Modern Graded Science - 9

Some other examples of refraction of light

1. The apparent upward bending of a pencil

when placed in water: A a pencil ABC placed

in a trough of water. The pencil appears bent

upwards from point B. Rays CE and CD from

the end C of the pencil pass from the water

to air, and are bent away from the normal

along EE' and DD' respectively, since they

are passing in a rarer medium. To the eye, Fig. 6.6 a straight pencil appears bent in water
they appear to come from point C' above C.

C' is thus the image of C as a result of refraction. The same reasoning applies to any point

on the immersed portion of the pencil BC, so that the observer sees an image apparently

in the position BC'.

2. The stars appear twinkle on the clear sky: The light from the stars passes through the

atmosphere before it reaches an observer on the

earth. The atmosphere is not still as there are

always currents of hot and cold air. The hot and air in different
temperatures
cold air form layers of air of different densities
and the speed of light in them is different. Due to

the current of air, the densities of the layer of air

keep on changing continuously. This causes the

position of the image of a star to change

continuously with time in space. As the change

in density is in a small range, the image of a star

appears to move within a small area in the space

due to refraction of light of the stars and this

gives rise to twinkling effect. Fig. 6.7 stars appear to twinkle on the sky

The given illustrations can be explained based on the principle of refraction of light.

a. An object is observed farther than the actual position when it is obeserved by a

swimmer inside the water of a swimming pool.

b. A stick held boliquely and partly immersed in water appears to be bent at the

water surface.

c. A pool of water appears to be less deep than it actually is.

d. The sun appears two minutes before the actual sunrise and two minutes after the

actual sunset.

e. A fisherman targets to a fish as it is seen in water but the fish escapes.

Light 97

f. The legs of a person seem shorter when they are standing in a swimming pool.

g. The birds dive straight down to catch fish from the lake.

h. The printed word PHYSICS written on the paper appears to be raised up when a glass
slab is kept on it.

Total internal reflection

In the diagram, there are to optical mediums. glass is a denser medium and air is a rarer

medium. Suppose a ray AO is passing from glass to air i.e. from a denser medium to a rarer

medium. It bends away from the M AI
normal MN, as OA1. Here ∠AON
is the angle of incidence and angle of refracted ray
refraction

∠A1OM is the angle of refraction. air (rarer medium) Or 90o refracted ray BI

When the angle of incidence greater than CI
critical angle
increases, the angle of refraction c critical
angle
also increases. If the angle of i

C

incidence is increased continuosly, B angle of
incident rays incidence
a condition may come, when the glass (denser medium)
AN

angle of refraction (∠B1OM) in Fig. 6.8 total internal reflection

the rarer medium is 90o. Now, the angle of incidence in the denser medium (∠CON) is called

a critrical angle. In this case ∠i= C and ∠r = 90o.

Thus, the critical angle is defined as the angle of incidence in a denser medium for which
the angle of refraction in rarer medium is 90o . It is denoted by C. Different substance have
different values of the critical angle.

If the angle of incidence is increased more than the critical angle, all the rays reflect in the
denser medium. The process is total internal reflection. Thus, when a ray of light passes from
a denser to a rarer medium with the angle of incidence greater than the critical angle, the ray
is reflected back into the same medium. This phenomenon is called total internal reflection.

Conditions for total internal reflection

1. The incident ray must pass from a denser medium to a rarer medium.

2. The angle of incidence in the denser medium should be greater than the critical angle
(C) for the given pair of media.

98 Modern Graded Science Class 9

Critical angles for some optical media with respect to air

Substances Ice Water Alcohol Kerosene Paraffin Glycerine Turpentine Glass Ruby Diamond
50o 49o 48o 46o 44o 43o 43o 42o 35o 24o
critical
angle (C)

Refractive index

The ratio of the speed of light (C) in air or vacuum to the Stars do not appear twinkling if
speed of light in the medium (v) is called refractive index. they are viewed from above the

It is denoted by (µ). The refractive index of a medium is also earth's atmosphere. Twinkling

defined in terms of velocity of light. effect is observed if the stars are
viewed from the earth's surface.

∴ R.I. of a medium (µ) = speed of light in air or vacuum (C)
speed of light in the medium (v)

For example, the velocity of light in air is 3 × 108 m/s
and that for glass is 2 × 108 m/s. Therefore, the refractive
index (µ) for the glass is given by,

µ = 3 × 108 = 1.5
2 × 108

According to Snell's law, sin i r= constant, where 'i' is the
sin
angle of incidence and 'r' is the angle of refraction. This

constant is called refractive index of the second medium Fig. 6.9

with respect to (w.r.t) the first medium.

If light passes from water to glass as shown in the given figure, the refractive index of glass w.r.t
water, denoted by wµg, is given by,

The refractive index of any medium w.r.t air (or vacuum) is called the absolute refractive index
of the medium. This is a measure of optical density. For example, absolute refractive index of
water is denoted by aµw or µw simply µ.

Absolute refractive indices of some substances

Substances Ice Water Alcohol Kerosene Paraffin Glycerine Turpentine Glass Ruby Diamond
1.31 1.33 1.36 1.39 1.44 1.47 1.47 1.5 1.76 2.42
R.I.(m)

Light 99

Example 1: The speed of light in a vacuum is 3 × 108 m/s. Find the speed of light in glycerine,
if its refractive index is 1.47.

Solution:

Here, Speed of light in vacuum (C) = 3 × 108 m/s

Speed of light in glycerine (v) = ?

Refractive index (µ) = 1.47

We know,

R.I = Speed of light in vacuum (C)
Speed of light in a medium (v)

or, 1.47 = 3 × 108
v

or, v = 3 × 108 = 2.04 × 108 m/s
1.47

Therefore, the speed of light in glycerine is 2.04 × 108 m/s

Example 2: A ray of light in air strikes a glass slab at an angle of incidence 20°. Find the angle
of refraction in glass. (µg = 1.5)

Solution:

Here, Angle of incidence (i) = 20°

Refractive index of glass (µ) = 1.5

Angle of refraction (r) = ?

We know that, from Snell's law.
ssiinn ri
µ =

or, 1.5 = sin20°
sin r
sin20°
or, sin r = 1.5

or, r = sin-1 0.3420 = 13.18°
1.5
Therefore, the angle of refraction in glass is 13.18°.

Example 3: Calculate the refractive index of water when the real depth and apparent depth of
the bottom of a beaker are 8 cm and 6 cm respectively.

100 Modern Graded Science Class 9

Solution:
Here,

Real depth = 8 cm
Apparent depth = 6 cm
Refractive index (µ) = ?
We know,

µ = real depth = 8
apparent depth 6

∴ µ = 1.33
Therefore, The refractive index of water is 1.33.

Prism

A prism is a transparent refracting medium bounded

by two plane surfaces inclined at the same angle. The

plane surfaces through which the light passes are called

refracting faces (ABCD and ABEF). The angle between Fig. 6.10 prism

two refracting surfaces is called the angle of the prism. It is usually denoted by A. The line

along which the two surfaces meet is called the refracting edge of the prism (AB). The plane

at which two refracting surfaces rest is called the base of the prism. The section of the prism

perpendicular to the refracting surface is called the

principal section of the prism. A prism is represented

by a triangle as shown in this diagram.

XYZ = a prism X Y

and XZ = refracting faces

∠YXZ = A, the angle of prism PQ

= incident ray Fig. 6=.11em- reefrrgacetniotnrathyrongh a prism
= normal to the face XZ at R
QR = refracted ray RS = angle of deviation (D)\
= angle of prism
N1N2 = normal to the face XY at Q N3N4
YZ
= base of the prism ∠MNR

Dispe rsion of lig ht ∠YXZ

When a beam of white light is passed through a prism, it
splits into its seven constituent colours: red, orange, yellow, The angle between the incident
green, blue, indigo and violet as shown in the figure. To ray and the emergent ray is
remember these colours, the word VIBGYOR should be called the angle of deviation in
kept in mind i.e. Violet, Indigo, Blue, Green, Yellow, Orange the case of the prism.

Light 101

and Red. The phenomenon of splitting up of white light into its constituent colours is called

dispersion of light. The band of X

seven colours produced on a screen A
as a result of dispersion is called the

spectrum. In the spectrum, there

is infra-red above the red colour

and ultraviolet below the violet

colour of light. Y Z
The angle between the incident ray

and the emergent ray is called the Fig. 6.12 dispersion of light through a prism

angle of deviation in the case of the prism. The angle between two refracting surfaces is called

the angle of the prism. It is denoted by A.

How is a rainbow formed in the atmosphere?

In a rainy season, a large number of water droplets in the
atmosphere function as small prisms. When the white light
rays from the sun meet these water droplets in the air, they
are refracted at the boundary of air and water, and enter the
droplets, where the light is dispersed into seven colours. These
light rays are reflected inside the droplets and finally refracted
out again into the air. The white light splits into seven colours
(VIBGYOR) which appear in the form of a rainbow in the sky.
In a rainbow, the violet colour appears at the lower side and
the red colour appears at the upper side while other colours
lie between violet and red colours of the spectrum. Thus, a Fig. 6.13 formation of rainbow
rainbow is formed by the dispersion of sunlight by small droplets of water in the atmosphere.

Causes of dispersion of light

Dispersion takes place because white light consists of seven colours and the different colours
are refracted in different amounts by the prism.

In a vacuum (or air), all constituent colours (VIBGYOR) of white light have the same speed
(i.e. 3 × 108 m/s). When white light enters a denser medium, each colour has different speed
but has less speed than that of the light in a vacuum. On leaving the denser medium, its speed
increases to its original value (i.e. 3 × 108 m/s). It is proved that red light has the maximum
speed i.e. 8×10-7 m/s and violet light has the minimum speed i.e. 4 ×10-7 m/s within a denser
medium. Thus, the red light deviates least and the violet light deviates most. The deviations of
the other colours occupy intermediate positions between that of the violet and the red colours.

102 Modern Graded Science Class 9

The causes of dispersion of light can be summed up in following two points.

1. White light consists of seven different colours of spectrum.
2. Different colours of light have different wave lengths, due to which they have a different

velocites in different mediums.

Recombination of spectrum of colours

When the spectrum produced by a Fig. 6.14 recombination of spectrum of colours
prism is made to pass through another
inverted prism as shown in the figure,
all the colours mix with each other
and white light emerges out from the
prism. This is called recombination of
the colours.

Activity 6.2
To prove that white light consists of seven colours.

1. Take a cardboard disc of 3 inch diameter and paste white paper at its one side.

2. Divide the surface of the white paper in seven equal parts and
colour them in seven different colours of spectrum.

3. Now, mount the disc in an electric motor that is adjusted in a
circuit on a stand as shown in the diagram.

4. Now, switch on and observe that disc.

The coloured disc appears white. It proves that white light also
consists of seven different colours and when all the colours effect
on our eyes at the same time, then it appears white.

Electromagnetic waves and their properties

A wave motion is a mean of transferring

energy from one point to another without

transferring any matter between these points.

Waves may be classified as being either

mechanical or electromagnetic. A mechanical

wave is the disturbance that requires a

material medium for its propagation e.g.

water waves, sound waves, waves in stretched

strings, etc. It is also known as elastic waves. energy by thFeig.v6a.1r6iations of electric and
Electromagnetic waves are the waves which carry

Light 103

magnetic fields. These waves can propagate through the vacuum at a high speed i.e. 3×108 m/
sec. e.g. visible light, radio waves, x–rays, γ- rays, etc. They can travel through a vacuum. The
solar radiation which we receive every day is also electromagnetic wave.
The main divisions of the electromagnetic spectrum are as shown in the figure. Infrared
radiation, visible light and ultraviolet radiation are all members of the same electromagnetic
spectrum. Visible light is that light which affects our eyes and produces sensation of visions.
Invisible light is that light which exists at both ends of the visible light spectrum and consists
of infrared and ultraviolet light.

Properties of electromagnetic waves

1. All electromagnetic waves travel through a medium or in a vacuum at the same speed i.e.
3 ×108 ms-1.

2. They are unaffected by electric and magnetic fields.
3. They are transverse waves.
4. They all, under suitable conditions, can be reflected, refracted and diffracted.
5. Electromagnetic waves are formulated by the formula:
Speed of light = Frequency × Wavelength i.e., C = f×λ. This can be applied on them.

Electromagnetic spectrums are characterized by their different frequencies or wavelengths.
Ultraviolet radiation has a greater frequency than visible light and infrared has a lower
frequency than visible light.

Visible light

1. The wavelength of visible light (red, orange, yellow, green, blue, indigo, violet light) is in
the range of 4 × 10-7 m to 8 × 10-7 m. The wavelength of red light is about 8 × 10–7m and
that of violet is 4 × 10–7 m.

104 Modern Graded Science Class 9

2. They are produced by flames and discharge tubes.
3. They can be detected by the human eye, photographic film and they initiate photosynthesis.

Infrared radiation

1. The wavelength of infrared radiation extends from about 10-5 m to 5 × 10-3 m, which is
higher than the wavelength of visible red light.

2. They are characterized by warmth. Heat radiation is produced from hot bodies such
as the sun, electric heaters and furnaces. They are used for cooling, drying, infrared
photography, etc.

3. They can be detected by a sensitive thermometer.
When we sit near a fire, we feel warmth because of invisible infrared instead of visible light.
When a body absorbs infrared, it gains energy and becomes hotter. From the sun, we get
visible, ultra-violet and infrared radiations. We feel warm in the sunlight because of the sun's
invisible infrared rays.

Radio waves

1. Beyond the infra-red region is a wide range of wavelengths from about 10-4 m to 5 ×
104 m known as radio waves. They are also further sub-divided into micro-waves, radar
waves, television waves, etc.

2. Radio waves are produced by electrical oscillations.
3. They can be detected by radio receivers or similar tuned circuits.
Radio waves are produced by radios, radars and television transmitters. They are used widely
in communication. They do not have any harmful effect on the body.

Ultraviolet radiation

1. Ultraviolet radiation is just beyond the violet end of the visible spectrum and has wave
length in the range of 5×10-9 m to 5×10-7m.

2. They are produced by electric sparks, discharge tubes, mercury vapour lamps, hot bodies
and the sun.

Ultra-violet radiation from the sun causes sun-tanning of the skin but it helps with the
formation of vitamin D in human body. It is used in burglar alarm, automatic door
openers and counters and for detecting real pearls, etc. Excessive ultra-violet radiations
are harmful to the eyes and the skin.

3. They can be detected by photographic film.
4. In the ozone layer of the atmosphere, it ionizes oxygen and produces ozone gas.

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X-rays

1. Their wavelength is less than about 5 × 10-8 m.
2. X-rays are produced when very fast moving electrons are stopped by a heavy metal and

γ- rays are produced in radioactive decay, nuclear fission and fusion. X-rays are used in
the medical field to produce pictures of internal organs of the body.
3. They can be detected by photographic films.
4. They have harmful effects on our body.

Difference between X-ray and ultraviolet ray

X-ray Ultraviolet ray

1. Its average wave length and frequen- 1. Its average wave length and frequen-

cy are 10-10m and 1018Hz respec- cy are 108m and 1018Hz respectively.

tively.

2. It is obtained by an X-ray machine. 2. It is obtained from the sun, tube

lights, etc.

The following table presents the wavelengths, sources, and usefulness of the members of the
electromagnetic spectrum.

Name of Average Wave- Source Use Harmful on
waves frequency length exposure

Ɣ - rays 1020Hz 10-13m - Radioactive elements Radio therapy Very harmful
10-11m (e.g. Uranium)

X-rays 1018Hz 10-11m - X-ray machine, color Radio therapy Very harmful
5×10-9m TV etc. Harmful to eyes
Ultraviolet 1016Hz
5×10-9m - The sun, very hot bod- Burglar alarm,
5×10-7m ies, mercury lamps vitamin D in body,
etc. automatic doors

Visible light 1015Hz 4×10-7m - The sun, electric For lighting No harmful
8×10-7m bulbs, oil lamps, flu- purposes, photog-
orescent substances raphy
8×10-7m - etc.
Infrared 1013Hz 8×10-5m Remote sensing Little harmful
rays The sun, hot bodies, like in a remote to the skin.
10-5m – fire, heaters controller
Microwaves 1011Hz 10-3m No harmful
Hot bodies, micro- For heating
wave ovens etc.

Radio 105Hz 10-3m – Radio transmitter, TV Telecommunica- No harmful
waves 103m transmitter, radar etc. tion

106 Modern Graded Science Class 9

S me Reasonable Facts

1. Red, blue and green colours are called primary colours because any other colour can be
obtained by mixing these colour.

2. A ray of white light is dispersed by a prism into a seven-colour spectrum (VIBGYOR)
because the white light ray consists of seven different colour lights having different
wavelengths. Due to the different wavelengths of these lights, they have different speeds
when passed through a prism.

3. X-rays can penetrate a thick layer as they have a very high frequency.
4. The paper gets burnt when sunlight is focused using a lens on it because the sunlight

consists of infrared radiations which carry heat energy. When they are focused on the
paper with the help of a convex lens, the paper burns.

Things To Know

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

2. The laws of refraction of light are:

i. The incident ray, the refracted ray and the normal all lie at the point of incidence in
the same plane.

ii. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is
constant for a given pair of media. This is also called Snell's law.

3. The ratio of the speed of light in air to the speed of light in a medium is called refractive
index (µ).

4. Depth of a pond appears shallower due to refraction of light.

5. The splitting up of white light into its constituent colours (VIBGYOR) is known as
dispersion of light.

6. The red-coloured light has maximum speed and violet-coloured light has minimum
speed. Due to this, red light is deviated the least and violet light is deviated the most.

7. The band of seven colours which appears on the screen after dispersion of light is spectrum.

8. The rainbow is formed by the dispersion of sunlight by small droplets of water in the
atmosphere.

9. The electromagnetic waves of shorter wavelength are dangerous as they carry
more energy.

Light 107

10. The band of all electromagnetic waves according to increasing wavelengths is called an
electromagnetic spectrum.

11. The members of the electromagnetic spectrum are gamma rays, X-rays, ultraviolet rays,
visible light, infrared rays and radio waves.

12. The wave of the electromagnetic spectrum which produces the sensation of vision in our
eyes is called visible light.

13. Microwaves, ultra high frequency (UHF) radio waves, very high frequency (VHF) radio
waves , short waves (SW) and medium waves (MW) are the kinds of radio waves.

Things To Do

Place a beaker covering its free surface with a paper over a coin as shown in the figure- I.
The coin can be seen through the sides of the beaker. (i) Fill the beaker with water. Is the
coin seen through the sides of the beaker? If not, why?

(i) Drop water over the coin. Is the coin seen through the sides of the beaker? If yes, why?

Test Yourself

1. Multiple Choice Questions (MCQs)

a. The speed of light in a vacuum is ………. .

A. 3×108 km/s B. 2.25 ×108 m/s

C. 3×108 m/s D. 3×108 cm/s

b. Which colour of light has high speed in a medium?

A. blue B. violet C. orange D. red

c. An object becomes hot when absorbs ………. .

A. UV rays B. infrared ray C. X-ray D. gamma ray

d. Which of the following the best refracting transparent medium?

A. glycerine B. water C. glass D. iron

108 Modern Graded Science Class 9

e. Which of the following is refractive index of water?

A. 1.33 B. 1.36 C. 1.47 D. 1.56

f. Which of the following is the critical angle of glass?

A. 50° B. 46° C. 49° D. 42°

g. Which of the following has maximum refractive index?

A. glycerine B. diamond C. paraffin D. water

2. Answer the following questions.
a. State the laws of refraction of light.
b. Why does light bend when it passes through one medium to another?
c. Draw a labeled diagram to show refraction of light through a glass slab.

d. Write any two properties of the electromagnetic spectrum.

e. How will you show the presence of infrared rays in the electromagnetic spectrum?

f. Explain the dispersion of light in a prism in brief with a labeled diagram.

g. Write the merits and demerits of ultraviolet rays.

h. What is visible light? Name the sequence of colours from up to down an a spectrum.

i. Define radio waves. Why are radio waves not harmful to us?

j. Write any two uses of X-rays.

3. Differentiate between.

a. Reflection of light and refraction of light.

b. Refraction of light and total internal reflection of light.

c. Refractive index and critical angle.

d. Radio wave and x-rays.

e. Electromagnetic wave and mechanical wave.
f. Rarer medium and denser medium
g. X-ray and ultraviolet ray

h. Ultraviolet ray and infrared ray.
i. Define

i. refraction of light ii. critical angle

iii. total internal reflection iv. refractive index

Light 109

3. Give reasons.
a. A stick partly immersed in water, appears bent.
b. The incident light deviates from its rectilinear path as it passes from one medium to
another.
c. White light is found to be broken into seven colours after passing through a prism
while no such thing takes place when light passes through a a glass slab.
d. A swimming pool appears shallower than it really is.
e. If a fish in an aquarium is observed from the bottom, it appears in air.
f. Stars twinkle but planets do not.
g. A person hits a spear to a fish as it is seen in water but the fish escapes.
h. In the spectrum of light violet is found at the bottom and red is found at the top.

5. Diagrammatic questions
a. Complete the following ray diagrams:

b. When a narrow beam of white light passes through a prism, the spectrum obtained
is as shown in the figure. Observing the figure, answer the following questions:
i. Write the names of the colours of rays QA
and QG.
ii. Why is the ray QG deviated most?
iii. When a thermometer is placed some
distance above A, the level of mercury in it
rises. Why?

c. A ray of light is travelling from medium 'A' to medium 'B' as
shown in the figure.
i. Which line does the refracted ray represent?
ii. Which medium do you think is denser and why?

110 Modern Graded Science Class 9

d. Sketch a ray diagram to show that the way by which the fish 'A' and Fish 'B' can see
each other eventhough they are at either side of a big stone inside water.

6. Numerical problems

a. A ray of light is incident in air on the surface of a glass block. The angle of incidence
is 30°; calculate the angle of refraction. (R.I. of glass w.r.t. air = 1.5) (Ans: 19.47°)

b. A ray of light in air makes an angle of incidence of 45° at the surface of a sheet of ice.

The ray is refracted within the ice at an angle of 30°. What is the refractive index of

the ice? (Ans: 1.414)

c. The refractive index of a glass is 1.5 and the speed of light in a vacuum is 3 × 108m/s.

Find the speed of light in the glass. (Ans: 2 × 108 m/s)

d. With what angle should a ray strike the surface of water to make an angle of refraction

20° in water? The refractive index of water is 1.33. (Ans: 27.057°)

Diamond : transparent precious stone of pure carbon in crystallized form, the
hardest substance known

Burglar alarm : automatic device that rings an alarm bell when a burglar tries to enter a
building

Tumour : abnormal mass of new tissue growing in or on the part of the body
Periscope : an apparatus used to see over the top of an obstacle e.g. it is also used in

submarines for observing the movements of ships

∆∆∆

Light 111

Chapter Electricity and Magnetism

7

Total estimated Pds: 11 (9T/2P

Competencies
On completion of this chapter, the students will be competent to:

interpret the units ampere, voltage, watt and ohm and use them in measurement.
point out the factors affecting resistance in circuit.
demonstrate Ohm's law and show the relation between volt and ohm.
demonstrate and define magnetic field and magnetic lines of force.
explain the components of terrestrial magnetism.
compare and measure conductivity of some substances.

This is the age of electricity. We cannot surrive easily without electricity in our modern
society. Electricity is used to operate radios, televisions, computers, electric fans, electric
heaters, telephones, etc. Electrical energy can be easily converted into various forms of energy
like heat, light and magnetic energy. In electric power stations, electricity is generated by
converting chemical energy or mechanical energy into electrical energy.

Some physical quantities of electricity

1. Electric Current

An electric current flows in a closed circuit made of a conducting wire, a switch, a source of
current and an electric load. The rate of flow of charge is an electric current which is measured
by using an ammeter and in the unit ampere. 1 A current is that in which 1 coulomb charge
flows per second (1 coulomb charge is equivalent to the charge of 6.242×1018).
1 coulomb
or, 1 A = 1 sec

Ammeters have less resistance and in series connection only the total current of the circuit can
pass through it. Thus, ammeters are connected in series with load.

2. Electromotive force and potential difference

The amount of energy converted from non-electrical energy into electrical energy by the
source (emf) of the circuit is called as electromotive force of the circuit.
The amount of energy converted from electrical to non-electrical by the electric load when
one unit charge flows in it is called potential difference (pd) or voltage (v) of the circuit. 1 V
pd is that in which 1 Joule work is done by the flow of 1 coulomb charge.

or 1 V = 1 1J
coulomb

112 Modern Graded Science Class 9

Emf and pd both are measured by using voltmeter and in unit volt. Voltmeters have
more resistance and are connected in
parallel with loads.
Consider battery, voltmeters, ammeters resistors
and a switch are connected in a circuit as shown
in the diagram. When the curcuit is closed, the
ammeters connected at different places show
the same value but the voltmeters show different
values. Consider, in this situation, V1 is showing
2V, V2 3V and V3 is showing 5V. V1 and V2 are Fig. 7.1 an ammeters and voltmeters in a circuit
showing the p.d of the apparatus, V3 is showing the pd of the battery i.e. the total pd of the
circuit. If the circuit is open except V3­ all the meters will show 0 value. V3 will show the emf of
the source and it will be greater than the pd shown by it before.

3. Electric 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 called 1Ω. Some bigger units than ohm are kiloohm (KΩ) and mega ohm (MΩ).
103Ω = 1kΩ
106Ω or 103 kΩ = 1 MΩ
We know that metals contain a large number of free electrons and thus an 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 (eureka) is an alloy of copper (55%) and nickel
(45%) and is used in the construction of pyrometer and thermocouple. Manganin is an alloy of
copper (83%), manganese (13%) and nickel (4%) that is used in the construction of rheostats
and resistors. Nichrome is an alloy of nickel (60%) and chromium (40%) and is used in the
construction of heating element of heaters (coil).
Insulators do not have free electrons. So, electric current does not flow through them.
Insulators have very high resistance.
Semiconductors contain a few free electrons and so the flow of current through them is small.
However, if their temperature rises, more electrons are ejected from the valance shell and they
become free electrons. So, they are better conductors at a higher temperature. Semiconductors
have higher resistance than conductors but they have lower resistance than insulators.

Electricity and Magnetism 113

Factors affecting resistance

Do the following activities to understand about the factors of resistance.

Activity 7.1
To demonstrate the factors affecting on resistance.

1. Place two nichrome wires of same thickness but different lengths. One of them is 10 cm

and other is of 100 cm long. Connect them one by
one in the gap PQ of the electric circuit, as shown in
the figure. Note the electric current indicated by the

ammeter for each wire. We find that higher current

flows for the wire of length 10 cm than that of the

wire of length 100 cm. We find that the longer wire Fig. 7.2 an incomplete ciruit

has high resistance than the shorter one. Thus, the resistance (R) of a conductor is directly

proportional to the length () of the conductor.
or, R ∝ 

2. Now repeat the activity 1 by using 2 nichrome wires of the same size and thickness. At first,

connect the gaps with a single wire and then by using both the wires. Find the difference.
We find that a thin wire has high resistance than the thicker one (two wires) even though

they have the same length. Thus, the resistance (R) of a conductor is inversely proportional

to the cross-sectional area (A) of the conductor.
or, r ∝ 1/A
3. Now, repeat the activity 1 with copper and nichrome wires of the same thickness and same

length. Note the electric current in the ammeter for each wire. We find that high current
flows in copper wire than in nichrome wire. It shows that different materials have different
resistances even though they have the same length and thickness. Thus, the resistance of a
conductor depends on the material of which it is made up of.
4. Note the electric current in the ammeter when a nicrome wire is connected the gap PQ.
Then, heat the wire with the help of a spirit lamp or burner. We find that the electric current

decreases as the wire becomes hotter and hotter. Thus, the resistance of a conductor is
directly proportional to the temperature of a conducting wire.

From the above activities, the relation of resistance with its factors can be summed up as
the following:
a. Resistance is directly proportional to the length of the conducting wire.

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

c. Resistance of a wire depends on the material of which it is made.

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

114 Modern Graded Science Class 9

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 'ι' is the length of a wire, 'A' is the cross-sectional
area of the wire and 'R' is its resistance, then

R ∝  .............. (i)

R ∝ 1/A .............. (ii) Thomas Edison did not invent the first light
From equation (i) and (ii) we have bulb but he did invent one that stayed lit for
more than a few seconds. Thomas Edison

R ∝ /A invented more than 2,000 new products
including almost everything needed for us to

∴ R =ρ  .............. (i) use electricity in our homes: switches, fuses,
A sockets and meters.

Where ρ (Rho) is the resistivity or specific
resistance of the material. The SI unit of resistivity (ρ) is ohm-metre (Ωm). If  = 1m, A =
1m2, then R = ρ

Thus, the resistivity of a material is the resistance in a conductor of unit length and unit cross-
sectional area.

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

∴ conductivity (σ) = 1 (ρ)
Resistivity

The SI unit of conductivity (σ) is Ω-1 m–1 (Per ohm per metre or mho). It is also called siemens (S).

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 AD.
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 condition [Temperature, cross-sectional area, length, nature of material etc.]

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 conductor, Electricity can be made from wind, water,
which is used as constant. the sun and even animal poop.

Electricity and Magnetism 115

From equation (i), we get
R = V/I
Here, R = Resistance, V = Potential difference and I = Electric 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 flowing through it.

Activity 7.2
Experimental verification of Ohm's law

The experimental set up for the verification of Ohm's law is as Fig. 7.3 verfication of ohm's law
shown in the figure. A voltmeter V is connected across a Fig. 7.4 result of the experiment
nichrome wire PQ. The wire PQ is then connected with an
ammeter A, a key K and cells. The ammeter measures the
current (I) through the circuit and the voltmeter measures the
p.d. (V) across the wire PQ. At first, take only one cell (join M
and N). When the key is closed, electric current flows in the
circuit. Record the readings of the ammeter and the voltmeter.
Repeat the experiment with two cells by joining M and O. Now,
join M and R and record the readings of ammeter and voltmeter.
If a graph is plotted between p.d. (V) and current (I), we get a
straight line through its origin as shown in the figure. This
shows that the current is directly proportional to the p.d. which
verifies Ohm's law.

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

Here, Resistance (R) = 4 Ω
Potential difference (V) = 8V
Current (I) = ?
From Ohm's law, we have
V = IR
8=I×4

116 Modern Graded Science Class 9

∴ I = 48 = 2A
Hence, the current in the circuit is 2A.

Example 2: What is the length of a manganin wire of diameter 0.44 mm required to construct
a 10Ω coil? Resistivity of manganin is 4.43×10-7Ωm.

Solution:

Here,

Diameter (d) = 0.44 mm = 4.4 × 10-4 m (∴1mm = 10–3 m)

Resistance (R) = 10 Ω

Resistivity (ρ) = 4.43 × 10-7 Ωm

Length () = ?

We know,

R = ρ A

Hence, the length of the manganin wire is 3.44 m.

Magnetic field

Magnetic substances are attracted strongly by a magnet near to it. If a magnet is kept gradually
away from such substances, magnetic force also decreases. Beyond a certain region, the magnet
has no influence on magnetic substances. The space around a magnet where magnetic force
affects a magnetic body is called the magnetic field of the magnet. The imaginary lines mapped
around a magnet are called magnetic lines of force. Any tangent drawn at the magnetic lines
of force gives the direction of the magnetic field at that point. The pattern of these lines tells
us about the characteristic of force caused by the magnet. These lines start from the north
pole and end to the south pole of the magnet. It is the path in which an imaginary free n-pole
travels if it is free to do so. However, a free north pole does not exist.

Electricity and Magnetism 117

Activity 7.3
To plot the lines of force.

To draw the magnetic lines of force, a bar magnet is kept in N-S directions on a sheet of paper
over a drawing board. A small magnetic compass is placed near the north pole of the magnet.
When the compass needle
has settled, the ends N and
S of the needle are marked
with a sharp pencil as dot A
and dot B. The compass is
now replaced so that the
S-pole points the dot B.
Now, put a dot C against the
N-pole. This process is
continued until the compass reaches the south pole of the magnet. A line connecting series of
dots obtained thus gives the magnetic line of force. An arrow is used on the line to indicate the
direction of magnetic lines of force from north pole of the magnet to south. Other lines are
plotted in the same way to obtain the entire magnetic field of the magnet. Any magnetic field
consists of infinite number of magnetic lines of force.

Properties of magnetic lines of force

Some important properties of the magnetic lines of force are as follows:

1. Magnetic lines of force start from the North pole and reach the South pole of a magnet
externally and from the South pole to the North pole internally. Thus, the lines of force
are in the form of a continuous curve.

2. Magnetic lines of force do not intersect.

3. Magnetic lines of force are crowded at the poles as the field is stronger and they are
widely separated in the middle where the field is weaker.

Neutral point

If the lines of force are drawn by pointing the N pole of a bar magnet's towards geographical
north, we get the magnetic field pattern as shown in the figure 7.6 'a'.

In the figure, it is seen that the horizontal component of the earth's magnetic field and the
magnetic field due to the magnet are in opposite directions. The magnetic field due to the
earth remains constant while the magnetic field due to the bar magnet is strong near the
poles and it weakens as the distance from the magnet increases. At the points indicated by
X, the compass needle will not point to any fixed direction. So, at these points, the horizontal

118 Modern Graded Science Class 9

component of the earth's magnetic field and that of the magnet are exactly equal and opposite.
These points are neutral points. Horizontal component is defined as the earth's magnetic field
along the horizontal directions.

Similarly, if a bar magnet is placed pointing to its S-pole towards the earth's geographical
N-pole, we get the magnetic lines of force as shown in figure b. In this case, the neutral points
lie towards the end of the magnet.
Thus, the neutral points in a magnetic field are the points at which the horizontal component
of the earth's magnetic field and that due to a magnet are exactly equal and opposite. No lines
of force will pass through the neutral point and the compass needle will not point to any fixed
direction at these points.

Terrestrial magnetism

The earth behaves as a huge magnet. The earth's magnetism is called terrestrial magnetism. It
was discovered by a British scientist named William Gilbert in 1600 AD. Declination, dip and
horizontal component of the earth's magnetic field are the elements of terrestrial magnetism.
They help to locate the correct direction. For example, the declination is helpful in steering
ships in the right direction with the help of a mariner's compass.

Evidence of terrestrial magnetism

1. A freely suspended magnet always points towards N-S direction by the influence of
terrestrial magnetism on it.

2. Some iron ores possess magnetism which may be due to the magnetic effect of the
earth.

3. The angle of declination and the angle of dip are due to the magnetic effect of the earth.
4. Neutral points are obtained on the magnetic field of a magnet which is due to the

combined effects of its magnetic field and the earth's magnetism.
5. If an iron rod is buried in the ground in the direction as indicated by a compass needle,

it gets magnetized after a long period.
Electricity and Magnetism 119

Earth's magnetic field

A freely suspended bar magnet always comes to
rest pointing towards the north-south direction.
This shows that the earth behaves as a huge
magnet. The N-pole of a compass needle points
towards geographical N and S poles and the
compass needle points towards geographical
south. It is because of the presence of the
magnetic poles of the earth at these places. It
proves that the earth also acts as a huge magnet
or the earth has a magnetic field around it.

The magnetic poles of the earth do not remain at the same place always but they wander.
Currently the North magnetic pole is close to Ellet Ringnes Island in the Queen Elizabeth
Island near the boundary of Canada. The south magnetic pole lies just off the coast of Wilkes
Land, Antarctica. The magnetic poles are about 1600 km away from the geographical poles. It
is very surprising that the magnetic poles reverse too. In average after 2,00,000 years the North
pole of the earth's magnet changes into the south pole and vice versa.

About the couse of earth's magnetism many theories are suggestion is dynamo theory. The
theory describe that the flow of liquid iron in the outer core of the earth sets up electric
currents which produce the earth's magnetic field. It is the most convincing theory.

Magnetic elements of the earth's magnetic fields

The quantities that are required to specify the magnetic field of the earth completely are
called the magnetic elements of the earth. They are the angle of declination, angle of dip and
horizontal component of the earth's magnetic field.

a. Angle of declination

The angle between the magnetic meridian and the
geographical meridian at a place is called the angle of
declination at that place. Its value is different at different
places on the surface of the earth. The knowledge of
declination is essential while flying planes and sailing
ships. The value of the angle of declination at the equator
is 17°. There is no perfect instrument to measure it, but
magnetic compass is used for this purpose.

120 Modern Graded Science Class 9

b. Angle of dip or inclination

The angle of dip is the angle made by a freely suspended magnet at rest with the horizon. This
angle is also different at different places on the surface of the earth. On the magnetic equator,
the angle of dip is 0°. This is because a freely suspended magnet rests horizontally at it. The
freely suspended magnet rests vertically at the
magnetic poles. Thus, the angle of dip is 90° at the
magnetic poles. At other places, its value lies between
0° and 90°. A dip circle/dip-needle is used to
determine the angle of dip at a place.

We should note that the angle of declination at any
place is not constant but it changes slowly as the years
go by. The change in the angle of declination at any
place is very small; hence the angle of declination
can be taken roughly as a constant at a place. In the
similar way, there is very small change in the angle of dip at a place. Approximately, the angle
of dip at a place can be taken as a constant.

c. Horizontal component

As the earth has magnetic property, it has magnetic field around the horizontal directions
which is called the horizontal component of the earth.

S me Reasonable Facts

1. Thick wires of same metals have less electric resistance than that of thin wires. It is because
electric resistance is inversely proportional to the cross sectional area of a conducting
wire. As thick wires have more cross sectional area, they have less resistance and thin wires
have less cross sectional area so they have more resistance.

2. A hot conducting wire has more resistance than cold one. It is because resistance is directly
proportional to the temperature of a conducting wire. It means that, at high temperature
electric resistance increases.

3. The angle of dip of Kathmandu is 42°. It means a freely suspended magnet in Kathmandu
makes an angle of 42° with the horizon.

4. A freely suspended bar magnet on the earth's surface always comes to rest pointing to the
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 the South pole of the earth's magnet and the S-pole of the magnet points towards
the North pole of the earth's magnet.

Electricity and Magnetism 121

5. The 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.

6. The value of the angle of dip is not zero at geographical poles; it is because the angle of dip
is caused by the magnetic poles of the earth not by the geographical poles. Magnetic poles
and geographical poles are about 1600 km distant. Thus, the value of the angle of dip is
0oat the magnetic poles, not at geographical poles.

7. Neutral points are found in magnetic field of a magnet, it is because of the equal magnetic
force of the similar poles of the magnet and the earth's magnet but in opposite directions.

Things To Know

1. Resistance is the property of a conductor by the virtue of which it opposes the flow of current.

2. The resistance of a conductor is (i) directly proportional to its length and (ii) inversely
proportional to its cross-sectional area.

3. The resistance of a conductor increases with increase in temperature.
4. The electric current passing through a conductor is directly proportional to the potential

difference across its ends at constant physical conditions. This is known as Ohm's law.
5. The space around a magnet, where magnetic force affects magnetic bodies, is called

magnetic field.
6. A line of force in a magnetic field is a line, straight or curved, tangent to which at any point

gives the direction of the magnetic field at that point.
7. A neutral point in a magnetic field is a point at which the horizontal component of the

earth's magnetic field and the magnetic field due to a magnet are exactly equal and opposite.
8. Magnetic property of the earth is called its terrestrial magnetic field.
9. The angle between the magnetic meridian and the geographic meridian at a place is called

the angle of declination at the place.
10. The angle of dip is the angle made by a freely suspended magnet with the horizon.

Things To Do

Make a model to demorstrat terrestrial magnetism.
1. Take a bar magnet and put in the dough of soil as shown in

the and dry it (as the model of the earth).

2. Make the equator and the meridian in the soil as shown in
the figure.

3. Study about the terrestrial magnetism and magnetic
elements of the earth's magnetic field using a magnetic compass and the earth's model.

122 Modern Graded Science Class 9

Test Yourself

1. Multiple choice questions (MCQs)

a. What is the rate of flow of charge?

A. voltage B. electric current C. resistance D. conductivity

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

A. 12° B. 15° C. 16° D. 17°

c. Which of the following factor does not affect resistance?

A. thickness of wire B. type of wire

C. pressure of wire D. temperature of wire

d. Which formula gives the value of conductivity?

A. ρ = RA/ B. ρ = R/A C. ρ = A/R D. ρ = /RA

e. Which formula can be used to calculate electric resistance?

A. R = VI B. R = V/I C. R = I/V D. R = VI/2

f. Where on the earth surface is the value of the angle of dip 0°?

A. geographical N B. N pole of the earth's magnet

C. geographical S D. equator

g. What is the angle formed between geo. N and mag. N called?

A. magnetic meridian B. geographical meridian

C. angle of declination D. angle of dip

h. Which of the following is given by 1/ρ

A. resistivity B. conductivity C. electric current D. resistance

i. If the lines of force are plotted by inclining the north pole of a bar magnet towards

the north, where will the neutral points lie ?

A. sides of the magnet B. ends of the magnet

C. centre of the magnet D. a and b both

j. If the length of a conducting wire increases, what will be the effect on the resistance

of the wire?

A. decrease B. no change C. increase D. both A and C

2. Answer the following questions:
a. Define resistance. What is meant by 1 Ω resistance?

b. Give the relation of resistance of wire with its length and thickness.

c. How does resistance of a conductor vary with temperature?

d. A uniform resistance wire is stretched till its length becomes four times. What
happens to the resistance?
Electricity and Magnetism 123

e. Define resistivity and conductivity.

f. Give any four evidences about the earth's magnetism.

g. Define the following:

i. magnetic field ii. angle of dip

iii. angle of declination iv. neutral point

v. magnetic lines of force vi. electric current

vii. electromotive force viii. potential difference

h. Is the value of the angle of dip 90° at the geographical North pole? Give reason.

i. Give any three properties of the magnetic lines of force.

3. Differentiate between: b. conductivity and resistivity
a. angle of dip and angle of declination d. electric current and voltage
c. ammeter and voltmeter f. electric current and resistance
e. emf and pd

4. Give reasons.
a. A freely suspended magnet shows N-S poles at rest.
b. The value of the angle of dip at the magnetic equator of the earth is zero.
c. A freely suspended magnet does not point exactly at the geographical N-S directions.
d. The angle of dip is 90° at the magnetic poles.
e. A compass needle does not point towards any fixed direction at neutral points.
f. Ammeters are connected in series.
g. Voltmeters are connected in parallel.

5. Diagrammatic questions
a. Study the diagram and answer the following questions:

i. What mistake is made in the diagram?

ii. Identify the North and South poles of the magnet.

iii. The magnetic lines of force are closer at poles. Why?

b. Answer the following questions on the basis of the given diagram.

i. Label A, C, D and H.

ii. What is the value of I?

iii. A freely suspended magnet inclines towards HF or AG. Why?

c. What angle is shown by a dip needle at H? Which end of the
compass inclines towards H?

124 Modern Graded Science Class 9

6. Numerical problems
a. What is the resistance of a material that draws 200 mA of current at 20 V?
(Ans: 100 Ω)

b. Which one of them has greater resistance: a 1 kW heater or a 100 W tungsten bulb
marked 230 V? [Hint : P = V × I, P = RI2, P = Rv2]
(Ans: 100W tungsten bulb)

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

filament and its resistance? [Use P = V × I] (Ans: 0.43 A, 529Ω)

d. Calculate the resistance of a bulb rated 60 W, 240 V. (Ans: 960 Ω)

e. An electrical appliance is marked 220 V, 550 W. What is the resistance when in use?
(Ans: 88Ω)

f. What is the length of a metallic wire of diameter 0.5 mm required to connect a 12 Ω

coil. Resistivity of the metal is 4.46 × 10–7 Ωm. (Ans: 5.82 m)

g. Resistivity of manganin is 4.43 × 10–7 Ωm. Calculate its conductivity.

(Ans: 2.3×106Ω-1m-1)

Free electron : any electron that is not attached to an ion, atom or molecule
Rheostat and is free to move under the influence of electric or magnetic
field

: variable resistor

Pyrometer : a thermometer designed to measure very high temperature

Thermocouple : a transducer consisting of two different metals welded together at
each end

Geographic meridian : the imaginary line connecting geographical north pole and south
pole

Magnetic meridian : the imaginary line connecting the magnetic North pole and South
pole

∆∆∆

Electricity and Magnetism 125

Chapter Classification of Elements

8

Total estimated Pds: 9 (8T/1P

Competencies
On completion of this lesson, the students will be comptent to:

define valency as the combining capacity of elements.
explain the structure of atom and its electronic configuration
describe radicals, ions and their types.
write the molecular formulae of some simple compounds.
describe the types of chemical bonds in brief.

The universe is composed of matter and energy. These matter and energy are inter-convertible
to each other. In nature, all the things are made up of matter. Matter is anything that has mass,
density, volume and weight. Book, wood, ice, etc. are the objects which are made up of matter.
The branch of science that deals with the study of composition and properties of matter is
called chemistry. A brief outline of the division of matter is given below.

Properties of matter: Mass,
Density, Volume and Weight.
It can be remembered using
mnemonic as: Monkeys Dance
Very Well.

Fig. 8.1 energy and matter

Pure substances

A pure substance is composed of similar kind of its unit particles. For example, copper
is a pure substance because it is composed of the smallest particles of copper, similar
in nature and properties. Pure substances can be classified into two classes. They are
elements and compounds.

126 Modern Graded Science Class 9

Element

An element is the simplest pure substance which can neither be split nor built up from other
simpler substances by any chemical reaction. Robert Boyle (1961 A.D.) was the first scientist
who used the term element. All the substances are made up of elements. Thus, elements are
said to be the basic units and building blocks of all the complex substances.

Till today, 118 elements have been discovered. Out of them, 92 elements have been found in

nature and the rest 26 elements have been synthesized

in the laboratory with the help of nuclear reaction.

Gold, silver, carbon, oxygen, helium, etc. are the On the basis of the modern concept,

elements that we find in our surroundings. matter exists in five states. They are

There are similar kinds of extremely small particles solid, liquid, gas, plasma and Bose-
called atoms in each element. The atoms of one element Einstein Condensate (BEC). Plasma
is a state of matter in which an
are exactly alike and the atoms of different elements ionised gaseous substance becomes
are different. For example, all the atoms of iron are highly electrically conductive. BEC

identical but they are different from the atoms of gold. is a state of matter of a dilute gas of

Thus, an element is a pure substance that contains only low densities called boscms cooled to
temperatures very close to absolute
one kind of atoms.
zero. (oK or - 273oC)
On the basis of properties, elements are classified into

metals, non-metals and metalloids.

1. Metals: Those elements which can conduct heat and electricity are called metals. They are
malleable, ductile and have metallic lustre. Copper (Cu), Gold (Au), Silver (Ag) Iron (Fe),
etc. are examples of metals.

2. Nonmetals: Those elements which show the properties of both metals and non-metals
and are bad conductors of heat and electricity and have no metallic lustre are called non-
metals. Examples include hydrogen (H), nitrogen (N), carbon (C), oxygen (O), etc.

3. Metalloids: Those elements which show common characters of metals and nunmetals
are called metalloids. They are also called semi-conductors. Examples include Boron (B),
Silicon (Si), Arsenic (As), etc.

Compound

A substance made from the chemical reaction of two or more atoms of elements in a fixed
proportion by weight is called a compound. For example, water is formed by the two elements
namely hydrogen and oxygen in the ratio of their mass 1:8 whatever the source of water.

Classification of Elements 127

Similarly, when we burn firewood, a gas is produced. This gas is called carbon dioxide. Some
other examples of compounds are sodium chloride (sodium and chlorine) and calcium
carbonate (carbon, calcium and oxygen). Carbon dioxide gas contains the atoms of carbon
and oxygen. From these examples we can say that the compounds are formed from their
constituent elements due to chemical change. So, properties of compound are different from
its constituent elements. The smallest part of a compound is called a molecule. A molecule
contains all the characters of the compound.

Molecule

The smallest particle of an element or compound which exists independently is called a
molecule. The molecule of an element contains the atoms of the same elements. Molecules
of the some elements are made up of only one atom and the molecules of some elements
are made up of two or more atoms. The number of atoms from which one molecule of the
element is made is known as atomicity of the element. The atomicity of elements is of two
types: monoatomic and diatomic elements. The molecule having only one atom is called
monoatomic molecule. For example, helium (He), neon (Ne), Argon (Ar), etc. The molecule
having two atoms of the same elements is called diatomic element. There are seven diatomic
elements: hydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2), chlorine (Cl2), bromine
(Br2) and iodine (I2).

The molecule of compounds is made up of the atoms of two or more different types of ements.
Such molecule is called polyatomic molcule. Sodium chloride (NaCl), ammonia (NH3), water
(H2O), calcium carbonate (CaCO3) etc. are examples of the molecules of compounds.

Symbols of elements

We have learnt that symbols are used in different fields. Likewise, in chemistry, chemists use
different symbols for the name of elements as well as radicals.

A symbol is defined as the abbreviation of the full name of an element. Symbols of elements
are used to make their study easy, fast and practicable. Using the English letters, the symbols
of elements are expressed. A symbol of an element is made up of either one or two letters.
If the symbol of an element has two letters, the first letter must be written in the capital and
the second letter in the small form. The symbols of some common elements, their atomic
numbers and atomic mass are given below:

128 Modern Graded Science Class 9

S.N. Elements Symbol Atomic Atomic S.N. Elements Symbol Atomic Atomic
1 Hydrogen number mass number mass

H 1 1 22 Manganese Mn 25 55

2 Helium He 2 4 23 Iron Fe 26 56
3 Lithium Li 3
4 Beryllium Be 4 [Ferrum (Latin)]
5 Boron B5 7

25 Nickel Ni 28 59
9

26 Copper Cu 29 64
11 [Cuprum (Latin)]

6 Carbon C 6 12 27 Zinc Zn 30 65

7 Nitrogen N 7 14 28 Bromine Br 35 80

8 Oxygen O 8 16 29 Krypton Kr 36 84

9 Fluorine F 9 19 30 Silver Ag 47 108
10 Neon
Ne 10 20 [Argentum (Latin)

11 Sodium Na 11 23 31 Tin Sn 50 119

[Natrium (Latin)] [Stannium (Latin)]

12 Magnesium Mg 12 24 32 Iodine I 53 127

13 Aluminium Al 13 27 33 Barium Ba 56 137

14 Silicon Si 14 28 34 Tungsten W 74 184
15 Phosphorus P
16 Sulphur S [Wolfram (German)]

15 31 Au 79 197
35 Gold

16 32 [Aurum (Latin)]

17 Chlorine Cl 17 35 36 Mercury Hg 80 201
18 Argon
Ar 18 40 [Hydrargyrum(Latin)]

19 Potassium K 19 39 37 Lead Pb 82 207

[Kalium (Latin)] [Plumbum (Latin)]

20 Calcium Ca 20 40 38 Radium Ra 88 226

21 Chromium Cr 24 52 39 Uranium U 92 238

Atom

The smallest particle of an element which takes part in a chemical reaction without its
division is called an atom. All the atoms of an element are the same. The atoms of different
elements are different. All the atoms of iron in an iron nail are the same. But the atoms of
iron are different from the atoms of copper. All the atoms of elements except hydrogen are
made up of electron, proton and neutron. A hydrogen atom is made up of only electron and
proton. Most of the atoms of elements do not exit independently except the atoms of inert
gases like helium, neon, krypton, etc.

Classification of Elements 129

Structure of the atom

According to scientists, an atom is a sphere

with a diameter of approximately 10-10 m. In the

19th century, it was found that there are other

smaller particles in an atom. These smaller

particles are called subatomic or elementary

or fundamental particles. The main subatomic

particles are electrons, protons and neutrons.

Among three subatomic particles of an atom,

protons and neutrons are located at the centre

of the atom called nucleus. The nucleus shows

the positive charge due to the positive charge Fig. 8.2 berylium atom

of protons. It also helps to find atomic weights. Electrons are revolving around the nucleus

in their respective orbits or shells.

The number of electrons present

in an atom determines the size of Phosphorus is an element. When it comes in contact

the atom as it occupies more space with air, it burns itself. Sometimes, in our villages

as compared to the nucleus. In any we hear people claiming to have seen a ghost burning
atom, the number of negatively a torch, which is called a fire ghost (rakebhoot). The
charged electrons in the orbits is burning is caused by the phosphorus of the bones
released from dead animals which changes into solid
equal to the number of positively phosphorus due to different chemical reactions. The
charged protons in the nucleus. solid phosphorus comes in contact with oxygen to
Thus, an atom is electrically neutral. burn.

[Note: 1 g= 6 × 1023 amu | 1 amu = 6×10-23 g

1 Coulomb = 6.25 × 1018 esu = charge of 6.25 × 1018 electrons]

Summary of subatomic particles of an atom

S.N. Name of the sub- Location Relative mass Charge
atomic particles [in atomic mass unit [in electrostatic
Shell
1. Electron (e-) Nucleus (amu)] unit (esu)]
2. Proton (p+) Nucleus 1/1837 amu - 1 esu
3. Neutron (no)
1 amu + 1 esu

1 amu 0 esu

130 Modern Graded Science Class 9

Shells, sub-shells and orbitals

a. Shells

The paths traced by electrons around the nucleus are called orbits or shells. The shells are
called energy levels because an electron belonging to any of these shells has a definite quantity
of energy associated with it. The energy levels of electrons are represented by the numbers 1,
2, 3, 4, 5, 6 and 7 (also called principal energy numbers or Bohr's Quantum number) whereas
the shells by K, L, M, N, O, P and Q. Now, the 1st energy level is called K-shell, 2nd energy
level is L-shell and so on. The K-shell has the minimum energy and lies close to the nucleus.
The L-shell has little more energy and it is a little bit far from the nucleus. The systematic
distribution of electrons in different shells is electronic configuration of an atom is called
electronic configuration of an atom.

The maximum number of electrons Fig. 8.3
that a shell can hold is given by the
formula 2n2, where n is the number
of shells. The law is applicable up to
N shell only. O, P and Q shells may
have 32 electrons but the last shell
of an atom cannot have more than
8 electrons. Similarly, the second
last shell cannot have more than
18 electrons. It represents the sub-
energy level occupied by the electrons.

For first or K-shell, n = 1, 2n2 = 2 × 12 = 2
For second or L-shell, n = 2, 2n2 = 2 × 22 = 8
For third or M-shell, n = 3, 2n2 = 2 × 32 = 18
For forth or N-shell, n = 4, 2 n2 = 2 × 42 = 32

b. Subshells

Each shell consists of sub-shells. A subshell is the pathway in which an electron moves within

a shell. In other word, a subshell represents the sub-energy level Shells Subshells
accupied by the electrons. They are designated by s, p, d and f
i.e. derived from the first letter of the words sharp, principal, K (n=1) 1s
L (n=2) 2s and 2p

diffused and fundamental lines of the spectra. An obital is M (n=3) 3s, 3p and 3d

defined as a region of space in which an electron can be found. N (n=4) 4s, 4p, 4d and 4f

Classification of Elements 131

c. Orbitals

The three dimensional space or the region around the
nucleus, where the probability of finding an electron is sub-shell orbital maximum num-
ber of electrons
maximum (∼ 95%) is called orbital. A subshell is made s 1
2
up of orbitals. The orbital can have different sizes and p 3
6
shapes. The maximum number of electrons present in d 5
10
an orbital is two. Both electrons should have opposite f 7
14

spin. The orbital is represented by magnetic quantatum

number and it indicates the orientation which a subshell takes place.

1. s - orbital : It is spherical in shape and represented by a circle. The size Fig. 8.4 s - orbital
of s-orbital increases with the increase in the principal energy number.
Due to its spherical shape, the chance of finding electrons is the same in
all directions.

2. p - orbital : It is in the shape of a dump-bell with two lobes on opposite Fig. 8.5 p - orbital
sides of the nucleus. In this orbital, the nucleus remains at the centre
between two lobes. The probability of finding the electrons is the same in
both the lobes but not the same at the nucleus. 'p' subshell contains three
orbitals. p-orbitals are denoted by px, py and pz.

3. d and f orbitals : Both d and f orbitals have more complicated shapes
or double dumb-bell shaped structures. There are five d-orbitals and
7f-orbitals.

Electronic configuration and its symbolic notation Fig. 8.6 d - orbital

The systematic representation for the distribution of electrons in different energy levels of an
atom or orbitals of an atom is called electronic configuration of any orbital. The electronic
configuration of an element is shown on the right side.

prinicipal shell
(Shell = 1, 2, 3, .............)

132 Modern Graded Science Class 9

For instance, helium (He)

The electronic configuration of helium is 1s2. It has only one shell, called K-shell.

K shell contains one s-sub-shell and s-sub-shell has one s-orbital. s-orbital can be
contained 2 electrons.
Electrons to s-orbital

Principal shell 1(K)

Orbital (s)
The stable electron configuration of any atom can be obtained by using certain rules and
building up principles. One of such principles is Aufbau principle.

Aufbau principle [Aufbau is a German word meaning build up]

The Afubau principle was proposed by Wolfgang Pauli and Neil
Bohr in the early 1920s. This principle explains a sequence of filing
of various orbitals by electrons. Aufbau principle states that "the
electrons occupy the orbitals of minimum energy first and then they
occupy the orbitals of maximum energy." This means, the electron
enters the orbital having the lowest energy first. The orbital will be
filled by the electrons in the following sequence.

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
→orbital being filled→

This sequence of filling electrons in various sub-shells can be shown
in the given diagram.

Electronic configuration of potassium Fig. 8.7

The potassium element (K) has 19 electrons in its various shells. Its electronic configuration
is as follows.

Name of shell KL M N

Subshell 1s 2s, 2p 3s, 3p, 3d 4s, 4p, 4d, 4f

Number of electrons 28 81

Electronic configurations 1s2 2s2, 2p6 3s2, 3p6 4s2
(Orbital)

The electronic configuration of potassium is 1s2, 2s2, 2p6, 3s2, 3p6, 4s1. According to the Aufbau
principle, the last one electron occupy the 4s orbital of N-shell instead of 3d orbital of M-shell
because 4s orbital has lower energy than the third (3rd) and hence is filled up first.

Classification of Elements 133

134 Modern Graded Science Class 9

Valance shell and valance electrons

An atom consists of various shells in Elements Atom- Electronic Valence Valency Remark
which different numbers of electrons are ic No. configuration electrons (V)
located. The outermost shell of an atom
KL M (Ve–)

is called a valance shell. The electrons Sodium 11 28 1 1 1 V = Ve–
Magne- 12 28 2 2 2 V = Ve–
present in the valance shell of one atom of sium

an element are called valance electrons. Alumin- 13 28 3 3 3 V = Ve–
ium 28 4 4 4 V = Ve–
The electrons which lie inner to the Silicon 14

valance shell are called core electrons. Phospho- 15 28 5 5 3 V=8
The part of the atom that lies inner to the rus 16 28 6 6 -Ve–
valance shell is called core of the atom. Sulphur 17 28 7 7
The core of the atom consists of core 2 V=8
Chlorine -Ve–

1 V=8
-Ve–

electrons, protons and neutrons. Argon 18 2 8 8 8 0 V=8
-Ve–

The valance electrons possess more [Remember : If ve– is 1 to 4, V = Ve– (except helium) and if
energy than the core electrons so they Ve– is 5 or more V = 8-Ve–.]
take part in a chemical reaction. The
valance electrons are those that determine how an atom will react with other atom in a
chemical reaction. In general, valence electrons of an atom are either transformed to the
valence shell of another atom to from a molecule during a chemical reaction. The electronic
configuration and valence electrons of some elements are given in the table above.

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

Shells KL MN

No. of electrons 2 8 81

Core electrons Fig. 8.8 atomic structure of potassium

eVlaecletnrocne

Electronic configuration: 1s2, 2s2, 2p6, 3s2, 3p6, 4s1

Core Valence
Shell

Information from valence electrons

1. Valence electrons of an atom give 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
table in which the representative element falls. For example, the number of valence electrons
of sodium is 1. Thus, sodium element belongs to the Group I A of the periodic table.

Classification of Elements 135

Valency

Valancy is the measure of the combining capacity of elements to form compounds. The
valency of an element depends on the number of valence electrons in its atom because valence
electrons take part in a chemical reaction. The valency of an element is either equal to the
number of valence electrons in its atom or equal to the number of electrons required to
complete 8 electrons in the valence shell.
The combining capacity of an element or the radical with the other element or radical to form
a molecule or a compound is called valency. Numbers such as 1, 2, 3, 4, 5 and 6 represent the
valency. The inert gases like helium (He), neon (Ne), argon (Ar), krypton (Kr) have valency
zero so they cannot take part in chemical combination. But other elements take part in
chemical combination and form a compound due to their valency.

Traditionally, the valency of an element was defined as the total number of hydrogen atoms
with which one atom of an element combines during a chemical combination. The following
are the common examples:
i. By using number of hydrogen atom present in a molecule

1. The valency of chlorine (Cl) in hydrochloric acid (HCl) is 1 because 1 atom of Cl combines with
1 atom of H.

2. The valency of oxygen (O) in water (H2O) is 2 because 1 atom of O combines with 2 atoms of H.
3. The valency of nitrogen (N) in ammonia (NH3) is 3 because 1 atom of N combines with

3 atoms of H.
4. The valency of carbon (C) in methane (CH4) is 4 because 1 atom of C combines with

4 atoms of H.
5. The valence of nitrate (NO3) in nitric acid (HNO3) is 1 because 1 nitrate radical to combines with

1 atom of hydrogen (H).
Many compounds do not have hydrogen atoms. So, this concept has been modified and a
new concept is given to define the valency. According to modern concept, the total number of
electrons lost, gained or contributed for sharing by an atom during a chemical combination
is valency. For example, a sodium atom loses an electron and chlorine gains one electron to
form a compound sodium chloride. So, the valency of sodium and chlorine is 1. On the other
hand, in chlorine (Cl2) molecule, a pair of electrons is shared between two chlorine atoms. So,
the valency of chlorine is one.

Ways of finding out valency

1. The valency of an element is equal to the number of valence electrons in the valence
shell if that number is four or less. Otherwise, the valency is equal to eight minus
the number of valence electron. For example, sodium atom has one electron in its
valence shell. So its valencey in one. Chlorine atom has seven electrons in its valence
shell. So its valency is 8-valence electrons (7) one.

136 Modern Graded Science Class 9

2. According to modern periodic table, the valency of an element is equal to the group
number of the first four group 1, 2, Group 1 (I)
3 and 4. 2 (II) 3 (III) 4 (IV) 5 (V)
3. The valency of elements belonging Valency 1 23 4 5

to group 5th, 6th, 7th, and 8th is 3, 2, 1, and 0 Group 5 (V) 6 (VI) 7 (VII) 8 (VIII)

respectively. Valency 3 2 1 0

4. The valency of radical is equal to the
number of cations or ions present in it. For example, valency of C1- = 1, valency of
Al+++ is 3.
5. The transition elements have two in- 1. Copper → Cuprous Cu+ = 1, Cupric Cu++ = 2
complete outer shells. So, the elec-
trons of these two shells take part in 2. Iron → Ferrous Fe++ = 2, Ferric Fe+++ = 3
Gold → Aurous Au+ = 1, Auric Au+++ = 3
chemical combination which deter- 3. Tin → Stannous Sn++ = 2, Stannic Sn+++=3
mines their variable valencies. 4.

Variable valency

Some elements have more than one valency. An element may show more than one valency.

For example, iron combines with chloride during a chemical reaction to from FeCl2 and FeCl3.
Thus, the valencies of iron in the
Elements Valencies Elements Valencies
two iron compounds (FeCl2 and Nitrogen (N) 3, 5 Cupper (Cu) 1, 2
FeCl3) are 2 and 3 respectively. Phosphorus (P) 3, 5 Lead (Pb) 2, 4

Similarly, the sulphur has more than Manganese (Mn) 2, 4 Tin (Sn) 2, 4
2 valencies: 2, 4 and 6 while forming Iron (Fe)
2, 3 Gold (Au) 1, 3
the compounds of H2S, SO2 and SO3. Silver (Ag) 1, 3 Mercury (Hg) 1, 2
Usually, the name of elements with

the lower valency ends with suffix–ous and that with the higher valency ends with suffix–ic.

Now, the new system of keeping the variable valency of metal in Roman numeric enclosed in

brackets. Example, ferrous chloride (inFenCelw2) and ferric chloride (FeCl3) are represented by iron
(II) chloride and iron (III) chloride naming system.

Radicals

Radical is defined as an atom or a
group of atoms having one or unpaired
electron. But this atom or group of atoms
does not carry a charge like an iron. It The valency of an atom or a radical is always a pure
is because the number of electrons still whole number without any +ve or –ve sign. Example:

matches the number of protons in the the valency of NH4+ radical is 1 not + 1. Similarly,
nucleus, however it is active. the valency of CO3––radical is 2 not –2.

If a radical consists of a single atom, it is called a simple radical. For example, O, H, N, Na,

Classification of Elements 137

K, Ca, etc. The radicals consisting of two or more atoms are called compound radicals. For
example. O2 NH4, NO2, NO3, CO3, HCO3, etc.

Radicals are classified into basic or electropositive radicals and acidic or electronegative
radicals. Electropositive radicals are those which lose electrons during formation of a molecule.
Examples are NH4+, Na+, Mg++, Ca++, Zn++, K+, etc. Electro negative radicals are those which
gain electrons during formation of a molecule. For example, CO3- -, HCO3-, O- -, O2- -, N- - -,
Cl-, NO3-, etc. Thus, radicals are a group of atoms of the same or diffrent elements combined
chemically which acts as a single unit with a positive or negative charge. Radicals are highly
reactive and unstable chemical species. So, they cannot be seen but we can see their effects only.
ymbols and valencies of electropositive radicals

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

Symbols and valencies of some electronegative radicals

Monovalent Valency = 1 Bivalent Valency = 2 Trivalent Valency = 3
Chloride Cl– Sulphide S – –
Nitride N – – –
Bromide Br– Sulphite (SO3) – –
Phosphate (PO4)– – –
Iodide I – Sulphate (SO4)– – Borate (BO3) – – –
Phosphite (PO3)– – –
Fluoride F – Oxide O– – Phosphide P – – –

Chlorate (ClO3)– Peroxide O2– –
Nitrite (NO2)– Silicate (SiO3)– –
Nitrate (NO3)– Carbonate (CO3)– –
Hydroxide (OH)– Zincate (ZnO)2– –
Peroxide O2– –
Bicarbonate (HCO3)– Dichromate (Cr2O7)– –
Thiosulphate (S2O3)– –
Cyanide (CN)–

Metaluminate (AlO2)–

138 Modern Graded Science Class 9

Some compounds and their acid and basic radicals

Compounds Basic radical Acid radical

NaCl Na+ Cl–
K+
K2SO4 Mg+ + SO4– –
Mg (NO3)2 Ca+ + NO3–
CaCO3 NH4+ CO3– –
NH4OH NH4+ OH–
(NH4)3 PO4
PO4– – –

Ion

Atoms are electrically neutral as they have equal number of positively charged protons in
the nucleus and negatively charged electrons orbiting in nucleus. When an atom loses an
electron (s) to other or gains an electron (s) from other, it becomes positively charged or
negatively charged to become stable. This kind of charged atom is called an ion. Hence,
an ion is defined as an atom that has either a positive charge or a negative charge. When an
atom loses one or more electrons from the valence shell, it becomes positively charged.
The positively charged ion is called cation. All the metals which lose electrons to other to
become an ion are called cations. Examples are Na+, Mg+ +, Al+ + +, Ca+ +, K+, etc. The cation
has the same name as the element. For example, Na+ → Sodium ion, Ca+ + → Calcium ion.
When an atom gains one or more electrons from the valence shell of other atom, it becomes
negatively charged. The negatively charged ion is called anion. Most of the nonmetals gain
electrons from other atom to become anion. Examples are Cl-, O- -, N- - -,­ Br-, I-, etc.
The anion has the same name as the element but with the ending (suffix) changed to -ide.
Examples are N- - - → Nitride, Cl- → Chloride, O - - → Oxide, etc.

Inert gases

The elements which 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 contains 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 following is the electronic configuration of inert gases:

S.N. Inert gases At. Electronic Nature Subshell electronic configura-
1. Helium (He) num- configuration tion
ber
2 Duplet 1s2
2

2. Neon (Ne) 10 2, 8 Octet 1s2, 2s2, 2p6
3. Argon (Ar) 18 2, 8, 8 Octet 1s2, 2s2, 2p6, 3s2, 3p6
4. Krypton (Kr) 36 2, 8, 18, 8 Octet 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6

Classification of Elements 139

From the above mentioned stable electronic configuration, we know that inert gases do not
allow the valence electron to take part in a chemical reaction. As they do not have combining
capacity with other elements or radicals, their atoms are stable and they can exist freely.

Duplet state and duplet rule

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. Thus, the presence of two electrons in the K-shell of an
atom with single shell is called duplet.

Elements such as hydrogen, lithium, beryllium and boron have the tendency to attain stable
electronic configuration similar to that of helium atom. It is called the duplet rule. Thus, the
tendency of an atom with a single shell to attain two electrons in its K-shell is called the duplet rule.

Octet state and octet rule

The inert gases such as neon, argon, krypton, xenon and radon except helium have 8 electrons
in their valence shell are called octet. These elements do not take part in chemical bonding.

All the atoms of the elements try to lose or gain electrons from their valence shell to attain
stable electronic configuration of the nearest inert gases except H, Li, Be, and B. This is called
the octet rule. Thus, the natural tendency of an atom of an element to make 8 electrons in its
valence shell by gaining, losing or sharing electrons between the combining atoms during the
formation of molecules is called the octet rule.

Chemical bonding

The number of electrons lost or gained or shared by an atom in the formation of a chemical
bond is called the valency. The force that is responsible to hold the atoms in a molecule is
called a chemical bond. Thus, the force of attraction by which atoms or group of atoms are
combined together to form a stable chemical molecule or compound is called a chemical
bond. For example, ammonia (NH3) is a compound which is formed by a chemical bond
between one atom of nitrogen and three atoms of hydrogen. Depending upon the nature,
there are three types of chemical bonds. They are:

1. Electrovalent or ionic bond
2. Covalent bond
3. Co-ordinate bond

Among these three, we will study about two common types of bonds: electrovalent bond and
covalent bond.

1. Electrovalent bond (Ionic bond)

Electrovalent bond is formed between two atoms in which one atom belongs to a metal while
the other to a non-metal. The metal atom loses its valance electron to attain stable electronic

140 Modern Graded Science Class 9

configuration and gains positive ion or cation. On the other hand, the non-metal atom gains
electrons from the metal to form stable electronic configuretaion and negative ion or anion.
Two oppositely charge ions i.e. cation and anion are bonded together by the force of attraction
to form the ionic bond. So, the chemical bond formed by the transfer of electrons from the
valence shell of one atom to the valence shell of another atom is called electrovalent bond or
ionic bond and the valency is electrovalency. The compounds which are formed as a result of
electrovalent bonding are called electrovalent compounds or ionic compounds.
The number of electrons gained or lost by an atom to attain stable electronic configuration is
known as electrovalency of the atom. For example, in the compound sodium chloride (NaCl),
the electrovalency of sodium (Na) is 1 as it loses its one electron and the electrovalency of
chlorine (Cl) is also 1 as it gains one electron.

Characteristics of electrovalent compounds

1. Electrovalent compounds have high boiling and melting points on account of the strong
inter-ionic forces of attraction.

2. They are found in the solid state and they are brittle.
3. In a molten or aqueous solution, they conduct electricity.
4. They are soluble in water but insoluble in organic solvents.
5. They contain metal in their molecule.
The formation of electrovalent bond is illustrated by the following examples.

Sodium chloride (NaCl)

Sodium chloride is an electrovalent compound, which is formed only when sodium reacts
with chlorine. The electronic configuration of sodium is 2, 8, 1, whereas that of chlorine is 2,
8, 7. The electronic configurations in sodium and chlorine show that sodium atom has only
one electron more than the nearest inert gas argon in their valence shell.

Fig. 8.9. molecular structure of sodium chloride

Thus, the sodium atom has a tendency to lose one electron from its valence shell to form Na+
(cation) having stable electronic configuration 2, 8. The chlorine atom has a tendency to gain
one electron in its valence shell to form Cl– (anion) having the stable electronic configuration

Classification of Elements 141

2, 8, 8. These two opposite charges (cation and anion) attract together and are held with each
other by the force of attraction between Na+ and Cl– ions, called electrovalent bond.

Molecular structure of some electrovalent compounds

i. Magnesium chloride (Mg Cl2)

ii. Potassium oxide (K2O)

2. Covalent bond

The chemical bond that is formed by the mutual sharing of electrons between two atoms
each contributing equal number of electrons to the electron pair is called covalent bond. By
sharing electrons both bonding atoms acquire a stable electronic configuration similar to the
nearest inert gas. A covalent bond is represented by a line (–) between the bonded atoms. The
chemical compounds/molecules formed by covalent bonding are called covalent compounds.

Covalency

It is defined as the number of the electrons which the atom contributes for sharing in a covalent
compound. In other words, covalency is equal to the number of covalent bonds formed by
an atom in a molecule. Thus, the covalencies of H, O and N are 1, 2 and 3 respectively. It is
always formed when two or more non-metallic elements combine together to form covalent
compoound.

Characteristics of covalent compounds/molecules

1. Covalent compounds have low melting and boiling points in comparison to electrovalent
compounds, because they are held by weak force which can easily be overcome by heat
except graphite, diamond and ethanol (alcohol).

2. They do not conduct electricity as they do not form ions in the aqueous solution.
3. They are usually insoluble in water but soluble in organic solvents.
4. They are found in solid, liquid and gaseous states. For example, graphite, diamond and

quartz are found in the solid state; water, methanol, etc. in the liquid state and carbon
dioxide, nitrogen, hydrochloric acid and oxygen in the gaseous state.
5. They do not contain metal in them as the covalent bond is formed between a non-metal
and a non-metal.

142 Modern Graded Science Class 9

Formation of covalent bond

The formation of covalent bond is illustrated by the following example.

i. Hydrogen Chloride (HCl)

A hydrogen atom has 1 electron in its valence shell, so it needs one more electron to attain
electronic configuration of a helium (He) atom. Chlorine has 7 electrons in its outer energy
level, so it also needs one electron to attain a stable electronic configuration like that of argon
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MMooleleccuulalarrsstrtruucctuturreeooffhhyyddrrooggeenncchhlolorrididee

Ȁ ȋȋȀ ȋȋ …‹ …‹‘ ʹ‘ ʹ ʹ‹ ʹŽ‹ȌȌƒŽȌ’ Ȍƒǡ’ ” ǡ‘”ƒ Ž‘‘ƒ›Ž‘—”›š—”ǡ‡š ›ǡ‡ ›†‘‰ †‘‰ˆ–•‡ˆ‘–•‡ŠǤ‘Š Ǥ”‡ ” ‡” ȋ”ȋ ‡ …‡†ʹ…‘†ʹȌ‘ Ȍǡ ǡ„ Š„’›Š’››‘ ›F‘† —i†…g—”…‘”.‘‘8‘˜†‰˜†.‰ƒ1•‡ƒ•0Ž‡Ȁ‡ŽȀm‡  ȋo–‘ȋ –l‘ e Ž ‡Žcʹ„‡ʹu„Ȍ…‘Ȍ…l —‘a —rŽ‡†Ž‹s‡†–‹t•‹–”r•‹ ”u‘ ‘Ž‰c‰‹Ž‰t ‰‹u‡ ‡‡rƒ‡ƒe  o †ȋf†ȋ h ‡ ‡yʹ––ʹd–Ȍ–ŠŠȌŠŠǡr‡ ƒǡo‡ ƒ…›g…›Še ‡Šn ‡Ž ƒ‘Ž ƒc‘ȋ””hȋ” ”‡‹l ‡‹o  r‡…iͶ‡d… ͶƒȌ eƒȌȋǡŽȋ ǡ ŽŽ ‡ŽƒŽ‡ƒ†ʹŽ†ʹȌ Ȍ  ƒ…ƒ…‘‘‘‘˜†˜†ƒ ‹ƒ ƒ‹Ž…ƒ‡Ž… ƒ‡ ƒȋ”ȋ” –„ – „‘ ‘ ͵͵ Ȍ‘ Ȍǡ‘† Žǡ† ‡Ž‹™‡‘‹™…‘…šƒ—šƒ—‹–Ž†‹–‡‡Ž†‡‡‡”•‡”•
MMMooloelcleueclcuaurllasatrrrussctttrruuurcectotuufrsreoemoofefscsooomvmaleeencctoomvvaoallleeecnuntltemsm/ocoollemecpcuuoluleenssd//sccoommppoouunnddss
ii1.1.. OOOxxyxygygegneen(nO((O2O)22))

OO ++ OO OO OO OO OO

(2(2, ,66)) (2(2, ,66))

ii2i2... WWWaataettreer(rH((H2HO22O)O)) ȋȋ ‡‡””‡‡ α α ” ”‡‡’’””‡‡••‡‡–– † †‘‘——„„Ž‡Ž‡ „ „‘‘††ȌȌ

HH ++ OO ++ HH HH OO HH HH OO HH

(1(1)) (2(2, ,66)) (1)

v.33.. NMMietertothhgaaennnee((N(CC2H)H44)) shared electron covalent
pairs bond
HH HH
×× HH
++ HH ×× CC ×× HH || 143
HH ++××××CC××××++ HH
×× HH––C|CC| –l–aHsHsification of Elements

HH HH HH

(1) (2, 6)

iv.3. MMetehtahnaen(eC(HC4H) 4)

H H ×H H
× |
+ H×C H–C–H
H + ××C×× + H |
×
H

HH

M ol e–Šc‡u „lƒa•r‹• f‘oˆ r–mŠ‡u l—a„‡” ‘ˆ …Š‡‹…ƒŽ „‘†• ˆ‘”‡† „‡–™‡‡ –™‘ ƒ–‘•ǡ …‘˜ƒŽ‡–
M„o‘lec†u l‹e• i‘sˆf o͵r –m›’ed‡•bǤy the combination of two or more atoms of the same or different types in
a fixed proportion. A molecular formula indicates the symbolic representation of the molecule
of a1s6u8bstaMncoed. eItrnalsGoragdiveeds SincfioenrmceaCtiloanssab9out the total number of atoms of different elements.
Molecular formula of elements

We learnt that the symbol of elements indicate There is no any role of nucleus in a
one atom. To represent one molecule of an chemical reaction. But life cannot be
element, we use a molecular formula. For originated without change in nucleus
example, a molecule of oxygen contains two during a nuclear reaction. The light and
oxygen atoms, thus its molecular formula is O2. heat are emitted continuously from the
Here, the number below its symbol (2) stands sun by a nuclear fusion reaction. In this
for the number of oxygen atoms in a molecule reaction, there is change in the number
of oxygen. Similarly, the molecules of hydrogen, of protons and neutrons in the nucleus
nitrogen, chlorine are H2, N2 and Cl2 respectively. and new elements are formed along a
Thus, the molecular formula of an element is large amount of heat, light and uv rays.
defined as the symbolic representation which
shows the actual number of atoms in one
molecule of the element.

The symbols and molecular formulae of inert gases like 2N represents two atoms of
helium (He), neon (Ne) and krypton (Kr) are just the same
because they are monatomic molecules which are stable nitrogen, whereas N2 represents
and exist freely in nature. All the metals do not have any one molecule of nitrogen.
separate formulae, so they are indicated by their symbols.

Molecular formula of compounds

To understand and have the practical knowledge about compounds, each compound has been
separately represented with a molecular formula. A molecular formula of each compound
contains the actual number of atoms of different elements. For example, one molecule of water
contains two atoms of hydrogen and one atom of oxygen. Thus, the molecular formula of
water, a compound liquid, is H2O.
Similarly, the combination of two atoms of nitrogen, four atoms of hydrogen and three atoms of

144 Modern Graded Science Class 9

oxygen form a molecule of ammonium nitrate. Thus, its molecular formula is NH4NO3. Thus,
the molecular formula of a compound is defined as symbolic representation which shows the
actual number of atoms of different elements present in one molecule of the compound.

To represent more than one molecule of a compound, the number of molecules is written
in front of the formula of the molecule. For example, H2O represents a molecule of water.
Similarly, 2H2O simply denotes the two molecules of water.

Information obtained from molecular formula

The following information can be obtained by a molecular formula.

1. A 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 helps to know the valency of elements or raticals.

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 a.m.u. (a.m.u = atomic mass unit).

Similarly, the molecular formula of carbon dioxide is CO2.

∴ Molecular weight of CO2 = 1C + 2O

= 1 × 12 + 2 × 16

= 22 + 32 = 44 a.m.u.

The molecular weight is defined as the sum of atomic weight of all atoms of the molecule.

Writing a molecular formula

To write a molecular formula, the following steps are usually adopted only when we know the
symbol and valencies of elements and radicals present in a molecule:

Step 1 : First, the name of the compound is written.

Step 2 : The symbols of basic and acidic radicals are written side by side.

Step 3 : The valency of each radical is written at the right upper corner of the symbol. The
valency of one radical is transferred to another radical and it is is written on the
right hand side at the bottom corner. If necessary, L.C.M. of the valencies is taken
to get a simple whole number.

Step 4 : If a compound radical takes part in the molecular formula, the radical is enclosed
in brackets and the valency number is written on the right side of the bracket at the
bottom of the formula.
Classification of Elements 145

Example

Calcium Sulphate (compound)

1. Ca SO4 (symbol of basic and acidic radicals)
2. Ca2 SO42 (valency at right upper corners of the symbols)
3. Ca2 (SO4)2 (valencies are exchanged and compound radical is enclosed

in bracket.)

4. CaSO4 → (L.C.M. is taken to get molecular formula of calcium sulphate)

Some other examples

1. Ammonium sulphate
NH41
SO42→(NH4)2 (SO4)1 →NH4)2SO4

2. Carbon dioxide

C4 O2→C2O4 →CO2

3. Aluminium sulphate

Al3 SO42 →Al2 (SO4)3

4. Potassium Sulphate

K1 SO42 → K2 (SO4)1 K2SO4

S me Reasonable Facts

1. Neon is an 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 an inert gas.

2. An atom is electrically neutral. This is because the number of negatively charged electrons
in the shells is equal to the number of positively charged protons in the nucleus of the
atom and they can neutralize together.

3. Sodium is a reactive metal. This is because sodium has 1 valence electron and high
tendency to lose the valence electron for stable electronic configuration of neon.

4. The valency of chlorine is 1. The electronic configuration of a 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 of argon. Hence, the valency of chlorine is 1.

5. CaO is an electrovalent compound. CaO contains metal Ca that loses electrons for non-
metal oxygen to come in octet stage. It produces electrovalent bonding thus CaO is an
electrovalent compound.

6. Sodium is more active than magnesium. It is because valency of sodium is 1 but that
of magnesium is 2. The element having lower valencies has more possibility of losing,
gaining and sharing electrons. Thus, sodium is more active than magnesium.

146 Modern Graded Science Class 9