New Creative Science Book 9

Optical fibres

Optical fibres are those highly transparent and flexible fibres made of fibre of
quartz, glass or plastic. The diameter of the fibres small i.e. about 10–4 cm. It is made
in such a way that the light entering light pipes (combination of optical fibres) is
transmitted although it is bent. This is due to total internal reflection because the
critical angle for the materials used to prepare optical fibre is less and in every case,
the angle of incidence is greater than the critical angle.

Light pipe

Uses of optical fibre

Optical fibre plays a very important role. Some of its uses are as listed below:
(i) Light pipes are used in endoscopies to study the internal organs like food

pipe, intestines, stomachs, etc.
(ii) It is used in communication purposes.
(iii) It is used for sending video signals.
(iv) It is used to study blood vessels and tissues.

MEMORY TIPS

Periscopes or binoculars are made by using a totally reflecting prism instead of
plane mirrors.

Dispersion

During the rainy season, you can see a band
of different colours called the rainbow. Do you
know why this occurs in the rainy season? It is
because the light rays entering the raindrop split
into different colours.

The phenomenon of the splitting of white
rays of light into different colours is called the
dispersion of light. The white beam of light is

New Creative Science, Class 9 | 97

dispersed into seven different colours: violet, indigo, blue, green, yellow, orange
and red i.e. VIBGYOR. These seven colours can be obtained on the screen called
the spectrum. The light is dispersed as it passes from the medium called dispersion
medium like prisms, rain drops, etc.

A

Q Least deviation S
P A(Sbuenalimghotfowr hbiutelblilBgighhtt) Glass prism CMaximum deviation
Red R
Orange O
Yellow Y
Green G
Blue B
Indigo I
Violet V

During the dispersion of light, the light rays of different colours overlap each
other and the colours cannot be distinct. This kind of spectrum is called an impure
spectrum. Similarly, the spectrum in which all the seven colours of light are distinct
is called a pure spectrum. To obtain a pure spectrum, the following things should be
considered:

(a) The light should pass into the prism through a narrow hole or point.
(b) The incident rays of light must be parallel to each other.
(c) The prism should be kept in a minimum deviation position.
(d) The parallel rays of the light emerging out from the prism must be focused

on the screen using a convex lens.

Cause of dispersion of light

In air or vacuum, the speed of light of all colours is equal so that they travel in
the form of white rays. As the medium is changed, the speed also changes because
the speed of light depends upon the wavelength. The wavelength of a red ray is the
longest (7 × 10–4 mm) whereas violet has the shortest wavelength (4 × 10–7 mm). So,
the speed of the red ray is the highest and that of violet is the lowest. Similarly, the
deviation of red is the lowest as the angle of refraction is the least and deviation of
violet is the most as the angle of refraction is the most. This brings red colour at the
top and violet at the bottom. These are the causes of the dispersion of light.

MEMORY TIPS
Wavelength and speed increase from violet to red whereas the deviation and the
angle of refraction decrease from violet to red.

98 | Light

Newton’s colour disc

The spectrum or dispersion of light was rope
shown by Newton for the first time and the disc wheel
spectrum was also called Newton’s spectrum.
The white ray of light consists of seven different
colours. It can be proved with the help of
Newton’s colour disc. It can be shown in the
diagram alongside.

Newton’s colour disc is a circular disc made of metals or cardboards which is
divided into seven equal segments. These segments are painted with seven different
colours i.e. violet, indigo, blue, green, yellow, orange and red. If the disc is rotated
rapidly, we can see white light. This proves that the white ray of light consists of
seven different colours.

No dispersion of light through glass slab

A glass slab can be formed by combining two prisms of an equal size. It means a
glass slab is a combination of two prisms as shown in the figure. The light is dispersed
by the first prism ABC and these splitted rays are recombined by the prism A’B’C’
before it emerges out. It means the white ray of light emerges out of the glass slab. As
a result, dispersion is not experienced.

MEMORY TIPS
Due to the dispersion of light, rainbow, a red sky, etc. during sunset and sunrise are
experienced.

Electromagnetic waves

Those waves which do not need any medium for their propagation are called
electromagnetic waves. It means it can travel even through the vacuum. These waves
carry the energy by the vibration of electric and magnetic fields in the direction

New Creative Science, Class 9 | 99

perpendicular to each other. They can travel by the means of radiation. It means they
propagate without the actual movement of material medium. X-rays, Gamma rays,
ultra-violet rays, visible light, infra rays, etc. are the electromagnetic waves.

Properties of electromagnetic waves

The energy, wavelength and nature of an electromagnetic wave differ from each

other. But some of their common properties are as follows:
(i) They are transverse in nature.
(ii) They can even travel through a vacuum.
(iii) They travel with the speed of 3 × 108 m/s in air.
(iv) They show the properties of reflection, refraction, dispersion, interference
and polarization.
(v) They can cast and form a shadow in the presence of obstacles.

Electromagnetic spectrum

S.N. Wave Wave Average Sources Uses Effects
length frequency

(m) (Hz)

1. Radio 106 to 10–3 105 to 1011 Radio Communication, Not

waves transistor radio, TV, radar harmful

2. Infrared 10–3 to 10–6 1013 Hot objects Heating, Not

thermograph harmful

3. Visible light 4×10–7 to 8 1015 Sun and other Photography, Not

× 10–7 hot objects seeing, harmful

transmission etc.

4. Ultraviolet 10–6 to 10–9 1016 Sun, mercury Fluorescent Harmful

rays lamps, lamps, food

fluorescence sterilizing

5. X-rays 10–8 to 10–12 1018 X-ray X-rays, Harmful
machine
m radiography, etc.

6. Gamma 10–12 and 1020 Radio active Radiography Harmful

rays less nuclei treatment of

cancer

ANSWER WRITING SKILLS

1. Why does the sky appear blue?
Ü The wavelength of violet ray and indigo in the visible spectrum is less so that

they scatter in a large amount. These two light rays of different colours mix to
form blue colour. So, sky appears blue due to the high scattering of light rays.

100 | Light

2. Diamond sparkles brilliantly. Why?
Ü The critical angle of diamond is 24. The edges of diamond are so cut in such a

way that the angle of incidence will always be greater than its critical angle. This
causes the total internal reflection. As a result, it sparkles brilliantly.
3. Why is the dispersion of light not seen in a glass slab?
Ü The glass slab is composed of two identical prisms. The rays of light dispersed by
one prism are combined i.e. recombined by the other prism emerging the white
ray of light. So, the dispersion of light is not seen in a glass slab.
4. A totally reflecting prism is used plane mirror in a periscope. Why?
Ü The plane mirror forms multiple images whereas the totally reflecting prism forms a
single image. So, to prevent the formation of multiple images, a totally reflecting prism
is used in a periscope.
5. We usually see rainbows in the rainy season. Why?
Ü During rainfall the droplets of water act as a
dispersing medium and disperse the light rays
into seven different colours in a spectrum form.
This spectrum is a rainbow. So, we usually see a
rainbows in the rainy season.

6. At sunset and sunrise, it appears red. Why?
Ü The wavelength of red ray of light is much more (7 × 10–7m) than other rays in a

visible ray spectrum. So, the sun appears red at sunset and sunrise. Similarly, it
deviates less than other rays.
7. The critical angle for water is 49°. What does it mean?
Ü The critical angle for water is 49°. It means the angle of incidence in a denser
medium (water) is 49° and the corresponding angle of refraction in a rarer
medium (air) is 90°. It can be illustrated as:

New Creative Science, Class 9 | 101

SUMMARY

” Light is a form of energy that provides us the sensation of vision to our eyes.

” The phenomenon of bending of the light as it passes from one medium to
another is called the refraction of light.

” Snell’s law states, “The ratio of sin of angle of incidence to the sin of angle of

refraction is constant” i.e.

m= sin i
sin r

” The angle of incidence in a denser medium for which the angle of refraction in

a rarer medium is 90°. It is called a critical angle.

” The phenomenon of the turning back of light in the same denser medium

when the angle of incidence is greater than the critical angle is called a total

internal reflection.

” The refractive index of any medium is defined as the ratio of velocity of light
in air to the velocity of light in the medium.

” Experimentally, the ratio of real depth to the apparent depth was found to be

equal to the refractive index.

” The scattering or splitting of the white ray of light into seven different colours
is called dispersion.

” Dispersion is caused due to the different velocities of different colour-rays in
any medium except air or vacuum.

” Newton’s colour disc practically proves the white ray consists of seven

different coloured rays.

” Electromagnetic waves are those which do not require any material mediums
for propagation.

” Gamma rays, x-rays, visible rays, ultraviolet rays, infra ray, and radio waves

are electromagnetic waves.

EXERCISE

1. What is the refraction of light? Mention its law.
2. Describe the refraction of light in a glass slab.
3. What is a refractive index? Write the value of the refractive index of water, glass and

diamond.
4. Explain the cause of the refraction of light.

102 | Light

5. State Snell’s law. Why is it important? (Hint: what does it calculate?)
6. What is the dispersion of light? Write its causes.
7. Draw a diagram to illustrate the dispersion of white ray and explain it briefly.
8. What do you mean by a total internal reflection? Mention its necessary conditions.
9. Explain how mirages are found.
10. What are electromagnetic waves? Give the names of four electromagnetic waves.
11. Give reason:

(a) Diamond shines brilliantly but an identical glass piece does not.
(b) The fish appears on the surface of water when viewed from the bottom of an

aquarium.
(c) The dispersion of light is not experienced in a glass slab.
(d) Red light deviates less whereas a violet ray deviates more.
(e) A finger dipped in water appears bent.
(f) The sky appears blue.
(g) The western horizon appears red at sunset.
(h) Mirage does not occur in the morning.
(i) Electromagnetic waves can even travel through a vacuum.
12. Write some general properties of electromagnetic waves.
13. Explain Newton’s colour disc in brief with a figure.
14. The critical angle of glass is 42°. What do you mean by this?
15. The refractive index of water is 1.5. What do you mean by this?
16. Complete the ray diagrams.

New Creative Science, Class 9 | 103

17. Study the given figure and answer the following questions.

(a) What is shown in the figure?

(b) Out of medium 1 and medium 2, which is
rarer and which is denser?

(c) Support your answer in (b) with a reason.

(d) Name M1M2, AO, BO and ON.

18. The absolute refractive index of medium A, B
and C are given below. Answer the questions given on the basis of the table.

(a) What happens to the ray of light as it passes from Medium m
medium A to B and A to C? A 1.50
B 1
(b) Show of the answer of (a) in diagram. C 2.4

(c) Calculate the critical angle for medium A, B and
C.

19. Among, infrared, ultraviolet, x-rays, radio waves and gamma rays, which has the shortest
and the longest wavelength? Also mention which has a higher speed and which has a lower
speed?

20. Write the differences between x-rays and infrared-rays.

Numerical problems

21. The refractive index of any medium is 1.33. Calculate its critical angle.

22. A coin is placed at the bottom of a vessel containing water to the depth of 20 cm. Find its
apparent position. (mw = 1.33)

23. The velocity of light in glass and water is respectively 2 × 108 m/s and 2.2 × 108 m/s.
Calculate their refractive index and the critical angle.

24. Calculate the speed of light in water if its refractive index is 4/3.

25. The real and apparent depth of the bottom of the pond is 3 m and 2.2 m. Calculate the
speed of light in water.

26. Calculate the angle of incidence so that the ray of light refracts with the angle of 20° in
glass. (mg = 1.5)

A

B GLOSSARY
C

Denser: having more density
Rarer: having less density
Normal: a perpendicular line on the surface
Optical: related to light

v

104 | Light

UNIT

7 ELECTRICITY AND MAGNETISM

About the Scientist Introduction

Hans-Christian Oersted The modern successful life is the gift of current
(1777-1851) electricity. In each and every place of our life, we use
a electric current. It is used for cooking, heating, in
During a classroom refrigerators, charging mobiles, etc. This is also used for
demonstration of an running large machines and equipment in factories and
experiment to his students, industries, press, hospitals, etc. It means a current is used
Oersted noticed that a for different purposes.
compass needle was deflected
when an electric current All the matters are made up of atoms containing an
was switched on nearby. equal number of positively charged protons and negatively
He went on to study this charged electrons. So, all the atoms are electrically neutral
effect further and discovered and in turn, every matter is electrically neutral. When
the connection between two substances are rubbed, the electrons flow from one
electricity and magnetism. body to another forming a positive and a negative charge.
It means all the bodies possess electricity at rest that can
be produced by rubbing (friction). This is called static
electricity. In this case, the substance which gains electrons
is negatively charged and the substance which loses
electrons is positively charged.

In any substances, the pole accumulating more
electrons is negatively charged and the pole accumulating
fewer electrons is positively charged. As these positive and
negative terminals are connected by using a conducting
wire, there is a flow of electrons. This is called current
electricity.

Current electricity

Current electricity or an electric current is simply

defined as the rate of flow of charge from the higher

potential region to the lower potential region. It is the

electricity in motion and can be used for several purposes.
Q
Mathematically, I = t

Where, I = electric current

New Creative Science, Class 9 | 105

Q = number of charge particles flowing through the circuit

t = time taken to flow the charged particles.

The SI unit of the electric charge is coulomb (c) and that of time is ‘second’ (s). So,
the SI unit of current is (A). And other units of current electricity are:

1A = 103 mA
1A = 106 mA

One Ampere current

One ampere current is the current produced in an electric circuit by the flow of 1

coulomb charge in 1 second.

i.e. 1A = 1C
1s

| NUMERICALS |

a. In an electric circuit 100 coulomb charge flows in 5 second. What is the current?

Solution: Given, amount of charge (Q) = 100 C

Time take (t) = 5 second

= 100 = 20 A
5
Hence, the current flown through the circuit is 20 A.

MEMORY TIPS

The current is a scalar quantity because it simply represents the rate of the flow
of electric change in an electric circuit and the total current flowing through the
branches of a circuit is added simply as scalars. They do not contain the property of
vector addition.

Differences between static electricity and current electricity

Static electricity Current electricity

1. It is the electricity at rest and 1. It is the flow of electrons from

produced by rubbing two non- the high potential to the lower

conductors. potential.

2. It cannot be brought into use. 2. It can be used for several purposes.

3. It cannot be transformed from one 3. It can be transformed from one

place to another. place to another.

Sources of electricity

Current electricity can be generated by using several devices. These devices
which generate electricity by changing other forms of energy into electrical energy
are called sources of electricity. Some of the sources of electricity are explained below:

106 | Electricity and Magnetism

1. Cell
The cell is one of the sources of electricity
that produces electricity by transforming
chemical energy into electrical energy. It is
portable in nature so that it is widely and
extensively used. It produces the direct
current (DC) which can be increased with
the help of the combination of cells. The
dry cell, simple cell, lead acid cell, etc. are
some examples of cells. The combination
of cells is called a battery. It is used in the
radio, watch, clock, calculators, torchlight, etc.

2. Photocell
The photocell is the advanced cell that converts solar energy into electrical
energy. Since, it produces less amount of current, it is quite impossible to use
it in factories, industries, electrification, etc. But, in less populated areas, solar
panels are used for generating electricity.

3. Dynamo or generator
It is a device that converts mechanical energy into electrical energy. The dynamo
produces a little amount of energy whereas the generator produces a high
amount of energy. The generator works or produces electricity using different
other forms of energy like thermal energy, atomic energy, hydro energy, etc.

Electric circuit

The conducting path of electricity consisting of an electrical source, load, switch,
connecting wire etc. is called an electric circuit. The current in an electric circuit
always flows from the higher potential to the lower potential. An electric circuit can
be divided into two types. They are:
(a) Open circuit
(b) Closed circuit

(a) Open circuit
It is an electrical circuit through which the current
does not flow and hence the electrical load does not
work. It is called an open circuit. In this circuit, the
switch is in an ‘OFF” position i.e. gap or discontinuity
is seen in an electrical circuit.

New Creative Science, Class 9 | 107

(b) Closed circuit
The electrical circuit in which there is a flow of
electricity and the electrical load works. It is called
a closed circuit. In this case, the switch is in an ‘ON’
position i.e. gap or discontinuity is not seen in an
electrical circuit.

Direction of current flow

In the past, it was supposed that positive Direction of flow Conventional flow
of electrons of current

charges flow through the circuit. It means the current

flows from positive to negative. This is called the

conventional flow of the current. Later on, after the

discovery of atoms, it was found that the current is

the flow of electrons i.e. negative to positive.

Some symbols used in an electric circuit diagram:

MEMORY TIPS
Rheostat is a resistor, whose resistance can be changed according to the need.

Conductors and insulators

Conductors are those substances through which, the current passes easily.
They have free electrons in the valence shells of their atoms. Different metals are the
examples of good conductors of electricity.

Insulators are those substances through which the current does not pass. They do
not have free electrons. All the non-metals (except graphite) rubber, plastic, thread,
etc. are non-conductors. They are also called bad conductors.

MEMORY TIPS
6 × 1018 electrons = 1 coulomb and 1 electron = 1.6 × 10–19 C

108 | Electricity and Magnetism

Ammeter

An ammeter is a device which is used to measure the amount of the current
flowing through the circuit. It measures the current flowing through the circuit. It
measures the current in milliampere, micro ampere, ampere depending upon the
calibration of the ammeter.

It is connected in series because it has very low resistance which does not affect
the current flowing through the circuit. The following points should be considered
while connecting the ammeter in an electric circuit.

(i) The ammeter should always be connected in series.
(ii) The positive and negative terminals of the ammeter should be connected

with positive and negative terminals of the current respectively.

Symbol of ammeter

Potential difference (p.d.)

In the figure aside, the body A is positively charged and body B is negatively
charged. These two bodies are connected with a conducting wire. The electricity
charges flow from body A to body B as the body A is in higher potential and body B
in lower potential. To move the unit charge from A to B, certain work is to be done.
This is called potential difference.

Potential difference is the difference of electrical potential (i.e. positive and
negatively potential) between any two points. In other words, potential difference
between any two points in an electric circuit is defined as the amount of work done in
moving unit positive charge from one point to another.

Mathematically,

VAB = W
Q

Where, VAB = the potential difference between two points A and B
W = amount of work done and Q = change

Its SI unit is Joule per coulomb (Jc–1) or volt (V). It is generally measured by using
a voltmeter. It is a scalar quantity.

New Creative Science, Class 9 | 109

MEMORY TIPS

The potential difference between any two points is to be maintained in an electric
circuit for the continuous flow of an electric current.

One volt potential difference (1V pd)

One-volt potential difference between any two points in an electric circuit is
defined as the pd developed when 1 Joule of work is done in moving 1 coulomb
positive charge.

i.e. 1 V = 1J
1C

Electromotive force (e.m.f)

Electromotive force is defined as the work done by a cell to move unit positive
charged particles across the two ends of the cell. It is the energy supplied by the cell
from low potential to high potential in order to move one unit positive charge. The
emf of the cell can be calculated by the formula,

E= W
Q

Where, W = amount of work done
Q = charge particles that move

Its SI unit is volt. It gives the potential difference of a cell in an open circuit.

Differences between potential difference and electromotive force

S.N. Potential difference (pd) S.N. Electromotive force (emf)

1. It is the amount of work done to 1. It is the amount of work done

move unit positive charge from to move unit positive charge

one point to another. throughout the circuit.

2. It is an effect of emf. 2. It is the cause of pd.

3. It is always less than emf. 3. It is always greater than pd.

4. It is the work done by the energy 4. It is the total amount of energy

supplied by the cell. supplied by the cell.

5. It is measured in closed circuits. 5. It is generally measured in open
circuits.

6. It is regarded as an irreversible 6. It is regarded as a reversible one.
one.

110 | Electricity and Magnetism

Relation between pd, emf and internal resistance

Let us consider a circuit in which the emf of a cell
is E, internal resistance r, external resistance R and pd
between the point A and B is V.

If the circuit is closed, the amount of the current
flowing through the circuit is:

I = E r [V = IR according to Ohm’s Voltmeter
R+
law]

i.e. E = IR + Ir

The potential difference across the point A and B is given by

V = IR

Then, E = V + Ir i.e. V = E – Ir

This is the relationship between potential difference, electromotive force and
internal resistance where Ir is called an internal potential drop.

MEMORY TIPS

In open circuit, I = 0 i.e. Ir = 0, so pd = emf

Voltmeter

A voltmeter is an instrument used to measure the potential difference across
any two points in a closed circuit. The resistance of the voltmeter should be high so
that the current may flow through the path across which potential difference is to be
measured. The following things should be considered while connecting a voltmeter
to an electric circuit.

(i) It should be connected in parallel across the device whose pd is to be
measured.

(ii) The positive and negative points of voltmeter should be connected with
positive and negative terminals of the circuit.

Symbol

Voltmeter

Voltmeter

New Creative Science, Class 9 | 111

Resistance

Conductors allow the electric current to pass easily through them whereas
insulators and semi-conductors create difficulty in flowing electric currents. This is
because of their resistance. Resistance is the property of any material.

The resistance of any conducting wire is defined as the property by virtue of
which it opposes the flow of the current through it. Generally, metals have low
resistance and insulators have high resistance. The high resistance materials are used
in heating and lighting purposes whereas low resistance materials are used in electric
circuits for the flow of the current. Different materials have different resistance. It is
also true that the same material has different resistance because of several factors. It
is represent by ‘R’ and its SI unit is ohm (W).

MEMORY TIPS
Resistance is due to the collision between free electrons and the ions of conductors.

Factors affecting resistance

Some of the factors affecting resistance are as explained below:

1. Length of the conductor (l): The resistance of any conducting wire is directly

proportional to its length i.e. R ∝ l

The higher the length of conducting wire, the higher is the resistance and

vice- versa.

2. Thickness of conduction: The thickness of conduction is inversely

proportional to the resistance. In this case, the thickness means cross-
(TAh)ei.ree. sRis∝tanA1ce
3. sectional area of a conductor is directly proportional to its
Temperature:

temperature (T) i.e. R ∝ T

4. Nature of material: The nature of materials from which the conductor is

made also affects its resistance. For example, nichrome has high resistance

than copper but lower than graphite.

Resistivity and conductivity

Let us consider a resistor of resistance ‘R’, length ‘l’ and cross sectional area ‘A’.
Then,

We have, R ∝ l ………………(i)

R∝ 1 ………………(ii)
A

112 | Electricity and Magnetism

Combining equation (i) and equation (ii), we get,
l
R∝ A

\R=r l
A

Where, r = constant called resistivity

\ Resistivity (r) = R × A
l

Hence, resistivity is the ratio of the product of its resistance and the cross sectional

area to its length. Its SI unit is ohmmeter (Wm).

If R = 1W, A = 1m2 and l = 1m, then

r = 1 W × 1m2
1m

i.e. r = 1 W m

Therefore, one-ohm metre is the resistivity of a conducting wire of length 1 m,
cross section area 1 m2 and resistance 1 ohm.

The reciprocal of resistivity is called conductivity of a conducting wire. It is
1
represented by ‘d’ i.e. d = r

= 1 A
× l
R

i.e. d = l
R×A
Its SI unit is per ohm per metre (W–1 m–1).

MEMORY TIPS

Resistivity of a conducting wire does not depend upon its shape and size but
depends upon the material from which it is made of.

Ohm’s law

A German inventor named George Simon Ohm propounded the relationship
between the current, resistance and potential difference at constant physical
conditions. This relation is called Ohm’s law.

It states, “The current flowing through the conducting wire is directly proportional

to the potential difference across its two ends at a constant temperature”. If ‘I’ is the

current flowing through the conducting wire and ‘V’ is the potential difference across

its two ends, then

I∝V

i.e. V = constant i.e. V = R (constant)
l l

New Creative Science, Class 9 | 113

Where, R is the resistance of the conducting wire which is constant for a given
conducting wire.

\ V = IR

This is mathematical formula of Ohm’s law.

Also, R = V
l

If V = 1V and I = 1A, then R = 1W

Hence, 1W resistance is the resistance of a conducting wire in which one ampere
current is flowing as the voltage of 1V is applied to it.

| NUMERICALS |

1. Calculate the voltage supplied to a circuit to flow 4A current through the
resistance of 2W.

Solution: Given, current (I) = 4 A
Resistance (R) = 2 W
Voltage (V) = IR = 4 × 2
\V=8V

2. The potential difference across two points of a conducting wire of resistance

4/3 W is 4V. Calculate the amount of the current flowing through it.

Solution: Given, resistance (R) = 4 W
3
Potential difference (V) = 4 V

Current (I) = ?

We have,V = IR

\ I = V = 4 = 3A
R 4

3

3. 4 A current flows in the headlight of a motorbike when the voltage of 12V is

connected to it. Calculate the resistance.

Solution: Given, current (I) = 4 A

Voltage (V) = 12 V

Resistance (R) = ?

We have,V = IR

\ R = V = 12
l 4

i.e. R = 3 W

Hence, the resistance of the bulb is 3 W.

114 | Electricity and Magnetism

Electric power

The rate of conversion of electrical energy into other forms of energy is called
electric power. It is the rate of doing work by an electric current.

Mathematically,

P = W
t

Where, P = electric power

W = work done by electricity

t = time taken

And, work done by current = V × I × t

Where, V = potential difference

I = current

\ P = V ×I × t
t

i.e. P = VI

This is the relationship between electric power, potential difference and the
current. Its SI unit is watt (W).

| NUMERICALS |

1. Calculate the resistance of an electric bulb marked with 100 W is connected to
the voltage of 220 V.

Solution: Given, Voltage (V) = 220 V

Power (P) = 100 W

Resistance (R) = ?

We have,P = VR [I = current]
V
or, P= V · R [ V = IR]

\ R = V2 = (220)2
P 100

i.e. R = 484 W
Hence, the resistance of the electric bulb is 484 W.

2. What is the amount of the current flow through an element of a heater of power
1000 W connected to 220 V? Also calculate its resistance.

Solution: Given, Power (P) = 1000 W
Voltage (V) = 220 V
Current (I) = ?
Resistance (R) = ?

New Creative Science, Class 9 | 115

We have,P = VI

\ I = P = 1000 = 4.5 A
V 220

Again, V = IR

\ R = V = 220
l 4.5

i.e. R = 48.89 W

Combination of resistors

Resistors of different or similar resistance are combined in two ways. They are;
(a) Series combination
(b) Parallel combination

a. Series combination of resistors

The end to end combination of resistors is called a
series combination of resistors. In this combination,
there is a flow of the same current but they have
different voltage. This type of combination of
resistors is shown in the figure.

Let us consider three resistors of resistance R1, R2 and R3 connected in series so
that the same current ‘I’ flows through all the resistors. The potential differences

across R1, R2 and R3 are V1, V2 and V3 respectively such that

V1 = IR1 V2 = IR2 V3 = IR3

If R is the total resistance across the circuit

and V is the potential difference supplied by

the source then,

V = IR

Also, V = V1 + V2 + V3

or, IR = IR1 + IR2 + IR3

\ R = R1 + R2 + R3

Hence, the total resistance of the circuit in which resistors are connected in series

is the algebraic sum of all the resistors.

116 | Electricity and Magnetism

| NUMERICALS |

1. Calculate the resistance in the given diagram.

Solution: Given, Resistance (R1) = 2 W
Resistance (R2) = 2 W
Resistance (R3) = 2 W

Total Resistance (R) = R1 + R2 + R3
R=2+2+2
R=6W

Hence, the resistance is 6 W.

MEMORY TIPS

In a series combination, the total resistance increases with the increase in the number
of resistance. The current decreases with the increase in the number of resistance.
All the loads stop working if one of them is fused.

b. Parallels combination of resistors

The type of combination of resistors in which all positive terminals are connected
at one point and all negative terminals are connected at another point is called a
parallel combination.

Let us consider three resistors of resistance R1, R2 and R3 connected in parallel.
In this electric circuit, the potential difference across the points of the cell is V

and there is a flow of the current I in the whole circuit. I1, I2 and I3 are the current
flowing through R1, R2 and R3 respectively.

If R is the total resistance, then

V = IR, i.e. I = V
R

Also, I = I1 + I2 + I3

or, V = V + V + V [ V = IR]
R R1 R2 R3

\ 1 = 1 + 1 + 1
R R1 R2 R3

| NUMERICALS | 2W
2W
1. Calculate the resistance in the given circuit. 2W

Solution: Given, Resistance (R1) = 2 W
Resistance (R2) = 2 W
Resistance (R3) = 2 W

New Creative Science, Class 9 | 117

( )Total Resistance 1 = 1 + 1 + 1
R R1 R2 R3

1 = 1 + 1 + 1
R 2 2 2

1 = 1+1+1
R 2

1 = 3
R 2

R= 2 W
3

Hence, the resistance is 2 W.
3

MEMORY TIPS
In a parallel combination of resistors, the total resistance decreases with the increase
in resistors, other loads go on working although if one is fused.

Magnet and its properties

Magnets are those substances which can attract magnetic substances like iron,
nickel, etc. A freely suspended magnet shows north and south of the earth. The end
point of the magnet which shows the geographical north is the north pole and which
shows the geographical south is the south pole. Magnets may be natural magnets like
loadstone or may be artificial like bar magnet, horseshoe magnet, etc.

Magnetic substances are those which are attracted by the magnet and non-
magnetic substances are those which are not attracted by the magnet. Some of the
properties of the magnet are as follows.

(i) A freely suspended bar magnet shows N-S direction at rest.
(ii) A magnet attracts magnetic substances like iron, nickel, etc.
(iii) Unlike poles of magnet attract each other whereas like poles repel each

other.
(iv) Magnetic poles cannot be separated.
(v) Magnets magnetize the magnetic substances when kept in contact with

them.

MEMORY TIPS
The properties of a magnet is called magnetism.

118 | Electricity and Magnetism

Magnetic compass

A magnetic compass is a simple instrument Magnetic compass
consisting of a magnetic needle free to rotate and rest
in the N-S direction. It is generally used to locate the
magnetic lines of force, identify the magnetic field, to
test the polarity of a magnet and so on.

It is also used to determine a direction during
voyage. It works with the influence of the earth’s
magnetic field while determining the direction.

Magnetic field and magnetic lines of force

When a magnetic substance is placed near the magnet, it gets attracted. If it is kept
away, the magnet does not attract. Can you guess why? It is because of the magnetic field.

The magnetic field is the area or space around

the magnet upto which the magnet influences the

magnetic substance. It is the area where the magnetic

influence can be felt. Only the magnetic materials

kept in the magnetic field are attracted but if they are

not kept in the magnetic field, they are not attracted.

The effect or the influence of the magnet decreases as

we move away from the magnet. The magnetic field

of any magnet is represented by the set of lines. These Magnetic lines of force
lines are called the magnetic lines of force. Therefore,

the magnetic lines of force are those imaginary lines drawn in order to show the

magnetic field of any magnet. The magnetic lines of force emerge out from the North

Pole of the magnet whereas terminate in the South Pole of the magnet. It can be

located with the help of a magnetic compass.

Properties of magnetic lines of force

Some of the important properties of the magnetic lines of force are as follows:
(a) Magnetic lines of force start from the North Pole and end in the South Pole

of a magnet.
(b) They are continuous closed curves.
(c) They repel each other.
(d) They never intersect each other.
(e) They have a tendency to contract.
(f) They are crowded near the poles as the magnetic force is strong and more

farther where magnetic force is weak.

(g) They can be obtained with a magnetic field.

New Creative Science, Class 9 | 119

Neutral points

Let us take two bar magnets and place them on a table with a certain distance
between them. At some points, the magnetic force of these two magnets is equal but
in an opposite direction. At this point, the compass does not show any fixed direction.

The point at which the needle of magnetic compass does not show any fixed
direction and no magnetic lines of force is obtained at that point is called a neutral
point. At the point, the net magnetic field is zero. It can also be obtained between the
magnetic field due to the earth’s magnet and a bar magnet. During this, if the N-pole
of the magnet shows the geographical north, neutral points are obtained on the east
and the west whereas if the N-pole of a bar magnet shows the geographical south,
neutral points are obtained towards the north and the south. It can be illustrated in the
following diagrams.

Neutral point Terrestrial magnet

Terrestrial magnetism

The earth is supposed to be a huge magnet. The
earth shows some magnetic properties. For example,
a freely suspended bar magnet shows geographical
N-S directions. This is called terrestrial magnetism.
In the geographical south, there is the north pole of
the earth’s magnet and in the geographical north,
these is the South Pole of the earth’s magnet. Some
of the evidences of terrestrial magnetism and the
earth’s magnetic fields are as follows:

(i) A freely suspended bar magnet always
shows the geographical N-S direction. It
is due to a terrestrial magnet.

120 | Electricity and Magnetism

(ii) The ores of iron or any buried magnet show magnetic property.
(iii) The angle of inclination and the angle of declination are shown by a magnetic

needle.
(iv) In the magnetic field of a magnet, the neutral point can be formed.

Magnetic and geographic meridian

The plane passing through a magnet i.e. south and north at a place is called a
magnetic meridian. It consists of magnetic south and magnetic north poles of the earth.

The plane passing through the geographical north and south pole of the earth is
called a geographic meridian. The magnetic meridian and geographic meridian do
not coincide each other but make a certain angle between them.

Angle of declination

The angle between the magnetic meridian and the geographic meridian at a place
is called the angle of declination. Its value differs from place to place on the surface
of the earth. It plays an important role in flying aeroplanes during the night and the
cloudy sky, sailing or going on a voyage, etc.

Angle of declination

Angle of dip (Angle of inclination)

The angle between a freely suspended
magnet and the horizontal component
of the earth’s magnetic field is called the
angle of the dip.

New Creative Science, Class 9 | 121

At the magnetic equator, a freely suspended magnet rests
horizontally whereas at two magnetic poles of the earth, it rests
vertically. Thus, the angle of the dip is 0° at the magnetic equator
and 90° at the magnetic poles of the earth. Hence, the value of
the angle of the dip lies between 0° to 90°. The angle of the dip is
generally measured by a dip circle.

ANSWER WRITING SKILLS

1. An electric bulb is marked with 60 W. What does it mean?

Ü An electric bulb is marked with 60 W. It means the electric bulb converts 60 J of
electrical energy into light energy and heat energy to some extent in 1 second.

2. An ammeter is connected in series in a electric circuit. Why?

Ü An ammeter has very low resistance. It measures the amount of current the
flowing through an electric circuit without any potential drop (Ir). So, an ammeter
is connected in series in an electric circuit.

3. A voltmeter is connected in parallel across the load. Why?

Ü A voltmeter measures the potential differences across any two points in an
electric circuit without any loss of current flowing through the circuit. It has very
high resistance. So, a voltmeter is connected in parallel across the load in an
electric circuit.

4. The angle of the dip is 0° at the equator and 90° at the poles. Why?

Ü At equator, a freely suspended magnet rests horizontally whereas it rests
vertically at the poles due to the effect of the earth’s magnetic field. And we
know that the angle of the dip is the angle between the magnetic meridian and
the horizontal component of the earth. So, the angle of the dip is 90° at the poles
and 0° at the equator.

5. The angle of the dip at Kathmandu is 42°. What do you mean by this?

Ü The angle of the dip in Kathmandu is 42°. It means a freely suspended magnet
or the dip needle makes an angle of 42° with the horizontal component of the
earth’s magnetic field. In other words, it means the earth’s magnetic poles lie at
42° from Kathmandu.

6. Calculate the total resistance of the given circuit.

Ü For AB, total resistance (RAB) = R1 + R2
[series

combination]

= 2 + 2 = 4W

For CD, total resistance
1 1 1
R = 2 + 2

122 | Electricity and Magnetism

1 = 2
R 2

R = 1W

For the whole circuit,

Total resistance 1 = 1 + 1 = 1+4
R 4 1 4

R = 4
5

\ R = 0.8 W
Hence, the total resistance of the circuit is 0.8 W.

SUMMARY

” Current electricity is the flow of electrons from the higher potential to the lower

potential.

” The current is defined as the rate of the flow of charge from one point to
another.

” The substances (atoms) which gain electrons are negatively charged and the
substances (atoms) which lose electrons are positively charged.

” The circuit in which an electric load is not working is called an open circuit.

” The circuit in which an electric load is working is called a closed circuit.

” Those substances which conduct electricity easily are called conductors.

” Those substances which do not allow the flow of the current through them are
called insulators.

” An ammeter measures the amount of the current flowing through the circuit

whereas voltmeter measures the potential difference across any two points in

a circuit.

” Resistance is the property by virtue of which the flow of the current is opposed.

” Ohm’s law states, “The current flowing through an electric circuit is directly

proportional to the potential difference across its two ends at a constant

temperature.”

” The length of the wire, the cross-sectional the area of the wire, the temperature

and the nature of the material of the wire are the factors affecting resistance.

” Electric power is defined as the rate of doing work by the electrical energy.

” In series combination of resistance, R = R1 + R2 + R3 + ………

Whereas in parallel combination, 1 = 1 + 1 + 1 + ……….
R R1 R2 R3

” The area of space around the magnet at which the effect of the magnet is seen

is called a magnetic field.

” The imaginary lines drawn to show a magnetic field are called magnetic lines

of force.

New Creative Science, Class 9 | 123

” The straight line the joining geographic south and the geographic north is
called a geographic meridian.

” The straight line joining the magnetic south and the magnetic north is called a
magnetic meridian.

” The point at which the effect of the earth’s magnetic field and a freely suspended
magnet is zero, is called as a neutral point.

” The angle between the geographical meridian and the magnetic meridian is
called the angle of declination.

” The angle between the dip needle and the horizontal component of the earth’s
magnetic field is called the angle of the dip.

EXERCISE

1. What is an electric current? Mention three sources of the current.

2. What is an electric circuit? Draw an electric circuit consisting of a cell, a switch, a
voltmeter, an ammeter and an electric bulb.

3. What are the things to be considered while using an ammeter and a voltmeter?

4. Derive the relation V = E – Ir, where the symbols have their usual meanings.

5. State Ohm’s law. Prove that V = IR.

6. Write the differences between:
(a) Closed and open circuit.
(b) Pd and emf.
(c) Resistivity and conductivity.
(d) Angle of dip and angle of declination.
(e) Voltmeter and ammeter.
(f) Conductors and insulators.

7. Define electric power. Prove that P = VI.

8. Define the terms: (a) 1 ampere (b) 1 volt

9. What do you mean by a series combination of resistance? Prove that in a series
combination of resistance the total resistance (R) = R1 + R2 + R3.

10. What do you mean by a parallel combination of resistance. Prove that 1 = 1 + 1 +
R R1 R2
1
R3 is parallel combination of resistance.

11. What are the factors that affect the resistance of any conducting wire? Also mention their
relationship with the resistance.

12. What are magnets? Enlist three properties?

13. Define a magnetic field and the magnetic lines of force.

124 | Electricity and Magnetism

14. Give reason.
(a) An ammeter is connected in series to an electric circuit.
(b) The angle of the dip is 0° at the equator and 90° at the poles.
(c) A freely suspend bar magnet shows the N-S direction.
(d) A voltmeter is connected in parallel to an electric load.
(e) Wood and plastic cannot conduct electricity.
(f) Thin wires have high resistance whereas thick wire have low resistance.

15. What is a neutral point? Draw a diagram to show neutral points.
16. List three properties of the magnetic lines of force.
17. What is terrestrial magnetism? Write two evidences.
18. What are geographic and magnetic meridians?
19. What are the uses of the angle of dip and the angle of declination?
20. What is potential difference? How can you calculate it mathematically?

Numerical problems

21. A resistor of 40 W resistance is connected to the voltage of 6V. Calculate the current
flowing through the circuit.

22. What is the resistance of the wire connected to 40 V through which 12 A current flows?
23. A wire carries 6 A current in 20 minutes. Calculate the amount of charge flowing through

the wire.
24. An electric bulb is marked with 220 V and 100 W. Calculate the current passing through

it and its resistance.
25. An electric heater is marked 125 V, 500 W. Calculate its resistance when the current flows.
26. Calculate the total resistance for the following case.

New Creative Science, Class 9 | 125

27. How much time does it take for the flow of 100 C charge through a circuit carrying 4 A
current?

28. 10 A current flows through an electric heater of 500 W. Calculate the resistance of the
heater and its potential difference across it.

A

B GLOSSARY
C

Resistor : which opposes the ow of electrons
a point where there is no e ect of a magnet
Neutral point : e ect of the ow of electrons
the unit of current
Current :

Ampere :



126 | Electricity and Magnetism

UNIT

8 CLASSIFICATION OF ELEMENTS

About the Scientist Introduction

Neils Bohr Around us there is nothing but only matter. Each and
(1885-1962) everything is made up of matter. To study this matter, we
have a separate branch in science which is called chemistry.
Neils Bohr (1885-1962) was The branch of main science which deals with matter, its
born in Copenhagen on 7 composition, properties and its relation with energy is
October 1885. He got the called chemistry.
Nobel Prize for his work
on the structure of atom The matters which we see, observe and use are not
in 1922. Among professors the same type. Some of them are made up of only one
Bohr’s was more famous type of atoms or compounds whereas some are made
for his writing the skill. after mixing two or more matter. Therefore matter can be
He wrote numerous books. categorized into two groups i.e. pure and impure matter.
The most popular three Iron rods, golden coins, aluminum pots, common salt,
appearing books are: glucose powder, etc. are the pure matter whereas soil,
(1) The theory of spectra clothes, food, etc. are the impure matter. A brief idea about
the division of matter is given below:
and atomic constitution
(2) Atomic theory, and Matter
(3) The description of nature
Pure Matter Impure Matter

Element Compound Homogeneous Heterogenous

Mixture Mixture

MEMORY TIPS
Matter is anything which has weight and occupies space.

Element

Copper, silver, gold, iron, etc. are pure substances.
They are made up of the same type of many atoms. About
one hundred and fourteen such substances are discovered
till now. They are called elements. These elements cannot
be divided into further simpler substances.

New Creative Science, Class 9 | 127

The simplest pure form of a substance which cannot be broken down into
simpler substances by the application of any physical and chemical methods is called
an element.

The chemical substances which are used by us or are present around us are either
elements or the compounds made from the chemical combination of the elements.
For example; the food which we eat contains maximum amount of carbohydrate.
Carbohydrate is made up of glucose molecules and a glucose molecule contains
carbon, hydrogen and oxygen elements.

Elements are divided into three groups. They are metals, non-metals and
metalloids. Metallic elements are electropositive in nature. They are malleable, ductile
and the good conductor of heat and electricity. Most of the metallic elements react
with oxygen to give metallic oxides. These metallic oxides are basic in nature. They
dissolve in water to give alkaline solution. However some are amphoteric in nature.
For example, Lithium (Li), Sodium (Na), Potassium (K), Magnesium (Mg), Calcium
(Ca), Gold (Au), Silver(Ag), Mercury (Hg), Nickel (Ni), Iron (Fe), Copper (Cu), etc.

The non-metallic elements are electronegative in nature. They are non-conductor
of heat and electricity, non-malleable and non-ductile in nature. They do not possess
luster. Non-metals form oxides when heated with oxygen. These oxides are acidic
in nature. When they are dissolved in water they give acid. Some examples of non-
metals are Hydrogen (H), Oxygen (O), Nitrogen (N), Chlorine (Cl), Phosphorus (P),
Carbon (C), Bromine (Br), Sulphur (S), etc.

Besides metals and non-metals, there is a third category of elements which show
the properties of both metal and non-metal. These elements are called metalloids.
Silicon (Si), Germanium (Ge), Arsenic (As), Antimony (Sb) , tellurium (Te), etc. are
the examples of metalloids.

Compound

Hydrogen and oxygen are gas but when they are combined, they give water
which is a liquid. Similarly sodium and chlorine are toxic elements. When they are
combined they give sodium chloride. It is a non-toxic and necessary in our food. So,
during the formation of a compound there occurs chemical change.

The chemical substance which is obtained by the chemical combination of two or
more elements in fix proportion by weight is called a compound.

In each compound, there is definite ratio of each combining element. For example,
in carbon dioxide (CO2) the ratio of carbon and oxygen is in 3 : 8 and in ammonia
(NH3) the ratio of nitrogen and oxygen is 14 : 3

128 | Classification of Elements

Structure of an atom s
p
We have discussed already that, an atom is the ultimate particle of an
element. It is the smallest and indivisible particle of the element. An atom d
is such a tiny structure, that it cannot be seen by naked eyes. Its diameter is
approximately 10–10m. An atom is the indivisible particle but it is made up of
three fundamental particles called electrons, protons and neutrons. Protons
and neutrons are located within a very small volume called the nucleus,
whereas electrons are revolving outside nucleus making the circular path
called shells. These shells are made up of sub-shells. The subshells have
different structures. Some are spherical and some are dumb-bell.

Sub-shells or Orbitals

According to Bohr and Burry, electrons are distributed by 2n2 formula where ‘n’ is
the number of shells. But electronic configuration of all elements cannot be explained
by this formula. So, the concept of sub-shells or orbitals is assumed. According to this
concept, within the main shell there are one or more sub-shells which are symbolized
by s, p, d and f. The structure of s-subshell is spherical and the structure of p and d
sub-shells is dumb-bell. The structure of f-subshell is not discovered till now. The
shells with their sub-shells are given below.

Shells Sub shells or orbitals
K (1) s
L (2)
M (3) s and p
N (4) s, p and d
s, p, d and f

In a s-subshell there is only one orbital. In a p-subshell there are three orbitals,
they are px, py and pz. Similarly in a d-subshell, there are five orbitals, they are dxy,
dyz, dzx, dx2 – y2 and dz2.

The “s” sub-shell may contain a maximum of two electrons, the “p” sub-shell
may contain a maximum of six electrons, the “d” sub-shell may contain a maximum
of ten electrons and the “f” sub- shell may contain a maximum of fourteen electrons.

Aufbau principle and electronic configuration

The systematic distribution of electrons around the nucleus in different shells
and subshells is called electronic configuration. In the electronic configuration the
coefficient shows the shell, the alphabet shows the subshell and the power of the
alphabet, shows the number of electrons.

shell electrons
sub-shell

New Creative Science, Class 9 | 129

We have already discussed that every shell
contains further smaller shells which are called
sub-shells. These sub-shells are represented by
the symbols s, p, d and f. These sub-shells have
different energy. Electrons always try to enter in
those sub-shells which have less energy. So,
according to Aufbau principle, the sub-shell
which has the lowest energy will be filled first
with electrons. The sequence of energy level of
sub-shells is shown in the diagram below.

According to the above diagram, the sequence of
energy level is given below.

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s . . .

K (n = 1) 1s 2 Electrons
L (n = 2) 2s, 2p 2 + 6 = 8 Electrons
M (n = 3) 3s, 3p, 3d 2 + 6 + 10 = 18 Electrons
N (n = 4) 4s, 4p, 4d, 4f 2 + 6 + 10 + 14 = 32 Electrons

According to sub-shell electronic configuration, some examples are:

1. Hydrogen (H1) = 1s1
2. Sodium (Na11) = 1s2, 2s2 2p6, 3s1
3. Chlorine (Cl17) = 1s2, 2s2 2p6, 3s2 3p5
4. Sodium (K19) = 1s2, 2s2 2p6, 3s2 3p6, 4s1
5. Calcium (Ca20) = 1s2, 2s2 2p6, 3s2 3p6, 4s2

Table: Electronic configuration based on sub-shells (s, p, d and f)

Element At. K L M N O P

No. 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f

H 11

He 2 2

Li 3 2 1

Be 4 2 2

B 5221

C 6222

N 7223

O 8224

130 | Classification of Elements

Element At. K L M N O P

No. 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f

F 9225

Ne 10 2 2 6

Na 11 2 2 6 1

Mg 12 2 2 6 2

Al 13 2 2 6 2 1

Si 14 2 2 6 2 2

P 15 2 2 6 2 3

S 16 2 2 6 2 4

Cl 17 2 2 6 2 5

Ar 18 2 2 6 2 6

K 19 2 2 6 2 6 1

Ca 20 2 2 6 2 6 2

Valence shell and valence electrons

From the given atomic structure, it is clear that the last shell of an atom is the
called valence shell and total number of electrons present in it are called the valence
electrons.

The outermost shell of an atom is called the

valence shell and the sum of electrons present in Valence
electrons
this shell is called valence electrons. 18n
In hydrogen the first shell is the valence shell 17p Valence
shell
and in nitrogen the second shell is the valence

shell. In hydrogen, there is one electron in its Chlorine
valence shell, in nitrogen there are five electrons in

the valence shell and in sulphur there are six electrons in the valence shell.

7n 16n
0n 7p 16p
1p

Hydrogen Nitrogen Sulphur

Valency

In our surrounding, we can observe various types of chemical substances. These
substances contain molecules. And each molecule is made from the different types of
elements. To make molecule, elements possess some force of attraction called valency.

New Creative Science, Class 9 | 131

The combining capacity of an element or the radical with other element or radical
to form a stable molecule is called valency.

In earlier time, the combining capacity was measured by counting the number of
hydrogen that is present along with an element. For example; In HCl, one hydrogen
atom is combined with one chlorine atom, in H2O two hydrogen atoms are combined
with one oxygen atom, in NH3 three hydrogen atoms are combined with one nitrogen
atom and in CH4 four hydrogen atoms are combined with one carbon atom. The above
molecules, the valency number of chlorine, oxygen, nitrogen and carbon is 1, 2, 3 and
4 respectively. So, the total number of hydrogen atoms which are combined with an
element is called its valency number. With the help of this definition, we can identify
the valency number of only hydrogen containing compounds. Thus, nowadays a new
definition of valency number has been given.

The total number of electrons which are gained or lost or shared by an atom
during the chemical reaction is called its valency number.

From the above definition, it is clear that the valency number of potassium is one
because it loses one electron and the valency number of nitrogen is three because it
gains three electrons. Elements having valency one are called monovalent, two are
bivalent, three are trivalent and four are called tetra valent.

MEMORY TIPS
 In radicals the total number of charge present in it, indicates the valency. For

example Ferric (Fe++) has valency 3 and Phosphate (PO4- - - ) has valency 3.

Element H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca
Valency No. 1 0 1 2 3 4 3 2 1 0 1 2 3 4 3 2 1 0 1 2

 Inert gases like He, Ne, Ar, Kr, Xe and Rn have zero valency.
 Some elements show variable valency like

Cuprous (Cu+) = 1 Ferrous (Fe++) = 2
Cupric (Cu++) = 2 Ferric (Fe+++) = 3

Inert gases

There are a total of six elements which have 8 electrons in the valence shell
(except helium) and do not participate in the chemical reaction due to zero valency
(combining capacity). These elements are called inert gases.

Those elements which have 8 electrons in their valence shell (except helium
which has two electrons) and are inactive in chemical combinations are called inert
gases.

132 | Classification of Elements

Characteristics of inert gases are given below:
1. They have 8 electrons in the valence shell except helium which has two
electrons.
2. They have zero valency. So, do not take part in the chemical reaction.
3. They occur as in atomic form in gaseous state.

S.N. Inert Atomic Electronic configuration
gas Symbol Number K L MNOP
2
1 Helium He 2 28
2 88
2 Neon Ne 10 2 8 18 8
2 8 18 18 8
3 Argon Ar 18 2 8 18 32 18 8

4 Krypton Kr 36

5 Xenon Xe 54

6 Radon Rn 86

Duplet and Duplet rule

The presence of two electrons in the K-shell of an atom is called duplet.
Helium is the single element, which has such condition. Due to the presence of
two electrons in this valence the shell it is stable.

The tendency of an atom to make two electrons in their K-shell is called the

duplet rule.

For example:

H1 +e— H—
Be4 – 2e— Be++
B5 – 3e— B+++

Octet and Octet rule

There are a total of five elements which have 8 electrons in their valence shell.
This type of arrangement is called octet. Thus, the arrangement of 8 electrons in
the valence shell of an atom is called octet.

The five elements which have 8 electrons in their valence shell are in stable

condition. So except inert gases, other elements try to maintain 8 electrons in their

last shell by gaining or losing electrons is called the octet rule. For example:

Na11 = 2, 8, 1 – 1 e— Na+ = 2, 8
Cl17 = 2, 8, 7 +1e— Cl– = 2, 8, 8

New Creative Science, Class 9 | 133

Radicals

Sodium loses one electron and becomes positively charged species. Similarly,
chlorine gains one electron and becomes negatively charged species. These positively
and negatively charged species are called radicals.

Na – 1 e— Na+ (Sodium radical)

Cl + 1 e— Cl— (Chloride radical)

The atom or group of atoms which have either positive or negative charge on
them and behave like a single unit are called radicals.

We cannot see the radicals but can be observed their effects because they are
highly reactive and the least stable chemical species. Hence, they immediately form
the stable compounds.

Depending upon charges, there are two types of radicals.
1- Electropositive radicals
2- Electronegative radicals

Electropositive radicals

When an atom or group of atoms lose electrons then they gain positive charge.
Those positively charged species are called electropositive radicals or basic radicals.

The atom or group of atoms which have positive charge on them are called
electropositive radicals.

Some electropositive radicals with their valency are given in the table.

Racicla Bivalent Trivalent Tetravalent

Radical Symbol Valency Radical Symbol Valency Radical Symbol Valency Radical Symbol Valency

Hydrogen H+ 1 Beryllium Be++ 2 Boron B+++ 3 Stannic Sn++++ 4

Lithium Li+ 1 Magnesium Mg++ 2 Aluminium Al+++ 3 Plumbic Pb++++ 4

Sodium Na+ 1 Calcium Ca++ 2 Ferric Fe+++ 3

Potassium K+ 1 Strontium Sr++ 2 Auric Au+++ 3

Rubidium Rb+ 1 Barium Ba++ 2 Chromium Cr+++ 3

Cesium Cs+ 1 Cupric Cu++ 2

Cuprous Cu+ 1 Mercuric Hg++ 2

Silver Ag+ 1 Ferrous Fe++ 2

Mercurous Hg+ 1 Stannous Sn++ 2

Aurous Au+ 1 Zinc Zn++ 2

MEMORY TIPS
Electropositive radicals have more number of protons than electrons.

134 | Classification of Elements

Electronegative radicals

When an atom or groups of atoms gain electrons then they acquire negative charge.
These negatively charged species are called electronegative radicals or acid radicals.

The atom or group of atoms which have negative charge in them are called
electronegative radicals.

Electronegative radicals with their valency are given below.

Monovalent Bivalent Trivalent

Radical Symbol Valency Radical Symbol Valency Radical Symbol Valency

Fluoride F– 1 Oxide O– – 2 Nitride N– – – 3

Chloride Cl– 1 Sulphide S– – 2 Phosphate PO4– – – 3
Bromide Br– 1 Carbonate 2
Iodide I– 1 Sulphate CO3– – 2
Cyanide CN– 1 Sulphite SO4– – 2
Hydroxide OH– 1 Silicate SO3– – 2
SiO3– –

MEMORY TIPS

 Electronegative radicals have more number of electrons then protons.
 NH3 is a molecule and NH4+ is a radical.

Chemical bond

There are millions of compounds in our surroundings. These compounds are
made by the combination of two or more elements except inert gases. Except inert
gases all other elements are unstable. They try to make stable electronic configuration
with the help of losing and gaining electrons. The loss and gain of electrons during
the chemical change is the main cause of chemical reaction.

While making chemical compounds, metals lose electrons and non-metals gain
electrons. When metals lose electrons they become positively charged chemical
species and when non-metals gain electrons they become negatively charged chemical
species. Between these positive and negative charged chemical species, there is a force
of attraction. As a result, they come together and form the compound.

Chemical bond is the attractive force through which two or more atoms are
present in a molecule.

MEMORY TIPS

Chemical bond is nothing but it is a type of force of attraction between two or more
atoms.

QUESTIONS >>
# What is duplet and duplet rule? Clarify with an example.
# Define octet and octet rule and explain with an example.
# Why does chemical reaction occur? Give reasons.

New Creative Science, Class 9 | 135

Electrovalent bond

Sodium is a metal. While making the sodium chloride, it loses its valence electron
and becomes positively charged. Chlorine being a non-metal, it gains one electron
and becomes negatively charged. Between these positively and negatively charged
radicals, there is a force of attraction which is called electrovalent bond.

Na11 = 2, 8, 1 Cl17 = 2, 8, 7
↓ — 1e— ↓ — 1e—

Na+ = 2, 8 Electrovalent bonding Cl— = 2, 8, 8

NaCl

The attractive force which is present between two opposite radicals is called
electrovalent bond.

The chemical compounds which are formed as a result of electrovalent bonding
are called electrovalent compounds. For example; NaCl, KCl, MgCl2, NH4Cl, CaCl2,
etc.

Characteristics of electrovalent compounds
1. They have a high melting as well as a high boiling point.
2. They are strong because they have strong electrostatic force of attraction.
3. Electrovalent compounds conduct electricity in molten or fused state.

Formation of sodium chloride (NaCl)

Sodium is a metal and chlorine is a non-metal. Sodium loses one electron
and becomes electropositive radical and chlorine gains one electron and becomes
electronegative radical. Between these opposite radicals electrovalent bonding occurs
to make sodium chloride.

12n 18n

11p 17p = Na+ + Cl– = NaCl

Sodium atom Chlorine atom

Shows the formation of sodium chloride

Formation of magnesium chloride (MgCl2)

Magnesium is a metal and chlorine is a non-metal. Magnesium loses two
electrons and becomes electropositive radical and chlorine gains one electron and
becomes electronegative radical. Between these opposite radicals (one magnesium

136 | Classification of Elements

and two chloride), electrovalent bonding occurs to give magnesium chloride.

18n 12n 18n = Mg++ + 2Cl– = MgCl2
17p 12p 17p

Chlorine atom Magnesium atom Chlorine atom
The formation of magnesium chloride

QUESTIONS >>

# Define electrovalent bond and describe the formation of AlCl3.
# What are electrovalent compounds? Write their characteristics.

MEMORY TIPS

Electrovalent bonding always occurs in between two opposite charged radicals.

Covalent bond

Between two or more non-metal atoms, there occurs a type of force of attraction,
which is called covalent bond. This bond is formed as a result of sharing of electrons.

The chemical bond which is formed as a result of mutual sharing of electrons
between two or more non-metal atoms is called covalent bond.

The chemical compounds which are formed as a result of covalent bonding are
called covalent compounds. For example H2, O2, N2, CH4, CO2, HCl, etc.

Characteristics of covalent compounds
1. They have a low melting and a boiling point.
2. They are weaker than electrovalent compounds.

When there is sharing of one pair of electrons, it is called single covalent bond
and the symbol used is single line (–). If there is sharing of two pairs of electrons, it
is called double covalent bond and the symbol used is double lines(=). Similarly for
triple covalent bond, the symbol used is triple lines (≡). For example; in H2 , H—H, in
O2, O = O and in N2, N ≡ N.

0n 0n 8n 8n 7n 7n
1p 1p 8p 8p 7p 7p

H–H O=O N≡N
Single covalent bond Double covalent bond Triple covalent bond

New Creative Science, Class 9 | 137

Formation of carbon dioxide (CO2)

Both carbon and oxygen both are non-metals. Carbon needs four electrons and
oxygen needs two electrons to fulfil their octet. So, both combine by the sharing of
two electrons each.

8n 6n 8n
8p 6p 8p

O=C=O
Molecular structure of CO2

Types of covalent bond

There are two types of covalent bonds. They are:
i) Non-polar covalent bond
ii) Polar covalent bond

i) Non-polar covalent bond
The covalent bond which is formed in between the same or nearly same type

of atoms is called non-polar covalent bond. The molecules of non-polar covalent
bond do not have charge because the combining atoms have same or nearly same
electronegativity. For example; H2, O2, N2, CO2, CH4, etc.

ii) Polar covalent bond
The covalent bond which is formed between different types of atoms is called the

polar covalent bond. The molecules of polar covalent bond have charge because the
combining atoms have different electronegativity. For example; HCl, H2O, NH3, etc.

Molecules

We can divide matter into small pieces. The smallest piece of matter which can exist
independently is called a molecule. This molecule cannot be seen by our naked eyes.

The smallest particles of a compound which can exist independently and contain
all the properties of the compound are called molecules.

Molecules of a matter are all alike but molecules of different matter are different.

In some molecules, there is one atom like helium, neon, argon, krypton, xenon,
radon, etc. They are called monoatomic molecules. If a molecule contains more than
one atom, they may be similar or dissimilar. For example H2 , O2, N2 , Cl2 , etc. Here
the same types of two atoms are present in each molecule. CaCO3 , NH3 , CH4, HCl,
etc. have different types of atoms.

138 | Classification of Elements

Molecular formula

H2 , O2 , CO2 , NH3 , etc. are the examples of molecular formula. Here, each
molecule is made up of two or more same or different atoms. Thus, the symbol of one
molecule of a substance is called its molecular formula.

Information obtained from molecular formula
1. Molecular formula is a symbol for one molecule of the substance.
2. It shows the number of atoms of each element.
3. The molecular weight of each molecule can be calculated from the molecular
formula.
4. The percentage composition of each element can be shown from it.

5. Molecular formula shows the valency of different elements

MEMORY TIPS

 Inert gases are monoatomic molecules.
 H2 denotes one molecule of hydrogen and 2H denotes two atoms of hyrogen.

QUESTIONS >>
# Define covalent bond and explain the formation of CH4.

Molecular weight

Molecular weight is calculated by adding the atomic weight of all atoms of the
molecule. For example:

Molecular weight of HCl = H + Cl = 1 + 35 = 36

Molecular weight of KClO3 = K + Cl + 3×O = 39 + 35 + 3 × 16 = 122
Molecular weight of H3PO4 = 3 × H + P + 4×O = 3 × 1 + 31 + 4 × 16 = 98

Methods of writing molecular formula

The following steps should be followed while writing the molecular formula.

1. Write the symbols of positive and negative radicals side by side with their

valencies.

For example:

Al3 Cl1

2. Exchange the valency of these radicals.

For example: Al3 Cl1

13
New Creative Science, Class 9 | 139

3. Combine these radicals.

For example: Al3 Cl1

13

AlCl3
4. When there is the same valency, it is canceled.

For example: Ca2 CO32

11

5. When radicals are made by two or more different atoms, they are enclosed
within a bracket. For example, Aluminium hydroxide Al(OH)3.

6. While writing the molecular formula, all the letters should be in small letters
except for the first letter. For example, Aluminium chloride.

Some examples of molecular formula

1. Sodium carbonate 2. Hydrogen phosphate

Na1 CO32 H1 PO43

21 31
Na2CO3 H3PO4
4. Aluminium hydroxide
3. Calcium carbonate Al3 OH1
Ca2 CO32

1 1 1 3
CaCO3 H3PO4

ANSWER WRITING SKILLS

1. What is octet and octet rule? Explain with an example.
Ü Octet: The arrangement of eight electrons in the last shell of an atom is called

octet. This type of arrangement is present in the inert gases. For example, Neon
(Ne) = 2, 8
Octet rule: The tendency of an atom to make eight electrons in the valence shell
of an atom either by gaining or losing electrons in called octet rule.
For example,

Na11 = 2, 8, 1 → Na+ = 2, 8

Cl17 = 2, 8, 7 → Cl— = 2, 8, 8

140 | Classification of Elements

2. Write any two differences between electrovalent and covalent bonds.

Electrovalent bond Covalent bond

1. It is formed by the transfer of 1. It is formed by the sharing of
electrons. electrons.

2. It is formed between metals 2. It is formed between two non-
and non-metals (or between two metal atoms.
opposite charges).

3. What is covalent bond? Explain with one example.

Ü The bond which is formed between two or more non- 0n 0n
metal atoms as a result of sharing of electrons is called 1p 1p
covalent bond.
For example; formation of methane (CH4). In methane, 6n 0n
one carbon and four hydrogen atoms are bonded 6p 1p
together by the sharing of electrons.
0n
1p

4. What are radicals? How are they formed? Molecular structure of methane

Ü Atoms or group of atoms which have either positive or negative charge on them
are called radicals. For example Na+, Cl–, Mg++, CO – –, etc. They are formed either
gaining or losing electrons.

5. Give a reason why oxygen has valency two and aluminium has three.

Ü Oxygen has six electrons in its valence shell. So, it gains two electrons to be stable.
Hence, its valency is two. Similarly aluminium has three electrons in its valence
shell and it loses these three electrons to follow the octet rule. Therefore, valency
of aluminium is three.

6. MgCl2 is an electrovalent compound but NH3 is a covalent compound. Why?
Ü In MgCl2 , magnesium and chlorine are bonded together due to transfer of

electrons where as in NH3 , nitrogen and hydrogen are bonded together due to
sharing of electrons. Hence MgCl2 is an electrovalent compound and NH3 is a
covalent compound.

SUMMARY

” Chemistry is a branch of science which deals with the study of matter its
composition and properties.

” An atom is the smallest indivisible particle of an element.
” A compound is formed by the combination of two or more elements.
” An element is the simplest pure form of a substance.
” Electrons, protons and neutrons are the fundamental or sub-atomic particles.

New Creative Science, Class 9 | 141

” The smallest particle of the compound which is capable of independent
existence is called a molecule.

” The atom or group of atoms which have either positive or negative charge in
them are called radicals.

” Those elements which have 8 electrons in their valence shell (except helium)
are called inter gases.

” The presence of 8 electrons in the valence shell or the last shell of an atom is
called octet.

” The force by which two or more atoms are combined together is called chemical
bond.

” Electrovalent bond is formed between opposite charges.

” Covalent bond is formed between or among the non-metals.

EXERCISE

1. Define the given terms with examples if required:

(a) Atom (b) Chemistry (c) Element (d) Molecule
(h) Octet rule
(e) Compound (f) Octet (g) Duplet (l) Inert gases

(i) Duplet rule (j) Electronic configuration (k) Radicals

2. Draw the molecular structure of the given molecules.

(a) H2 (b) CO2 (c) NH3 (d) H2O
(e) CH4 (f) CaCl2 (g) AlCl3

3. Write two differences between:

(a) Electrovalent and covalent bond

(b) Ammonia and ammonium

(c) Atom and element

4. Identify whether the given compounds are electrovalent or covalent?

(a) Water (b) Aluminium chloride

(c) Magnesium chloride (d) Ammonia

5. Give reason:
(a) Magnesium has valency two.
(b) NH3 is a covalent compound and MgCl2 is an electrovalent compound.
(c) An atom is a neutral particle.
(d) H2 is a non-polar molecule and NH3 is a polar molecule.

6. Write the names of given compounds.

(a) AlCl3 (b) MgCl2 (c) Mg3(PO4)2 (d) CaCO3 (e) Ca(HCO3)2
(f) H2CO3 (g) NH4OH (j) AuCl3
(h) AlPO4 (i) NaOH

142 | Classification of Elements

7. Write the molecular formula of the given compounds.

(a) Aluminium sulphate (b) Calcium chloride

(c) Ferrous hydroxide (d) Aluminium phosphate

(e) Ferric nitrate (f) Gold chloride

(g) Silver nitrate (h) Potassium chlorate

8. Define octet and octet rule with an example.

9. Define molecular formula with four examples.

10. What information do you get from molecular formula?

11. Define duplet with an example.

12. Define duplet rule with an example.

13. What are the radicals? Write their types with four examples.

14. Define electrovalent bond and describe it with an example.

15. What is meant by molecular weight?

16. What are sub-shells? Write down the total number of electrons present in them.

17. Show the subshell electronic configuration of sodium, magnesium, aluminium, carbon,
chlorine, potassium, calcium and neon.

18. What are polar and non-polar molecules? Write down two examples of each.

19. What is the shape of different sub-shells? Draw their diagrams.

20. How many electrons can be accommodated in different sub-shells?

21. What is Aufbau principle? Write down the sequence to show sub-shell electronic
configuration.

22. Calculate the molecular weight of the given molecules.

(a) AlCl3 (b) MgCl2 (c) Mg3(PO4)2 (d) CaCO3 (e) Ca(HCO3)2
(f) H2CO3 (g) NH4OH (j) BCl3
(h) AlPO4 (i) NaOH

A

B GLOSSARY
C

Atom : which cannot be cut

Aufbau : the German word to construct

Split up : to break into smaller pieces

s, p, d and f : sharp, principle, di use and fundamental

Electronegativity : the force of attraction of the bonded electrons in the covalent bond

Polar covalent bond : the bond in which bonded electrons are partially shifted towards one

atom



New Creative Science, Class 9 | 143

UNIT

9 CHEMICAL REACTIONS

About the Scientist Introduction

Amedeo Avogadro In nature, various types of changes are occurring every
time. Some of these changes are spontaneous whereas
Amedeo Avogadro was born some are made by human beings. Evaporation of water
August 9, 1776 and died on July from the water bodies, formation of clouds in the sky and
9, 1856. He was born and died in condensation of this vapour into water, etc. are the natural
Turin, Italy. Amedeo Avodagro, processes. Making different types of articles from soil,
conte di Quaregnae Ceretto, was construction of buildings, dissolving of sugar into water,
born into a family of distinguished etc. are the man made changes. Among these changes,
lawyers (Piedmont Family). some are permanent whereas some are temporary. Some
Following in his family’s footsteps, are reversible whereas some are irreversible. Based on these
he graduated in ecclesiastical law characteristics, the changes are broadly divided into two
(age 20) and began to practice types. They are physical changes and chemical changes.
law. However, Avogadro also
was interested in the natural Physical changes
sciences and in 1800 he began
private studies in physics and Water becomes
mathematics.
Avogadro’s law states that equal steam when it is boiled.
volumes of gases, at the same
temperature and pressure, contain The steam again
the same number of molecules.
Avogadro’s hypothesis wasn’t becomes water when
generally accepted until after 1858
(after Avogadro’s death), when it is condensed at low
the Italian chemist Stanislao
Cannizzaro was able to explain temperature. This water
why there were some organic
chemical exceptions to Avogadro’s changes into ice when it
hypothesis
is kept in the refrigerator

and so on. Salt gets

dissolved into water to give salt solution. This salt can be

re-obtained after evaporation. In these changes no new

substance gets formed and they are occurring in forward

as well as in backward directions. Such types of changes

are called physical changes. During physical change,

only physical characteristics are changed. Therefore, the

types of temporary and reversible changes where no new

substances are formed are called physical changes.

144 | Chemical Reactions

Characteristics of physical change
i) Physical change is a temporary change; it means that the product obtained in this

change is not permanent in nature.

ii) No new substance (with different chemical composition) is formed during this
change.

iii) Generally, it is a reversible change, so that the product obtained can be reversed
back into the reactant.

iv) In the physical change, there occurs only change in the physical characteristics.
It does not make any change in the chemical nature.

ACTIVITY Kettle

Objectives Water drop
To observe the water cycle.
Vapour
Materials required
Water, kettle, source of heat, etc.

Procedure
1. Keep some amount of
water in a kettle.
2. Supply heat with the help
of source of heat.

Observation
Water turns into steam when it is heated. This steam makes the drops of water on the lower
surface of the lid of kettle.

Conclusion
This activity shows that water cycle is a physical change.

Examples of physical change
i. Changing of water to ice, ice to water and water to water vapour.
ii. Dissolving of common salt, sugar and ammonium chloride into water to
make a solution.

iii. Magnetizing of iron into magnet and demagnetizing of magnet into iron.

MEMORY TIPS

Interconversion of water to ice, ice to water and water to water vapour is a physical
change because no new substance is formed here.

New Creative Science, Class 9 | 145

Chemical change

When we burn paper, wood, fuel, etc. then they give ash and smoke. If we want
to convert this ash and smoke into their respective substance, it is impossible. This
type of process is called irreversible process and the change is called chemical change.
In this change, new substance is formed and the product is entirely different from the
original substance.

The irreversible and permanent change, in which a new substance gets formed
is called chemical change.

Characteristics of chemical change
i) Generally, a chemical change is a permanent change where a new substance
cannot be reversed back.

ii) During a chemical change, new substance with an entirely different chemical
formula is formed.

iii) It occurs only in one direction. It means that the product once formed cannot be
reversed back.

iv) During chemical change, physical as well as chemical properties are
changed.

Examples of chemical change
i) Burning of wood, paper, coal, fuel, candle, etc.
ii) Formation of rust over the surface of metal.
iii) Digestion of food within living body and photosynthesis in the plant.
iv) Conversion of milk into curd.

Differences between physical and chemical change

Physical change Chemical change

1. It is a reversible and temporary 1. It is an irreversible and
change. permanent change.

2. During this change, no new 2. During this change, a new substance
substance gets formed. gets formed.

3. In this change, generally mass 3. Generally, mass and energy are
and energy is not changed. changed.

Chemical reaction 2NaCl
CaO + CO2
2Na + Cl2
CaCO3 ZnCl2 + H2
Zn + 2HCl

146 | Chemical Reactions