magnet at a place in
Kathmandu makes an angle of 42° with the horizontal.

6. Angle of dip varies from place to place, it is because magnetic poles of the earth do not
lie at the same distance from each part of the earth. It causes different amount of magnetic
force on the dip needle kept at different places.

7. The value of angle of dip is not zero at geographical equator, it is because the angle of dip
is caused by the magnetic poles of the earth not by the geographical pole Thus the value
of angle of dip is 0° at the magnetic equator not at geographical equator.

8. Neutral points are found in magnetic field of a magnet, it is because of the equal magnetic
force of the south pole of the earth and the magnet is same but in opposite direction.

Blooming Science Book 9 119

Main Points to Remember

1. The electricity in motion is called current electricity.

2. Simple cell, dry cell, accumulators, solar cells and generators are the sources of electricity.

3. The conducting path through which an electric current can flow is called electric circuit.

4. An electrical circuit through which an electric current flows is called a closed circuit and
an electrical circuit through which no current flows is called an open circuit.

5. Those substances which allow the electric current to pass through them are called
conductors and substances which do not allow the electric current to pass through them
are called insulators.

6. The potential difference is the amount of work done in moving a unit positive charge from
one point to another. Its SI unit is volt.

7. The potential difference between two points is said to be 1V if the amount of work done
in moving 1 coulomb charge from one point to another is 1J.

8. Electromotive force is the energy supplied by a cell to drive unit charge around the whole
circuit. It is also measured in unit volt.

9. Ammeter is used to measure the electric current while voltmeter is used to measure the
potential difference.

10. Electric charge is measured in coulomb unit. 1 coulomb = 6 × 1018 electrons. Current is
said to be 1 ampere if 1 coulomb charge flows even in 1 second.

11. Resistance is a quantity by which any material tries to resist the flow of current. Its symbol
is R and unit is Ohm (Ω).

12. Ohm’s law states that if the temperature and other physical conditions remain constant
the current flowing through a conductor is directly proportional to the potential difference
across its ends. According to this, V = IR.

13. The region around a magnet upto which it can influence the magnet and other magnetic
substances is called magnetic field.

14. A neutral point is a point in which the resultant of magnetic field due to magnet and the
earth’s magnetic field is zero.

15. The magnetism related to earth is called terrestrial magnetism.

16. The angle between the magnetic meridian and geographic meridian at any place on the
surface of the earth is called angle of declination.

17. Angle of dip is the angle made by the dip needle with the horizontal axis.

PRO J ECTWORK

Take a bar magnet & compass. Then draw the magnetic lines of force in a white sheet of paper
and submit to your teacher

120 Blooming Science Book 9

Exercise

A. Choose the correct answer from the given altenatives:

1
1. σ = P gives the value of :
a. resistivity b. electric current c. conductivity d. resistance

2. resistance can be calculated by:

V VV
a. R= I b. VI cR= I d. R = I

3. What is the value of the angle of declination at the equator of the earth?

a. 15° b. 12° c. 16° d. 17°
4. Which of the following doesnot affect resistance?

a. type of wire b. temperature of wire c. thickness oe wire d. pressure on wire

5. What is the value angles of dip at kathmandu valley?

a. 0° b. 45° c. 60° 42°

B. Answer the following questions.

1. What is an electric circuit? Classify it.

2. On what factors does resistance of conductor depend upon?
3. State Ohm’s law. Prove that V = IR.

4. Define current and static electricity.

5. Prove that ρ = R × A (ρ = resistivity, R = resistance)
l
6. Write any two properties of magnetic lines of force.

7. What is terrestrial magnet? Write down any two effects of earth’s magnetism..

8. Define: b. Open circuit c. Electromotive force
a. Closed circuit

d. Potential difference e. Angle of dip f. Angle of declination

g. Resistance h. Magnetic field i. Magnetic lines of force j. Neutral point

9. Differentiate between:

a. Electromotive force and potential difference b. Closed circuit and open circuit

c. Ammeter and Voltmeter e. Resistivity and conductivity

f. Angle of dip and angle of declination
10. Give reason:

a) Ammeter is always connected in series.
b) Voltmeter is always connected in parallel across a circuit.
c) Angle of dip is 90o at magnetic poles and 0o at the magnetic equator.
d) A freely suspended bar magnet always points north and south at rest.
e) A coiled wire produces more resistance than a straight wire of equal length.
f) A freely suspended magnetic doesnot show the geographical poles.
g) A magnet doesnot make angle of 90o with horizon at geographical poles.
h) A magnet fails to show any direction at neutral points.

Blooming Science Book 9 121

11. Which meters are represented by X and Y in the given diagram cells
of an electric circuit? Write reasons for answer. Yx

12. Sketch a closed circuit diagram containing a voltmeter, bulb switch
ammeter a cell and a load. Also indicate the direction of current
flow in the circuit.

13. For what purpose the following appliances are used?

a. Voltmeter b. Ammeter

14. What do you mean by earth’s magnetic pole?

15. What does it mean by saying that the angle of dip Magnetic Geographical
of Kathmandu is 42o? south pole north pole
X
16. a. Name the angle x shown in the diagram.

b. What is the use of this angle?

Numerical problems

1. The power of a bulb of a car is 72W. Calculate the current flowing through wire if a battery

of 3V is connected into it. Also calculate resistance. [Hint: I = VP, R = VI ][ 24A, 0.125Ω]

2. If a cell of emf 20V is connected across a resister of 10Ω, what is its current? [2A]

3. If a 220V is supplied in a heater of power 1000W, calculate the current flow in it? [4.54A]

4. If 360 coulombs charge flows in a circuit in 2 minute calculate the amount of current
[Hint: I = Qt ] [3A]
flow?

5. Calculate the potential difference across the resistance 108 ohm if current of 1.5A is

flowing through it. [162V]

6. How much charge flows through a wire carrying 3A of current in 10 minutes ? [1800C]

7. A 4V battery is connected to a lamp of resistance 40Ω. Calculate the current through the
lamp. [0.1A]

8. What is the resistance of a material that draws 200mA of current at 20V? [100Ω]

9. Which of the two has greater resistance: a 1KW heater or a 100W tungsten bulb marked

230V ? [100W]

10. An electric bulb is marked 230V, 100W. Calculate the current that passes through the

filament and its resistance? [0.43A, 529Ω]

11. Calculate the resistance of a bulb rated 60W, 240V. [R = V2 ] [960Ω]
I

12. An electrical appliance is marked 220V, 550W. What is the resistance when in use? [88Ω]

122 Blooming Science Book 9

Chapter

8 CLASSIFICATION OF ELEMENTS

Learning Outcomes Estimated Periods: 9+2

On the completion of this unit, the students will be able to:

• describe atomic structure and electronic configuration of elements.

• define valancy and radicals.

• describe formation of compound by electrovalent and covalent bonds with examples

• write molecular formula of some simple compounds.

Pure and Impure Substances

A pure substance is composed of its unit particles of similar kinds. For example, a copper plate
is a pure substance because it is composed of the smallest particles of copper, similar in nature
and properties.

An impure substance is composed of unit particles of different nature. In impure substance, two
or more than two different substances are mixed together. So, such mass is also called mixture.
A mixture is a substance composed of two or more elements or compounds in any proportion by
weight. In it, each of the substances retains its identity and specific properties e.g., air, salt solution
etc. The components of mixture can be separated by easy mechanical means like evaporation,
distillation, crystallization, etc. which we have already studied in lower classes. Pure substances
can be classified into two classes. They are elements and compounds.

Matter

Pure substances Impure substances (mixture)

Elements Compounds Homogeneous Heterogeneous
Mixture Mixture

Element

An element is the simplest form of a pure substance, which cannot be splited up into two or more
simpler substances by any chemical reaction. All the substances are made up of elements. Thus,
elements are said to be the basic units and the building blocks of all the complex substances.
Hydrogen, oxygen, carbon, gold and iron are some of the examples of elements. Out of millions
of substances, more than 111 has been identified as elements in nature. Out of them, 92 elements

Blooming Science Book 9 123

have been found in nature and the rest have been synthesized in laboratory by artificial means.
For easy understanding and practicable study of elements, each element has been separately
symbolized with different symbols consisting of one or two letters.

Compound

Compound is a substance of atoms of two or more elements formed by the chemical combination
in a definite proportion by weight. For example, water is a liquid compound of hydrogen and
oxygen in which two elements are present in the fixed ratio of 1:8 by weight. The properties of
compound are always different from the original substance from which it is formed. Some other
examples of compounds are sodium chloride, carbon dioxide, methane, etc.

A compound is defined as a form of matter every sample of which is homogeneous and composed
of two or more than two elements, combined in fixed proportion by weight. CO2 is a compound
of carbon and oxygen in a fixed proportion in which they are combined in the ratio 3:8 by weight.

The main differences between compound and mixture are tabulated below:

Differences between Compound and Mixture

Compound Mixture

1. A chemical compound consists of the same 1. In a mixture, constituents can be present

elements in a fixed proportion by weight. in any proportion.

2. A chemical compound is formed by a 2. A mixture is formed without a chemical
chemical action. action.

3. The constituents of a compound cannot be 3. The constituents of mixture can be

separated by mechanical means. separated by mechanical means.

4. The properties of a compound are 4. The properties of a mixture are midway
always different from the properties of between those of constituents and
constituents. maintain their identity.

5. When a chemical compound forms, energy 5. Generally, no energy is evolved or absorbed

is evolved or absorbed. when a mixture is formed.

Molecule

It retains the properties of its own. The molecule of an element is made up of one or more atoms
of the same type while the molecule of a compound is made up of dissimilar atoms. It is very
small. It is difficult to estimate the size of a molecule or to determine its shape. For example,
hydrogen gas consists of two atoms of hydrogen (H) and ammonia is a compound gas which
consists of one atom of nitrogen and three atoms of hydrogen.

Molecule is the smallest particle of a substance that has the properties of that substance and can
exist in free state.

Atom

The smallest particle of an element, which can take part in chemical change is called atom. It
may or may not be capable of independent existence. Atoms of same elements are identical in
all respect while the atoms of different elements have different kinds of atoms. For example, all
atoms present in an iron wire are alike in all respect. But an atom of sodium is different from an

atom of magnesium.

124 Blooming Science Book 9

According to scientists, an atom is a sphere with a diameter of Nucleus Shell
approximately 10-10m . In the 19th century, it was found that there
Electrons

are other smaller particles in an atom. These smaller particles

are called sub-atomic particles or elementary particles. They are

electrons, protons and neutrons. Among three sub-atomic particles

of an atom, the protons and neutrons are located at the centre of

an atom called nucleus. Nucleus shows the positive charge due to

the presence of protons. It helps to find the atomic weights. The

electrons are revolving around the nucleus in their respective orbit Proton Neutron
or shell. The number of electrons present in an atom determines
the size of the atom as it occupies more space as compared to the Structure of the beryllium atom

nucleus. For example, the position of protons and electrons of the beryllium is shown in the given

figure:

In any atom, the number of electrons in the orbits is equal to the number of protons in the
nucleus. Then, the atom is called electrically neutral.

Atomic Number Rutherford discovered the proton ?DO
in nucleus of atom and electrons
The number of protons or the number of electrons You
in an electrically neutral atom of an element in the orbit of atom in 1911AD. Know
is called atomic number. It is denoted by Z.
Atomic number always determines the chemical For that he was awarded the Nobel
characteristics of the elements. Each element has
its own atomic number. Prize.

Mathematically,
Atomic number = No. of protons = No. of electrons
i.e., Z = p + = e -

Atomic Mass

The sum of the protons and the neutrons in the nucleus of an atom is called its atomic weight. It
is also called mass number. It determines the mass of an atom. It is denoted by A. It has no unit.

Mathematically,

Atomic weight = No. protons + No. of neutrons

i.e., A = p+ + n0

The weight of electrons is negligible as compared to the weight of protons or neutron. So, atomic
weight does not attempt the weight of electron. Knowing the atomic weight and the atomic number
of an element, the number of electrons, protons and neutrons in an atom of the element can be found.

Solved Numerical Problem
The atomic number and the atomic weight of sodium are 11 and 23 respectively. Find the
number of electrons, protons and neutrons of sodium atom.
Here, Atomic number (Z) = 11
Atomic weight (A) = 23

Blooming Science Book 9 125

Number of protons (p+) = ?

Number of electrons (e-) = ? Scan for practical experiment
Number of neutrons (n0) = ?

We have,

Atomic number = No. of protons

ie Z = p+

or, 11 = p+ visit: csp.codes/c09e08
p+ = 11

∴ e- = 11 [ p+ = e-]

Therefore, the number of protons = number of electrons = 11

Again, atomic weight = No. of protons + No. of neutrons

ie A = p+ + n0

or, 23 = 11 + n0

or, n0 = 32 - 11

or, n0 = 12

Therefore, number of neutrons = 12.

Electronic Configuration of Atoms

The electrons are arranged according to their potential energy in different energy levels or shells or
orbits. The energy levels of electrons are denoted by the number 1, 2, 3, 4, 5 and 6 where as shells are
represented by the letter K, L, M, N, O and P. First energy level is K shell, 2nd energy level is L shell
and so on. The shells are counted from the centre outwards K shell has the minimum energy and is
nearest to nucleus. L shell has little more energy and it is a bit far from the nucleus and so on.

An atom is more stable when it has the minimum energy. So, the electrons occupy the low energy
levels first (This makes the atoms more stable). K shell is the lowest energy level, so first of all
the electrons fill K shell, then L shell, M shell and so on. To write the electronic configuration
of an element, we should remember the number of electrons in an atom of the element given by
its atomic number. The maximum number of electrons which can be put in a particular shell can
be calculated by using 2n2 rule. According to which K shell can accommodate 2 electrons, L can
accommodate 8, M can accommodate 18 and so on. The arrangement of electrons in the various
shells or orbits of an atom of the element is known as electronic configuration.

To explain the arrangement of electrons in different shells, Bohr and Bury (1921 A.D.) proposed
a scheme for the distribution of electrons in different shells as given below:

1. The maximum number of electrons in each shell or the energy level is determined by the
2n2 rule, where ‘n’ is the number of the shell.

For example
For the K shell, the maximum number of electrons will be 2n2

126 Blooming Science Book 9

= 2.(1)2 = 2 × 1 = 2 (where n = 1) Maximum number of electrons 2 8 18 32 32 18 8 Shells
are accommodated in different
For the L shell, the maximum K L M N O P Q
number of electrons will be 2n2 shells according to 2n2 rule
= 2.(2)2 = 2 × 4 = 8 (where n = 2)

For the M shell, the maximum 7 6 5 4 3 2 1
number of electrons will be 2n2

= 2.(3)2 = 2 × 9 = 18 (where n = 3)

For the N shell, the maximum
number of electrons will be 2n2

= 2.(4)2 = 2 × 16 = 32 (where n = 4)

Arrangement of electrons in different shells of an atom

But this rule is not applicable to the 5th, the 6th and the 7th shells of an atom. In these shell, the
numbers of electrons are: O = 32, P = 18 and Q = 8

2. The maximum number of electrons in outermost orbit is 8 and the maximum number of
electrons in the second last orbit is 18.

3. It is not necessary for an orbit to be completed before another begins to be formed. In fact,
a new orbit commences when any second outermost orbit attains 8 electrons.

4. The energy level or shell nearer to the nucleus is called lower energy level.

The outermost shell of an atom cannot accommodate more than 8 electrons, even it has the
capacity to accommodate more. This is due to the fact that having 8 electrons in the outermost
shell, makes the atom very stable.

The electronic configuration of sodium whose atomic number is 11 is 2, 8, 1. But the electronic
configuration of potassium whose atomic number 19 is 2, 8, 8, 1. Though M shell can accommodate
up to 18 electrons, it has only 8 electrons and the next electrons enter the N shell.

Structure of Sodium Atom

Atomic No. Number of electrons in each Electronic
shell configuration

11P K L M N

12N

Sodium 11 2 8 1 - 2, 8, 1

Sub-shell (orbitals)

The shells of atoms are made up of sub-shells. Every shells have sub-shell, they are names as s,
p, d, f. The first shell ‘K’ has one sub shell (s) , the ‘L’ shell has 2 sub-shells 2s and 2p, similarly,
‘M’ has 3 sub-shells 3s, 3p, 3d and N-shell has 4 sub-shells 4s, 4p, 4d, 4f. The maximum number
of electrons that every sub-shell can accomodate are given below:

s=2e- , p = 6e- , d = 10e- , f = 14e-

Blooming Science Book 9 127

Electronic configuration and its symbolic notation

The distribution of electrons in sub-shell is given by Aufbau Principle. It states that “ the
electrons occupy the orbitals of minimum energy first then they occupy the orbitals of maximum
energy.” The squence is given as:

4s 4p 4d 4f

3s 3p 3d

2s 2p

1s

i.e 1s, 2s, 2p 3s, 3p, 4s, 3d, 4p and so on. No. of
For example: Electrons

Electronic configuration of H: 1s1

Shell Orbital

Number of protons, neutrons and electrons of first twenty elements with sub-shell configurations.

S.N. Name of the Element Atomic No. of No. of No. of KL M N
Protons
and symbol number Neutrons Electrons

1. Hydrogen (H) 1 1 0 1 1s1

2. Helium (He) 2 2 2 2 1s2

3. Lithium (Li) 3 3 4 3 1s2 2s1

4. Beryllium (Be) 4 4 5 4 1s2 2s2

5. Boron (B) 5 5 6 5 1s2 2s2 2p1

6. Carbon (C) 6 6 6 6 1s2 2s2 2p2

7. Nitrogen (N) 7 7 7 7 1s2 2s2 2p3

8. Oxygen (O) 8 8 8 8 1s2 2s2 2p4

9. Fluorine (F) 9 9 10 9 1s2 2s2 2p5

10. Neon (Ne) 10 10 10 10 1s2 2s2 2p6

11. Sodium (Na) 11 11 12 11 1s2 2s2 2p6 3s1

12. Magnesium (Mg) 12 12 12 12 1s2 2s2 2p6 3s2

13. Aluminium (Al) 13 13 14 13 1s2 2s2 2p6 3s2 3p1

14. Silicon (Si) 14 14 14 14 1s2 2s2 2p6 3s2 3p2

15. Phosphorus (P) 15 15 16 15 1s2 2s2 2p6 3s2 3p3

16. Sulphur (S) 16 16 16 16 1s2 2s2 2p6 3s2 3p4

128 Blooming Science Book 9

17. Chlorine (Cl) 17 17 18 17 1s2 2s2 2p6 3s2 3p5
18. Argon (Ar) 18 18 22 18 1s2 2s2 2p6 3s2 3p6
19. Potassium (K) 19 19 20 19 1s2 2s2 2p6 3s2 3p6 4s1
20. Calcium (Ca) 20 20 20 20 1s2 2s2 2p6 3s2 3p6 4s2

Valence Shell and Valence Electrons

The outermost orbit or shell of an atom is called valence shell and the number of electrons present
in the valence shell of an atom are called valence electrons. The valence electrons determine the
valency of an element. Beside this, the valence electrons possess more energy than the electrons
of inner shell so they take part in chemical reaction. The electronic configuration and valence
electron of some elements are given below:

Elements Atomic No. Electronic Configuration Valence Electrons
K LMN
Sodium 11 281 1
Magnesium 12 282 2
Aluminium 13 283 3
Silicon 14 284 4
Phosphorus 15 285 5
Sulphur 16 286 6
Chlorine 17 287 7
Argon 18 288 8

Let’s consider potassium. The atomic number of potassium is 19. Its electronic configuration and
atomic structure are given below:

Electronic configuration Valence electron
Valence shell
Shells K LMN
Nucleus
No. of electrons 2 8 8 1
Atomic structure of potassium
The outermost shell of potassium contains one electron
and is called valence electron. During chemical reactions,
this valence electron of potassium atom is transferred to
the valence shell of another atom.

In term of sub-shell, K = 1s2 2s2 2p6 3s2 3p6 4s1 valence electron

Core sub-shell valence shell

Information from Valence Electrons

1. Valence electrons of an atom gives us information about the combining capacity i.e.,
valency of the element.

2. The number of valence electrons of an atom gives us an idea about the group of the periodic

Blooming Science Book 9 129

table in which the element falls. For example, the number of valence electron of sodium is 1.
Thus, sodium element belongs to the Group I of the periodic table. It is applicable only for
representative elements but not for transition elements of periodic table.

3. The number of shells determine the period to which the element falls in the periodic table.
For example, the number of shell of sodium atom is 3. Thus, the sodium element is present
in the period 3 of the periodic table.

Valency

The combining capacity of elements or the radicals with the other elements or radicals to form
a molecule of an element or a compound is called valency. Numbers such as 1, 2, 3, 4, 5 and 6
represents the valency. The inert gases do not have valency so they cannot form the compound.
But other compounds are formed due to the valency of elements or radicals.

Ways of finding valencies

A. To find the valency of an element or a radical, hydrogen, oxygen and chlorine are taken
as the standard elements. The number of hydrogen atoms or chlorine atoms or double
number of oxygen atoms with which one atom of a element combines is known as its
valency. The following are the common examples to find out the valencies of elements or
radicals:
a. The valency of chlorine (Cl) in hydrochloric acid (HCl) is 1 because 1 atom of Cl
combines with 1 atom of H.
b. The valency of oxygen (O) in water (H2O) is 2 because 1 atom of O combines with
2 atoms of H.
c. The valency of nitrogen (N) in ammonia (NH3) is 3 because 1 atom of N combines
with 3 atoms of H.
d. The valency of carbon (C) in methane (CH4) is 4 because 1 atom of C combines
with 4 atoms of H.

From these examples, we can say that different elements have different combining capacity with
respect to hydrogen. In case the element hydrogen does not combine with all other elements. In such
condition, the valency of an element is determined by comparing it with that of chlorine or oxygen.
The following are the examples to find out the valencies of elements or the radicals with the help
of chlorine:
a. The valency of sodium (Na) in sodium chloride (NaCl) is 1 because 1 atom of Na combines

with 1 atom of Cl.
b. The valency of magnesium (Mg) in magnesium chloride (MgCl2) is 2 because 1 atom of

Mg combines with 2 atoms of Cl.
c. The valency of aluminium (Al) in aluminium chloride (AlCl3) is 3 because 1 atom of Al

combines with 3 atoms of Cl.
Similarly the valency of an element combines with oxygen can be found out by taking the

valency of oxygen to be 2.
d. The valency of calcium (Ca) in calcium oxide (CaO) is 2 because 2 atoms of oxygen

combine with 1 atom of calcium.

130 Blooming Science Book 9

Examples 1. Find the valency of phosphorus in P2O5.

Here, P2O5

In P2O5, 2 atoms of ‘P’ combines with 5 atoms of ‘O’

= 10 atoms of hydrogen.

or, 1 atom of ‘P’ = 5 atoms of ‘H’

∴ Valency of ‘P’ = 5

Therefore, the valency of phosphorus in P2O5 is 5.

Example 2. Find the valency of ‘X’ in XCl5

Here, XCl5

In XCl5, 1 atom of ‘X’ combines with 5 atoms of ‘Cl’ so the valency of the ‘X’ in XCl5 is
5. Here the valency of ‘X’ is found out by comparing it with the number of chlorine.

B. The number of electrons present in the outermost orbit of an atom of element determines
the valency of that element. For example, the number of electron in the outermost orbit of
sodium is 1. Therefore, the valency of sodium is 1. Similarly, the number of electrons in
the outermost orbit of magnesium is 2. Therefore, the valency of magnesium is 2.

C. The number of electrons lost or gained or number of pairs of sharing electrons between the
combining atoms during molecule formation determines the valency of the elements. For
example, sodium atom loses an electron and chlorine gains it. So, the valency of sodium
and chlorine is 1. On the other hand one pair of electrons is shared between two chlorine
atoms. So, the valency of chlorine is 1.

Different elements have different valencies. The Greek prefixed mono for one, di for two, tri for
three, tetra for four, penta for five and hexa for six are used to explain the number of valency.
For example, hydrogen, sodium, potassium, etc. are monovalent because they all have valency
one. (Some metallic elements, sometimes show variable valencies. For example, mercury show
valency 1 in mercurous chloride (HgCl) and valency 2 in mercuric chloride (HgCl2). Usually, the
name of elements with the lower valency ends with suffix - ous and that with the higher valency
ends with suffix - ic. The variable valencies of an element depends upon the number of electrons
in the outer most orbit from other.)

Some Elements and their Valencies

Monovalent Bivalent Trivalent Tetravalent Pentavalent Hexavalent
Valency = 1 Valency = 2 Valency = 3 Valency = 4 Valency = 5 Valency = 6

Hydrogen H Oxygen O Nitrogen N Carbon C Nitrogen N Sulphur S
Chlorine Cl Magnesium Mg Boron B Silicon Si Phosphorous P
Bromine Br Calcium Ca Phosphorous P Tin (-ic) Sn
Iodine I Barium Ba Aluminium Al Lead (-ic) Pb
Potassium K Zinc Zn Chromium Cr Sulphur S
Sodium Na Mercury (-ic) Hg Iron (-ic) Fe

Blooming Science Book 9 131

Mercury (-ous) Hg Iron (-ous) Fe Gold (-ic) Au

Copper (-ous) Cu Tin (-ous) Sn Manganic (-ic) Mn

Silver Ag Lead (-ous) Pb

Gold (-ous) Au Sulphur S

Copper (-ic) Cu

Manganese (-ous) Mn

Variable Valency

Some elements can combine with different number of atoms to form different compounds. These
elements have more than one valency which is known as variable valency. For example, Copper
combine with chlorine to form two different compounds such as CuCl, CuCl2. The valency of
copper is 1 in compound CuCl (Cuprous Chloride) and 2 in compound CuCl2 (Cupric Chloride).
Similarly, iron combines with chlorine to form two different compounds such as FeC12, FeCl3.
The valency of iron in FeCl2 (Ferrous Chloride) is 2 while the valency of iron in compound FeCl3,
(Ferric chloride) is 3. Here, copper and iron show different valencies. Hence, these valencies are
known as variable valency. The name of the compound refers according to the valencies they
show in the compound. If an element forms compounds showing its lower valency, then the name
of the compound becomes ‘ous’ like CuCl, which is named as cuprous chloride. Similarly, if an
element shows higher valency in a compound, its name becomes ‘ic’, for example, the name of the
compound for CuCl2, changes as cupric chloride. In some elements one or two electrons of second
last orbit can also be used during chemical reaction. This causes variable valancy in the elements.

Radicals

A radical is an atom or a group of atoms of different elements having positive or negative charge
and acts as single symbols and valencies. They do not exist freely. They are present in combined
form with other atoms of elements or another radicals. On the basis of their electric properties,
radicals can also be classified into electropositive and electronegative radicals.

a. Electropositive Radicals: The radicals, which have the capacity of losing electrons and
carry positive charge are called electropositive radicals. They are also called basic radicals
or metallic radicals, e.g., Ca++, Mg++, Na+, K+, Al+++, etc.

b. Electronegative Radicals: The radicals, which have the capacity of gaining electrons
and carry negative charge are called electronegative radicals. They are also called acidic
radicals or non-metallic radicals, e.g., O– –, NO3–, CO3– –, HCO3–, etc.



On the basis of construction, radicals (whether basic radicals or acid radicals) can also be
classified into simple radical and compound radical. Simple radical is that radical which
contains only one atom of different elements. It does not exist freely. It presents only a part
of molecule, e.g., Na+, K+, Mg++, Ca++, A+++, Cl–, Br–, I–, etc. On the other hand, compound
radical consists of a group of atoms of different element. It can pass into or pass out of
compound as a unit without undergoing decomposition, e.g., CO3– –, NO3–, SO4– –, HCO3–,
etc.

132 Blooming Science Book 9

The number of charge carried by the radical indicates the valency of a radical.
Note: The valency of an atom or a radical is always a pure whole number without any +ve or

-ve sign. Example, the valency of NH4+ radical is 1 not +1. Similarly the valency of CO3– –
radical is 2 not -2.

Symbols and Valencies of Electropositive Radicals.

Monovalent Bivalent Trivalent Tetravalent
Valency = 1 Valency = 2 Valency = 3 Valency = 4
Hydrogen H+ Magnesium Mg++ Aluminium Al+++
Sodium Na+ Zinc Zn++ Ferric Fe+++ Stannic Sn++++
Potassium K+ Calcium Ca++ Auric Au+++
Cuprous Cu+ Cupric Cu++ Chromium Cr+++ Plumbic Pb++++
Mercurous Hg+ Nickel Ni++
Silver Ag+ Ferrous Fe++
Aurous Au+ Mercuric Hg++
Ammonium NH4+ Stannous Sn++
Barium Ba++

Symbols and Valencies of Some Electronegative Radicals

Monovalent Valency = 1 Bivalent Valency = 2 Trivalent Valency = 3
Nitride N---
Chloride Cl- Sulphide S-- Phosphate (PO4)---

Bromide Br- Sulphite (SO3)--
Sulphate (SO4)--
Iodine I- Oxide O--

Fluoride F- Peroxide O2--
Silicate (SiO3)--
Chlorate (ClO3)- Carbonate (CO3)--
Nitrite (NO2)-
Nitrate (NO3)-
Hydroxide (OH)-

Bicarbonate (HCO3)-
Cyanide (CN)-



Note: The valency of some radicals can also be determined with the help of valencies of
elements present in them.

Let’s consider, ammonium (NH4+) radical. In NH4+, there are one nitrogen atom and four
hydrogen atoms. The valency of one nitrogen is -3 and that of four hydrogen is (4 x 1) = + 4:

So the valency of NH4+ is -3 + 4 = +1 but it is written as 1.
Similarly, bicarbonate (HCO3)-, there are one hydrogen atom is +1, one carbon atom is +4 and
that of three oxygen atom is (3 × 2) = -6. So the valency of (HCO3)- is +1 + 4 - 6 = -1 but it is
written as 1.

Blooming Science Book 9 133

Inert Gases

The elements fall in the zero group of the periodic table are inert gases or noble gases. They are
helium, neon, argon, krypton, xenon and radon. These elements are chemically inert because of
their stable electronic configuration. That is, helium consists of 2 electrons in the valence shell
while the rest inert gases (Ne, Ar, Kr, Xe and Rn) contain 8 electrons in their valence shell. The

Duplet and Octet (Stable Configuration)

Helium is an inert gas which has K-shell only and has two electrons. This arrangement of two
electrons in the K shell is said to be duplet. It is chemically inert because it has a complete
number of electrons in its K-shell or duplet.

Except helium, the other inert gases have 8 electrons in their valence shell. Such arrangement
of stable group of 8 electrons in their valence shell is said to be octet. The inert gases are stable,
chemically inert and having no combining capacity to the other because they neither gain
electrons from others nor transfer their electrons to other.

Except inert gases, the atom of other elements have no stable group of 2 or 8 electrons (duplet or
octet) in their valence shell. So, they are unstable, chemically reactive and having the combining
capacity to the other. They try to achieve the duplet of the octet during the formation of molecules
of the same element or the molecule of a compound by different elements. During the formation
of molecule every atom of an element tries to achieve the octet in its valence shell by transferring
electrons to other or by sharing electrons between the combining atoms of elements. This process
is called octet rule. In this way, octet rule is a process by which atoms of the elements try to
maintain 8 electrons in their valence shell by transferring electrons to the other atom or by sharing
electrons between the combining atoms during the formation of molecule.

Note: The elements (like H, He, Li and Be) upto atomic number four have tendency to maintain
two electrons in their outermost orbit.

Chemical Combination or Bond Formation

The atoms ne with one another to achieve the stable electronic configurations of the nearest noble
or inert gases. Thus, when elements combine they either attain the stable configuration of helium
that is they attain a duplet of electrons and have two electrons in the outermost shell, or they
attain an octet of electrons and have eight electrons in their outermost shell.

The atoms having less than 8 electrons in their outermost shell are unstable. So these atoms
have a tendency to achieve the inert gas electronic configuration of 8 electrons (or, 2 electrons)
in their outermost shells and become more stable. Hence, the cause of chemical combination
between elements is the tendency of their atoms to have a duplet or an octet of electrons in
their outermost shell. A set of eight valence electrons which makes atom in the molecule more
stable in chemical combination is called octet. The noble or inert gases have stable electronic
configurations.

Atoms tend to achieve the stable inert gas electronic configuration either by transfer of electron
(electrovalent bond) or sharing the electrons (covalent bond) of the outermost shells or the atoms.

134 Blooming Science Book 9

Electrovalent Bond

The chemical bond formed by the transfer of electrons from one atom to another is known as an
electrovalent bond or ionic bond. The transfer of electrons takes place in such a way that the ions
formed have the stable electronic configuration of an inert gas.

For example, sodium and chlorine atoms combine to form a sodium chloride molecule. The sodium
atom loses its one valence electron and acquires a positive charge and forms a sodium ion (Na+).
Sodium ion has an electronic configuration similar to the inert gas neon (Ne), so it is very stable.

The chlorine atom has seven electrons in its outermost shell and accepts one electron (from the
sodium atom) to achieve the stable inert gas configuration. The chlorine atom after accepting one
electron becomes a negatively charged ion (Cl-). Chloride ion has an electronic configuration
similar to the inert gas argon (Ar), so it is also very stable.

+11p 17p +11p 17p

12n 18n 12n 18n

Na Cl Na+ Cl-

or Na + Cl →[Na]++� Cl - →Na+Cl-→ NaCl
�
2, 8,1 2, 8,7 2,8
2, 8,8

The sodium ion has a positive charge and chloride ion has a negative charge. Due to opposite
charges, sodium ion and chloride ion are held together by an electrostatic force of attraction to
form a molecule of sodium chloride (NaCl).

The compounds formed by electrovalent bonds are called electrovalent compounds.
Electrovalent Compounds are highly soluble in water but insoluble in organic solvents. They
ionize into ions in water. Electrovalent compounds are good conductors of electricity. They are
generally solids. On account of the strong interionic forces of attraction, they have high melting
and boiling points.

Covalent Bond

The chemical bonds formed by sharing of one or more pairs of electrons between the atoms are
known as covalent bonds. In the covalent bond, both the atoms are in need of electrons in their
valence shells and thus, both of them have tendency to share electrons. The atoms mutually share
electrons to complete their duplet or octet and this results in the formation of a definite bond
between the atoms. The sharing of one, two or three pairs of electrons forms single, double or
triple bonds.

Non-metals usually have 5, 6 or 7 valence electrons in their atoms (except hydrogen that has
1 and carbon that has 4 valence electrons). Atoms of non-metal mutually share electrons to
complete their octet i.e. attain the stable inert gas configurations. The compounds formed by
covalent bonds are called covalent compounds.

Blooming Science Book 9 135

Covalent compounds are usually soluble in organic solvents but insoluble in water. The smallest
particle of covalent compound is formed by sharing of electrons. So they do not ionize in water.
The solutions of these compounds do not conduct electricity. Covalent compounds are mostly
gases, a few are liquids and a very few are solids. On account of intermolecular forces of attraction
between the molecules, they have low melting and boiling points. Some examples of formation
of compounds by covalent bond are given below:

a) Formation of Methane

One carbon atom and four hydrogen atoms combine to form a molecule of methane (CH4).
Carbon atom has four valence electron and each hydrogen atom has one valence electron. So
carbon atom shares its four valence electrons with four electrons from four hydrogen atoms and
forms a methane molecule.

In a methane molecule, both carbon atom and hydrogen atoms have stable inert gas configuration.
Carbon attains an octet of electrons and the hydrogen attains a duplet of electrons. There are four
shared pairs of electrons.

Hydrogen

1p+
1p+

1p+ 6p+ 1p+ 1p+ 6p+ 1p+ H
Hydrogen 6n 6n HCH

Hydrogen H
Carbon
1p+

1p+

Hydrogen

H
H

or, H C H H C H CH4

H
H

b) Formation of Water

Oxygen atom has six electrons in its outer energy level. Four of these electrons are grouped
into two pairs and are generally unavailable for covalent bonding. The other two electrons are
unpaired and each can be shared with another atom in a covalent bond. In the water molecule
(H2O), one of these electrons participates in a covalent bond with one hydrogen atom, and the
other in one forms a covalent bond into another hydrogen atom. Two single bonds are formed,
and all three atoms have stable electronic configuration.

136 Blooming Science Book 9

1p+ 1p+ 1p+ 8p+ 1p+ HOH
Hydrogen 8p+ Hydrogen 8n
8n

Oxygen

or,

HO H

c) Formation of Nitrogen (N2) HOH
H2O
N
N

d) Formation CO2 NN
N2

O CO

O = C =O → CO2

Differences between Electrovalent Compounds (ionic) and Covalent Compounds

Electrovalent Compounds Covalent Compounds

1. They are formed by the complete transfer 1. They are formed by the mutual sharing of

of electrons. electrons.

2. They are generally crystalline solids. 2. They are generally liquids or gases under
ordinary conditions.

3. The molecules of these compounds have 3. The molecules of these compounds have

no definite shape. definite geometrical shapes.

4. They have high melting point and boiling 4. They have low melting point and boiling

point. point.

5. They are easily soluble in water but they 5. They are insoluble in water but they are

are insoluble in organic solvents such as usually soluble in organic solvents such as

benzene, chloroform etc. benzene, chloroform etc.

6. They can be electrolysed. 6. Generally they cannot be electrolysed.

7. They are bad conductor of electricity in the 7. They generally do not conduct electricity.
solid state but they can conduct electricity
in molten (liquid) state or in aqueous
solutions.

8. These compounds react very fast in solution. 8. These compounds react very slow in solution.

Blooming Science Book 9 137

A group of symbols of elements that represent a molecule of a compound or an element is called
a molecular formula. It can be written by applying the following steps.

Step 1: Write the symbols of the two radicals side by side. The positive radical first and the
negative radical to the right of it and note their valencies at the top.

Step 2. If necessary, divide the valencies by HCF to get a simple ratio.

Step 3: Shift the valencies crosswise to the lower right of the compound radical. The radical is
enclosed in a bracket and the number placed outside the bracket to its lower right-side.

When writing a molecular formula, one has to follow the steps given above these steps are
illustrated below:

Name of compound Symbol with valencies Criss-cross Formula

Aluminium chloride Al3 Cl1 Al3 Cl1 AlCl3
Al1 Cl3

Magnesium Oxide Mg2 O2 Mg2 O2 MgO
Mg2 O2

Sodium Phosphate Na1 PO34 Na1 PO43 Na3PO4
Na3 (PO4)1

Information conveyed by molecular formula

1. Molecular formula represents the percentage of composition of each element present in
the given compound.

2. It represents one molecule of the substance.

3. It tells about the number of atoms of each element present in one molecule of the compound.

4. It shows the combining capacity of its constituent elements with hydrogen i.e., valency.
For example in water, the valency of oxygen is two and the valency of hydrogen is one.

5. It helps to know about the molecular weight of the substance. For example, the molecular
formula of oxygen is O2.
∴ Molecular weight of oxygen = O + O = 16 + 16 = 32.
Similarly, the molecular formula of carbon dioxide is CO2.
∴ Molecular weight of CO2 = 1C + 2O

=1 × 12 + 2 × 16
= 22 + 32
= 44

138 Blooming Science Book 9

Let’s Learn

1. Ammonia is a covalent compound because there is sharing of e- between Nitrogen and
Hydrogen atom.

2. Neon is inert gas. Neon has the stable electronic configuration 2, 8. The outermost orbit
of neon is filled with 8 electrons so it cannot gain or lose any other electron. Hence neon
is said to be stable and it is called inert gas.

3. An atom is electrically neutral. This is because the number of negative charged electrons
in the different shells is equal to the number of positive charged protons in the nucleus of
an atom and they can neutralize together.

4. Sodium is reactive metal. This is because the sodium has 1 valence electron and has the
tendency to lose valence electron for stable electronic configuration.

5. The valency of chlorine is 1. The electronic configuration of chlorine atom is 2, 8, 7. The
chlorine atom gains one electron from other and becomes 2, 8, 8 which has the similar
electronic configuration reaction. Hence the valency of chlorine is 1.

Main Points to Remember

1. Matter occupies space; has weight and volume.
2. Matter exists in three states: solid, liquid and gas.
3. There is a space between the molecules. The space between the molecules is known as

intermolecular space.
4. Matter can be changed from one state to another. Conversion of solid into liquid is known

as fusion or melting.
9 . John Dalton gave a comprehensive atomic theory.
10. An atom is defined as the smallest particle of an element, which can take part in a chemical

reaction.
11. A molecule of an element or compound is the smallest particle of it that is capable of self

existence.
12. An element is a distinct kind of matter which can neither be broken down into anything

simpler by ordinary means nor built from any other form of matter.
13. A compound is defined as a form of matter every sample of which is homogeneous and is

composed of two or more than two elements combined in a fixed proportion by weight.
14. The smaller particles in the atom are known as sub-atomic particles.
15. The major sub-atomic particles are: Proton, Neutron & Electron.
16. Proton and neutron are situated in the nucleus of the atom and constitute the atomic weight.
Atomic weight = No. of Protons + No. of Neutrons
17. Electrons revolve round the nucleus in different shells or orbits. The path of the electron

is known as the shell or orbit.
18. An atom is electrically neutral since the number of protons and electrons are equal and

have opposite charges.
19. The mass of proton and neutron is equal to one atom of hydrogen.

Blooming Science Book 9 139

20. The mass of electron is 1/1839 of that of a hydrogen atom.
21. The atomic number = No. of protons = No. electrons (in an electrically neutral atom).
22. The name of the element is represented by the first letter of its name or the first letter of its

Latin name is called its symbol.
23. The symbol signifies one atom of the element and atomic weight compared to the weight

of one carbon atom considered as 12.
24. A radical may be an atom of element or combination of two or more elements. The former

is simply called a radical and the latter is called a compound radical.
25. The radicals are of two types: basic or electronegative radical and acid or electropositive

radical.
26. The electronegative radicals move towards the positive electrode during electrolysis.

They carry a negative charge.
27. The electropositive radicals move towards the negative electrode during electrolysis.

They carry a positive charge.
28. The representation of the molecule in the form of a formula is known as a molecular

formula.
29. When the electron transfer takes place from one atom to the other and they are charged,

such a bond relation is known as electrovalent bond, eg. Sodium chloride.
30. When the two atoms of a bond relation share the electrons, this is called a covalent bond,

eg. Hydrogen molecule, water molecule.
31. When electrons required to fulfill the outer octet is contributed by a single atom in

combination, then the bond relation is called co-ordinate bond or co-ordinate covalency.

PRO J ECTWORK

Make a table to show the sub shell configuration of elements with atomic number 1 to 20 in a
chart paper and keep this in your study room/ class room.

Exercise

A. Choose the correct answer from the given alternatives:

1. .................................... is in the shape of a dumb bell.

a. d-orbital b. f-orbital c. p-orbital d. s-orbital

2. Which one is polar?

a. H2 b. O2 c. H2O d. O
3. The electronic configuration of Helium is 2 and its valency is

a. 1 b. 2 c. 4 d. O

4. The noble gases lie Ne, Ar, etc are ...............................gases.

a. monoatomic b. diatonic c. reactive d. metallic

5. Energy required to break C C bond in acetylene is............................ than that required to
break C C bond in ethane.
a. more b. less c. some d. cnone of the above

140 Blooming Science Book 9

B. Answer the following questions.

1. What is an atom? Is it stable? Give reasons to support your answer.

2. What is a molecule? Is it stable?

3. Define the following terms:

(a) Element (b) Valency (c) Ions

(d) Sub-shell (e) Octet (f) Duplet

4. What is a compound? Name five compounds.

5. Distinguish between a compound and a mixture.

6. Identify whether these are an element or a compound.

Sugar, water, hydrogen, chlorine, salt, mercury, gold, sand

7. What is meant by sub-atomic particles?

8. Write short notes on:

(a) Electrovalent compound (b) Co-valent compound

9. What is a symbol? What is the significance of a symbol?

10. Give the symbol and Latin name of the following elements.

(a) Antimony (b) Copper (c) Gold (d) Mercury

(e) Iron (f) Lead (g) Potassium (h) Sodium

(i) Silver (j) Tin

11. What is a radical? Write with examples.

12. Explain the meaning of following with examples:

(a) Electropositive radical (b) Electronegative radical

13. Write the electronic configuration of following in term of sub-shell.

a) Li b) O c) Na d) P e) K

14. What is molecular formula?

15. Write the molecular formula of the following chemical compounds as given in the
example.

Name of compound Symbol with valencies Criss-cross Formula
Sodium sulphate Na1 SO42 Na2SO4
Na1 SO42

Na2 (SO4)1

Water Carbon dioxide
Aluminium chloride Magnesium chloride
Sodium phosphate Chromium sulphate
Ammonium chloride Ammonium hydroxide
Ammonium nitrate Ammonium nitrite
Ammonium sulphate Barium sulphate
Barium hydroxide Barium nitrate

Blooming Science Book 9 141

Barium nitrite Barium phosphate
Calcium chloride Calcium oxide
Calcium hydroxide Calcium silicate
Calcium sulphate Calcium carbonate
Calcium bicarbonate Calcium nitrate
Calcium nitrite Carbon tetrachloride
Cupric oxide Copper sulphate
Copper chloride Cuprous oxide
Ferrous sulphate Ferric oxide
Ferrous oxide Ferric hydroxide
Hydrogen chloride (hydrochloric Hydrogen bromide (hydrobromic acid)
acid)
Magnesium oxide Magnesium chloride
Magnesium hydroxide Magnesium nitrate
Magnesium nitrite Magnesium carbonate
Magnesium bicarbonate Magnesium nitride
Magnesium sulphate Chromium oxide
Nitrous oxide Nitric acid
Potassium chloride Potassium oxide
Potassium hydroxide Potassium nitrate
Potassium nitrite Potassium chlorate
Sodium oxide Sodium hydroxide
Sodium carbonate Sodium nitrate
Sodium bicarbonate Sodium nitrite
Sodium sulphate Sodium bisulphate
Sodium bisulphate Silver chloride
Silver nitrate Silver bromide
Zinc chloride Zinc oxide
Methane Ammonia
Phosphorus pentoxide Silica
Hydrogen Bromide Carbon tertrachloride

25. Write the molecular structure or show the molecular formation of following
compounds:

a) H2O b) CO2 c) N2 d) H2 e) NaC l
f) CaO g) MgCl2 h) K2O i) AlCl3

142 Blooming Science Book 9

Chapter CHEMICAL REACTIONS

9

Learning Outcomes Estimated Periods: 2

On the completion of this unit, the students will be able to:

• tell the method of writing chemical equation and write chemical equation.

• balance the chemical equations.

Everything around us changes with time. During such changes one form of substances are
changed into another form. In our daily life we observe many changes around us. For example,
germination of seeds, digestion of food, burning of coal, melting of ice on heating, making of salt
solution, rusting of iron, dissolving salt in water, etc. The changes can be classified into physical
change and chemical change.

Physical change Scan for practical experiment

Physical change is a temporary change in which no new substance
is formed. It is usually reversible. The chemical composition of the
substance remains the same but only the physical properties like state,
colour, speed, motion etc. are changed. Some of the examples of
physical change are as follows:

1. Switching on an electric bulb. visit: csp.codes/c09e09

2. Magnetizing an iron nail.

3. Preparing a solution in laboratory.

4. Changing of ice into water and water vapour.

When ice is heated, it changes into water. On further heating, water changes into water vapour or
steam. Here, the state is changed but not the composition. This is because; each molecule of ice,
water and water vapour is composed of two atoms of hydrogen and one atom of oxygen.

Ice Heat Water Heat Steam
Cool Cool

Heat Heat Where,

Cool Cool - Oxygen atom

- hydrogen atom

Molecules of ice Molecules of water Molecules of steam

Blooming Science Book 9 143

Activity

Put a little sugar in a test tube. Heat the test tube slowly for some time. You will see that sugar
turns black and some water droplets collect in its mouth. Sugar turns into carbon and water
due to heat. Such a change is known as a chemical change.

Test tube Water drops
Tong

Sugar Black carbon
(a) Test tube with sugar
Burner
(b) Heating the content

Chemical Change

The permanent change in which completely new substances are formed and is irreversible is
called chemical change. Chemical changes are undergoing in nature. Due to chemical changes
that take place in our surroundings, we have seen change in some substance. Some of these
changes are irreversible whereas some are reversible. A chemical change takes place under some
conditions. When these conditions are fulfilled, then chemical changes take place. In a chemical
change, the following things are possible.

1. In a chemical change, smallest particles of matter such as atoms or molecules take part.
Because of chemical reaction, change takes place in the matter and a new matter forms.

2. Heat plays a vital role in a chemical change. Heat is required in some changes which is
generally provided by external heating systems whereas some reactions autonomously
generate heat. The former type of reaction is called endothermic reaction whereas the
latter is called exothermic reaction. In other words, endothermic reactions require or
absorb heat whereas heat is given out in exothermic reactions.

3. Due to chemical changes, the substances lose their properties and new substances are
formed which have different properties than that of reactants.

Chemical reaction and Chemical Equation

The combination decomposition, displacement or exchange of atoms or molecules during a
chemical change is called chemical reaction. A chemical reaction can be expressed interms of an
equation. The equation used to represent a chemical reaction is called chemical equation.

A chemical equation represents a chemical reaction with the help of symbols and formula. A
chemical equation is symbolic representation of an actual change.

A chemical equation representing in words is called word equation. While writing such equation,
more time, space and effort is consumed. Therefore, symbolic equations are used for writing
chemical reactions in chemistry and this is called the language of chemistry.

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In a chemical equation, the chemicals which take part in reactions are called reactants and the
substances which are formed due to a certain condition present are called products. In the
representation of chemical equation, the reactants are written on the left hand-side of the arrow-
head and the products on the right hand-side. The arrow-head shows the direction in which the
reaction is going. (→) shows irreversible reaction, ( ) shows reversible reaction and (=) shows
equality. The gases are shown by (↑) and precipitates by (↓). A solid is given by (s), liquid by (l)
and gas (g) in some cases.

What Happens During Chemical Changes?

Generally the following things happen during chemical changes:

1. Atoms of molecules can take part in a chemical reaction to form a new substance.

2. Heat can play important role in the chemical reaction.

In some conditions, external heat is needed for the chemical reaction; while in some
condition, heat is produced itself due to chemical reaction.

3. New substances are formed. The reactants lose their properties and products gain new
properties.

Essentials of Chemical Equations

A chemical equation must satisfy the following conditions:

1. It should represent an actual chemical change which is practically possible.

2. It should be balanced i.e. the total number of atoms of elements in a reactant on the left
hand side should be equal to the total number of atoms of some elements on the product
on the right hand side.

3. It should be molecular i.e. all the substances should be represented in the form of their
molecules.

Limitations of a Chemical Equation

A chemical equation can’t give the following informations:

1. Heat is given out or absorbed during the chemical reaction.

2. The condition under which the chemical reaction takes place.

3. The physical states of substances concerned.

4. The time taken by the reaction to complete it.

5. The rate of reaction whether slow or violent.

6. The concentration of reactants.

An equation, which indicates the evolution or absorption of heat in the reaction is called a thermo
chemical equation. In an endothermic reaction, minus sign is used to indicate the absorption of

Blooming Science Book 9 145

heat while in an exothermic reaction, plus sign in employed to show its evolution. For example,

N2 + O2 → 2NO -43,000 cals
C + O2 → CO2 + 94,300 cals

Balancing Chemical Equation

Before we go on balancing chemical equations, a sound knowledge of the chemical formulae
of substances is required. The chemical equation may be in word equation form or skeleton.
Primarily, there are two methods for balancing chemical equations. They are:

(a) Hit and trial method

(b) Partial equation method

You will study about hit and trial method in this section. How to balance equations by hit and
trial methods?

1. Write the word equation clearly.
?2.
Write down the molecular formula DO
of each chemical substance. Elements like H, N, O, Cl, F, Br, I are called You
diatomic element because they exist in Know

3. Count the number of atoms or two atomic state(molecular form ) so in

molecules on both sides. chemical reaction they must be written in

4. Increase the coefficients of the molecular form eg: H2, N2, O2 Cl2, Br2,
molecules that are not equal on both I2, F2

sides.

5. Do not change the molecular formula.

6. Write the balance equation in molecular form.

Example

Word equation: Potassium chlorate → Potassium chloride + Oxygen

Symbolic equation: KClO3 → KCl + O2 (Skeleton equation)
KCl + 3O2
KClO3 → 2KCl + 3O2 (balanced equation)
Sodium hydroxide + Hydrogen
2KClO3 →

Word equation: Sodium + Water →

Symbolic equation: Na + H2O → NaOH + H2 (Skeleton equation)
Na + H2O → NaOH + 2H2
2Na + 2H2O → 2NaOH + H2 (balanced equation)

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Information from the balanced chemical equation
The following information can be obtained from the balanced equation.
(a) The names of the reactants and products and their molecular formula can be seen.
(b) The number of atoms and molecules that take part in the chemical reaction.
(c) The ratio of weight of molecules of reactants and products.
(d) The type of the chemical reaction.

Catalyst

A catalyst is a substance which alters the rate of a chemical reaction but itself remains unchanged,
chemically and in mass as well.
The catalyst may be positive and negative. A catalyst which accelerates the rate of chemical
reaction is called a positive catalyst. For example, MnO2 is a positive catalyst. This increases the
rate of reaction during decomposition of potassium chlorate.
A catalyst which retards the rate of chemical reaction is called negative catalyst. For example,
Glycerine is negative catalyst which decreases the rate of reaction during decomposition of
hydrogen peroxide.

Characteristics of catalyst
1) Catalyst remains unchanged in its physical and chemical composition in the reaction.
2) Catalyst is always required in small amount.
3) Catalyst can’t make impossible reaction possible.

Endothermic and Exothermic Reaction

Endothermic reaction: When limestone is heated at high temperature in a furnace, it undergoes
a chemical change. As a result, lime (calcium oxide) and carbon dioxide are produced. Such a
reaction which absorbs heat from surrounding is called as endothermic reaction
CaCO3 ∆ CaO + CO2↑

Decomposition involves breaking bonds. So decomposition reaction are usually endothermic.
Some more examples include:
2NaNO3(s) ∆ 2NaNO2(s) + O2(g)
NH4Cl(s) ∆ NH3(g) + HCl(g)
2HgO(s) ∆ 2Hg(l) + O2(g)

Exothermic reaction: Some reactions take place when two or more substances are brought
together and they generate heat. Such reactions that liberate heat due to chemical reactions are
called exothermic reaction. For example,

C + O2 → CO2 + 94,300 cals

Bond making is exothermic. When you add water to calcium oxide, the particles in the water
and the particles in the calcium oxide form bonds. The bonds hold the fine atoms together as a
molecule of calcium hydroxide.

CaO(s) + H2O(l) → Ca(OH)2 (aq)

As the bonds are made, heat is released into the surroundings. This reaction is exothermic.

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Let’s Learn

1. Acid base reaction is called neutralization reaction because the by products of this
reaction water and salt are chemically neutral.

2. Physical change is called temporary change because no new substances will be formed
and it is reversible also.

3. Burning of carbon with oxygen is exothermic because large amount of heat will be
evolved after this reaction. C + O2 → CO2 + Heat

Main Points to Remember

1. In a chemical change, atoms or molecules take part.

2. A chemical change is more or less permanent and gives rise to a new substance having a
composition and properties altogether different from those of the original substances.

3. A physical change is a change of state only and is not accompanied by an alteration in
composition, weight or chemical properties of substances.

4. When external heat initiates the reaction, it is called endothermic reaction.

5. When heat is evolved during the reaction, then it is called exothermic reaction.

6. A chemical equation is the representation of an actual reaction with the help of symbols.

CaCO3 + 2HCl → CaCl2 + H2O + CO2↑
7. Chemical reactions can be represented with the words called word equation.

Zinc + Sulphuric acid Zinc sulphate + Hydrogen

8. The compounds written on the left of arrowhead or equal sign is known as reactants.

9. The compounds or elements formed due to a reaction are written on the right of arrowhead
or equal sign. These are the products.

10. For balancing the chemical equation, follow the steps given below:

(a) Write the skeleton of the reaction.

(b) Increase the coefficient of product to balance and count it.

(c) Write the molecular formula in balanced form.

11. When the name of the two elements start with the same letter, a second significant letter is
added to the first to specify it.

12. The valency of the element is determined with respect to Hydrogen or Oxygen or Chlorine.

13. The number of electrons lent, borrowed or shared determines the valency of the atom.

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Exercise

A. Choose the correct answer from the given alternatives:

1. Cu+FeSo4 CuSO4 + Fe is ............... reaction.

a. combination b. displacement c. decomposition d. neutralisation

2. Heat is released in ..................... reaction.

a. endothermic b. isothermic c. exothermic d. all of them

3. Which one is the character of catalyst?

a. changes the properties with reactions b. should be only in solid state

c. changes the products d. alter the rate of reaction

4. The exchange of ions take places in .....................reaction.

a. single displacement b. double displacement c. neutralisation

d. all of the above

5. MnO2 is example of .......................catalyst.

a. position b. Negative c. simple d. complex

B. Answer the following questions.

1. Describe the following terms with an example each.

(a) Endothermic reaction (b) Exothermic reaction

2. What are the essential characteristics of a chemical equation?

3. a. What is the chemical equation?

b. What is physical change?

c. What is chemical change?

d. Give an experiment to show that chemical change is irreversible.

e. Identify whether the following are physical change and chemical change.

i. Rusting of iron ii. magnetizing an iron piece

iii. vaporization iv. condensation

v. preparation of gas in the laboratory vi. drying clothes

vii. cooking food

f. What distinction do you get in a chemical change and a physical change?

g. Define reactants and products.

h. What is a skeletal equation?

4. Rewrite the chemical reaction in the language of chemistry and balance them.

(a) Calcium carbonate → Calcium oxide + Carbon dioxide

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(b) Magnesium + Oxygen → Magnesium oxide
(c) Zinc + Sulphuric acid → Zinc Sulphate + Hydrogen
(d) Iron + Copper Sulphate → Iron Sulphate + Copper
(e) Carbon + Oxygen → Carbon dioxide
(f) Sodium + Chlorine → Sodium chloride
(g) Zinc + Sulphur → Zinc sulphide
(h) Iron + Oxygen → Ferric oxide
(i) Sulphur + Oxygen → Sulphur dioxide
(j) Magnesium + Sulphuric acid → Magnesium sulphate + Hydrogen
(k) Aluminium + Hydrochloric acid → Aluminium chloride + Hydrogen
(l) Magnesium + Copper sulphate → Magnesium sulphate + copper
(m) Sulphuric acid + Sodium hydroxide → Sodium sulphate + Water
(n) Lead oxide + Hydrogen → Lead + Water
(o) Ammonium chloride + Sodium nitrite → Sodium chloride + nitrogen + Water
(p) Magnesium + Nitrogen → Magnesium nitride
(q) Ammonium chloride + Calcium hydroxide → Calcium chloride + Water + Ammonia
(r) Ammonia + Water → Ammonium hydroxide
(s) Ammonium hydroxide + Sulphuric acid → Ammonium sulphate + Water
(t) Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon

dioxide
(u) Lime (Calcium oxide) + Water → Calcium hydroxide
(v) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(w) Calcium hydrogen carbonate → Calcium carbonate + Carbon dioxide + Water
(x) Magnesium bicarbonate → Magnesium carbonate + Carbon dioxide + Water
(y) Magnesium chloride + Sodium carbonate → Magnesium carbonate + Sodium Chloride
(z) Magnesium sulphate + Sodium carbonate → Magnesium carbonate + Sodium

sulphate

5. Write word equation first and balance chemical equations of the following chemical
reactions.

(a) Sulphur dioxide gas is produced when sulphur is burnt in air.

(b) When calcium carbonate is burnt in a furnace, it gives carbon dioxide and calcium
oxide (lime).

(c) When charcoal is burnt in air, carbon dioxide is produced.

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