In Chapter 3, we have learnt that atoms and
molecules are the fundamental building
blocks of matter. The existence of different
kinds of matter is due to different atoms
constituting them. Now the questions arise:
(i) What makes the atom of one element
differ
ent from the atom of another element?
and (ii) Are atoms really indivisible, as
proposed by Dalton, or are there smaller
constituents inside the atom? We shall find
out the answers to these questions in this
chapter. We will learn about sub-atomic
particles and the various models that have
been proposed to explain how these particles
are arranged within the atom.
A major challenge before the scientists at
the end of the 19th century was to reveal the
structure of the atom as well as to explain its
important properties. The elucidation of the
structure of atoms is based on a series of
experiments.
One of the first indications that atoms are
not indivisible, comes from studying static
electricity and the condition under which
electricity is conducted by different
substances.
4.1 Charged Particles in Matter
For understanding the nature of charged
particles in matter, let us carry out the
following activities:
Activity ______________ 4.1
A. Comb dry hair. Does the comb then
attract small pieces of paper?
B. Rub a glass rod with a silk cloth and
bring the rod near an inflated balloon.
Observe what happens.
From these activities, can we conclude
that on rubbing two objects together, they
become electrically charged? Where does this
charge come from? This question can be
answered by knowing that an atom is divisible
and consists of charged particles.
Many scientists contributed in revealing
the presence of charged particles in an atom.
It was known by 1900 that the atom was
indivisible particle but contained at least one
sub-atomic particle – the electron identified by
J.J. Thomson. Even before the electron was
identified, E. Goldstein in 1886 discovered the
presence of new radiations in a gas discharge
and called them canal rays. These rays were
positively charged radiations which ultimately
led to the discovery of another sub-atomic
particle. This sub-atomic particle had a charge,
equal in magnitude but opposite in sign to that
of the electron. Its mass was approximately
2000 times as that of the electron. It was given
the name of proton. In general, an electron is
represented as ‘e
–
’ and a proton as ‘p
+
’. The
mass of a proton is taken as one unit and its
charge as plus one. The mass of an electron is
considered to be negligible and its charge is
minus one.
It seemed that an atom was composed of
protons and electrons, mutually balancing
their charges. It also appeared that the protons
were in the interior of the atom, for whereas
electrons could easily be removed off but
not protons. Now the big question was:
what sort of structure did these particles of
the atom form? We will find the answer to
this question below.
4
SS
SS
S
TRUCTURETRUCTURE
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TRUCTURE
OF
OF
OFOF
OF
THETHE
THETHE
THE
A A
A A
A
TOMTOM
TOMTOM
TOM
C
hapter
Rationalised 2023-24
uestions
1. What are canal rays?
2. If an atom contains one electron
and one proton, will it carry any
charge or not?
4.2 The Structure of an Atom
We have learnt Dalton’s atomic theory in
Chapter 3, which suggested that the atom
was indivisible and indestructible. But the
discovery of two fundamental particles
(electrons and protons) inside the atom, led
to the failure of this aspect of Dalton’s atomic
theory. It was then considered necessary to
know how electrons and protons are arranged
within an atom. For explaining this, many
scientists proposed various atomic models.
J.J. Thomson was the first one to propose a
model for the structure of an atom.
4.2.1 THOMSON’S MODEL OF AN ATOM
Thomson proposed the model of an atom to
be similar to that of a Christmas pudding.
The electrons, in a sphere of positive charge,
were like currants (dry fruits) in a spherical
Christmas pudding. We can also think of a
watermelon, the positive charge in the atom
is spread all over like the red edible part of
the watermelon, while the electrons are
studded in the positively charged sphere, like
the seeds in the watermelon (Fig. 4.1).
Thomson proposed that:
(i) An atom consists of a positively
charged sphere and the electrons are
embedded in it.
(ii) The negative and positive charges are
equal in magnitude. So, the atom as a
whole is electrically neutral.
Although Thomson’s model explained that
atoms are electrically neutral, the results of
experiments carried out by other scientists
could not be explained by this model, as we
will see below.
4.2.2 RUTHERFORD’S MODEL OF AN ATOM
Ernest Rutherford was interested in knowing
how the electrons are arranged within an
atom. Rutherford designed an experiment for
this. In this experiment, fast moving alpha
(α)-particles were made to fall on a thin
gold foil.
• He selected a gold foil because he wanted
as thin a layer as possible. This gold foil
was about 1000 atoms thick.
• α-particles are doubly-charged helium
ions. Since they have a mass of 4 u, the
fast-moving α-particles have a
considerable amount of energy.
• It was expected that α-particles would be
deflected by the sub-atomic particles in
the gold atoms. Since the α-particles were
much heavier than the protons, he did
not expect to see large deflections.
Q
Fig.4.1: Thomson’s model of an atom
J.J. Thomson (1856–
1940), a British
physicist, was born in
Cheetham Hill, a suburb
of Manchester, on
18 December 1856. He
was awarded the Nobel
prize in Physics in 1906
for his work on the
discovery of electrons. He
directed the Cavendish Laboratory at
Cambridge for 35 years and seven of his
research assistants subsequently won
Nobel prizes.
STRUCTURE OF THE ATOM 39
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SCIENCE40
Fig. 4.2: Scattering of
α
-particles by a gold foil
But, the α-particle scattering experiment
gave totally unexpected results (Fig. 4.2). The
following observations were made:
(i) Most of the fast moving α-particles
passed straight through the gold foil.
(ii) Some of the α-particles were deflected
by the foil by small angles.
(iii) Surprisingly one out of every 12000
particles appeared to rebound.
In the words of Rutherford, “This result
was almost as incredible as if you fire a
15-inch shell at a piece of tissue paper and it
comes back and hits you”.
He will hear a sound when each stone strikes
the wall. If he repeats this ten times, he will
hear the sound ten times. But if a blind-folded
child were to throw stones at a barbed-wire
fence, most of the stones would not hit the
fencing and no sound would be heard. This is
because there are lots of gaps in the fence
which allow the stone to pass through them.
Following a similar reasoning, Rutherford
concluded from the α-particle scattering
experiment that—
(i) Most of the space inside the atom is
empty because most of the α-particles
passed through the gold foil without
getting deflected.
(ii) Very few particles were deflected from
their path, indicating that the positive
charge of the atom occupies very little
space.
(iii) A very small fraction of α-particles
were deflected by 180
0
,
indicating that
all the positive charge and mass of the
gold atom were concentrated in a very
small volume within the atom.
From the data he also calculated that the
radius of the nucleus is about 10
5
times less
than the radius of the atom.
On the basis of his experiment,
Rutherford put forward the nuclear model of
an atom, which had the following features:
(i) There is a positively charged centre in
an atom called the nucleus. Nearly all
the mass of an atom resides in the
nucleus.
(ii) The electrons revolve around the
nucleus in circular paths.
(iii) The size of the nucleus is very small
as compared to the size of the atom.
Drawbacks of Rutherford’s model of
the atom
The revolution of the electron in a circular orbit
is not expected to be stable. Any particle in a
circular orbit would undergo acceleration.
During acceleration, charged particles would
radiate energy. Thus, the revolving electron
would lose energy and finally fall into the
nucleus. If this were so, the atom should be
highly unstable and hence matter would not
exist in the form that we know. We know that
atoms are quite stable.
E. Rutherford (1871–1937)
was born at Spring Grove
on 30 August 1871. He was
known as the ‘Father’ of
nuclear physics. He is
famous for his work on
radioactivity and the
discovery of the nucleus of an atom with
the gold foil experiment. He got the Nobel
prize in chemistry in 1908.
Let us think of an activity in an open field
to understand the implications of this
experiment. Let a child stand in front of a
wall with his eyes closed. Let him throw
stones at the wall from a distance.
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STRUCTURE OF THE ATOM 41
4.2.3 BOHR’S MODEL
OF ATOM
In order to overcome the objections raised
against Rutherford’s model of the atom,
Neils Bohr put forward the following
postulates about the model of an atom:
(i) Only certain special orbits known as
discrete orbits of electrons, are allowed
inside the atom.
(ii) While revolving in discrete orbits the
electrons do not radiate energy.
uestions
1. On the basis of Thomson’s model
of an atom, explain how the atom
is neutral as a whole.
2. On the basis of Rutherford’s
model of an atom, which sub-
atomic particle is present in the
nucleus of an atom?
3. Draw a sketch of Bohr’s model
of an atom with three shells.
4. What do you think would be the
observation if the
α
-particle
scattering experiment is carried
out using a foil of a metal other
than gold?
4.2.4 NEUTRONS
In 1932, J. Chadwick discovered another sub-
atomic particle which had no charge and a
mass nearly equal to that of a proton. It was
eventually named as neutron. Neutrons are
present in the nucleus of all atoms, except
hydrogen. In general, a neutron is
represented as ‘n’. The mass of an atom is
therefore given by the sum of the masses of
protons and neutrons present in the nucleus.
uestions
1. Name the three sub-atomic
particles of an atom.
2. Helium atom has an atomic mass
of 4 u and two protons in its
nucleus. How many neutrons
does it have?
4.3 How are Electrons Distributed
in Different Orbits (Shells)?
The distribution of electrons into different
orbits of an atom was suggested by Bohr
and Bury.
The following rules are followed for writing
the number of electrons in different energy
levels or shells:
(i) The maximum number of electrons
present in a shell is given by the
Neils Bohr (1885–1962)
was born in Copenhagen
on 7 October 1885. He was
appointed professor of
physics at Copenhagen
University in 1916. He got
the Nobel prize for his work
on the structure of atom in
1922. Among Professor
Bohr’s numerous writings, three appearing
as books are:
(i) The Theory of Spectra and Atomic
Constitution, (ii) Atomic Theory and,
(iii) The Description of Nature.
These orbits or shells are called energy
levels. Energy levels in an atom are shown in
Fig. 4.3.
Q
Fig. 4.3: A few energy levels in an atom
These orbits or shells are represented by
the letters K,L,M,N,… or the numbers,
n=1,2,3,4,….
Q
Rationalised 2023-24
SCIENCE42
formula 2n
2
,
where ‘n’ is the orbit
number or energy level index, 1,2,3,….
Hence the maximum number of
electrons in different shells are as
follows:
first orbit or K-shell will be = 2 × 1
2
= 2,
second orbit or L-shell will be = 2 × 2
2
= 8,
third orbit or M-shell will be = 2 × 3
2
= 18,
fourth orbit or N-shell will be = 2 × 4
2
= 32, and so on.
(ii) The maximum number of electrons that
can be accommodated in the outermost
orbit is 8.
(iii) Electrons are not accommodated in a
given shell, unless the inner shells are
filled. That is, the shells are filled in a
step-wise manner.
Atomic structure of the first eighteen
elements is shown schematically in Fig. 4.4.
• The composition of atoms of the first
eighteen elements is given in Table 4.1.
uestions
1. Write the distribution of electrons
in carbon and sodium atoms.
2. If K and L shells of an atom are
full, then what would be the total
number of electrons in the atom?
4.4 Valency
We have learnt how the electrons in an atom
are arranged in different shells/orbits. The
electrons present in the outermost shell of
an atom are known as the valence electrons.
From the Bohr-Bury scheme, we also know
that the outermost shell of an atom can
Activity ______________ 4.2
• Make a static atomic model displaying
electronic configuration of the first
eighteen elements.
Fig.4.4: Schematic atomic structure of the first eighteen elements
Q
accommodate a maximum of 8 electrons. It
was observed that the atoms of elements,
completely filled with 8 electrons in the
outermost shell show little chemical activity.
In other words, their combining capacity or
valency is zero. Of these inert elements, the
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STRUCTURE OF THE ATOM 43
Table 4.1: Composition of Atoms of the First Eighteen Elements
with Electron Distribution in Various Shells
helium atom has two electrons in its outermost
shell and all other elements have atoms with
eight electrons in the outermost shell.
The combining capacity of the atoms of
elements, that is, their tendency to react and
form molecules with atoms of the same or
different elements, was thus explained as an
attempt to attain a fully-filled outermost shell.
An outermost-shell, which had eight electrons
was said to possess an octet. Atoms would
thus react, so as to achieve an octet in the
outermost shell. This was done by sharing,
gaining or losing electrons. The number of
electrons gained, lost or shared so as to make
the octet of electrons in the outermost shell,
gives us directly the combining capacity of the
element, that is, the valency discussed in the
previous chapter. For example, hydrogen/
lithium/sodium atoms contain one electron
each in their outermost shell, therefore each
one of them can lose one electron. So, they are
said to have valency of one. Can you tell, what
is valency of magnesium and aluminium? It
is two and three, respectively, because
magnesium has two electrons in its outermost
shell and aluminium has three electrons in
its outermost shell.
If the number of electrons in the
outermost shell of an atom is close to its full
capacity, then valency is determined in a
different way. For example, the fluorine atom
has 7 electrons in the outermost shell, and its
valency could be 7. But it is easier for
Name of Symbol Atomic Number Number
Number Vale-
Element Number of of of ncy
Protons Neutrons Electrons K L M N
Hydrogen H 1 1 - 1 1 - - - 1
Helium He 2 2 2 2 2 - - - 0
Lithium Li 3 3 4 3 2 1 - - 1
Beryllium Be 4 4 5 4 2 2 - - 2
Boron B 5 5 6 5 2 3 - - 3
Carbon C 6 6 6 6 2 4 - - 4
Nitrogen N 7 7 7 7 2 5 - - 3
Oxygen O 8 8 8 8 2 6 - - 2
Fluorine F 9 9 10 9 2 7 - - 1
Neon Ne 10 10 10 10 2 8 - - 0
Sodium Na 11 11 12 11 2 8 1 - 1
Magnesium Mg 12 12 12 12 2 8 2 - 2
Aluminium Al 13 13 14 13 2 8 3 - 3
Silicon Si 14 14 14 14 2 8 4 - 4
Phosphorus P 15 15 16 15 2 8 5 - 3,5
Sulphur S 16 16 16 16 2 8 6 - 2
Chlorine Cl 17 17 18 17 2 8 7 - 1
Argon Ar 18 18 22 18 2 8 8 0
Distribution of
Electrons
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SCIENCE44
fluorine to gain one electron instead of losing
seven electrons. Hence, its valency is determined
by subtracting seven electrons from the octet
and this gives you a valency of one for fluorine.
Valency can be calculated in a similar manner
for oxygen. What is the valency of oxygen that
you get from this calculation?
Therefore, an atom of each element has a
definite combining capacity, called its valency.
Valency of the first eighteen elements is given
in the last column of Table 4.1.
uestion
1. How will you find the valency
of chlorine, sulphur and
magnesium?
4.5 Atomic Number and Mass
Number
4.5.1 ATOMIC NUMBER
We know that protons are present in the
nucleus of an atom. It is the number of
protons of an atom, which determines its
atomic number. It is denoted by ‘Z’. All atoms
of an element have the same atomic number,
Z. In fact, elements are defined by the number
of protons they possess. For hydrogen, Z = 1,
because in hydrogen atom, only one proton
is present in the nucleus. Similarly, for
carbon, Z = 6. Therefore, the atomic number
is defined as the total number of protons
present in the nucleus of an atom.
4.5.2 MASS NUMBER
After studying the properties of the sub-
atomic particles of an atom, we can conclude
that mass of an atom is practically due to
protons and neutrons alone. These are
present in the nucleus of an atom. Hence
protons and neutrons are also called
nucleons. Therefore, the mass of an atom
resides in its nucleus. For example, mass of
carbon is 12 u because it has 6 protons and
6 neutrons, 6 u + 6 u = 12 u. Similarly, the
mass of aluminium is 27 u (13 protons+14
neutrons). The mass number is defined as the
sum of the total number of protons and
neutrons present in the nucleus of an atom. It
is denoted by ‘A’. In the notation for an atom,
the atomic number, mass number and symbol
of the element are to be written as:
Mass Number
Q
Symbol of
element
Atomic Number
For example, nitrogen is written as
14
7
N
.
uestions
1. If number of electrons in an atom
is 8 and number of protons is also
8, then (i) what is the atomic
number of the atom? and (ii) what
is the charge on the atom?
2. With the help of Table 4.1, find
out the mass number of oxygen
and sulphur atom.
4.6 Isotopes
In nature, a number of atoms of some
elements have been identified, which have the
same atomic number but different mass
numbers. For example, take the case of
hydrogen atom, it has three atomic species,
namely protium (
1
1
H), deuterium (
2
1
H or D)
and tritium (
3
1
H or T). The atomic number of
each one is 1, but the mass number is 1, 2
and 3, respectively. Other such examples are
(i) carbon,
12
6
C and
14
6
C, (ii) chlorine,
35
17
Cl
and
37
17
Cl, etc.
On the basis of these examples, isotopes
are defined as the atoms of the same element,
having the same atomic number but different
mass numbers. Therefore, we can say that
there are three isotopes of hydrogen atom,
namely protium, deuterium and tritium.
Q
X
A
Z
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STRUCTURE OF THE ATOM 45
Q
Many elements consist of a mixture of
isotopes. Each isotope of an element is a pure
substance. The chemical properties of
isotopes are similar but their physical
properties are different.
Chlorine occurs in nature in two isotopic
forms, with masses 35 u and 37 u in the ratio
of 3:1. Obviously, the question arises: what
should we take as the mass of chlorine atom?
Let us find out.
The average atomic mass of chlorine atom,
on the basis of above data, will be
35
75
100
37
25
100
× + ×
=
+ = =
105
4
37
4
142
4
35 5. u
The mass of an atom of any natural element
is taken as the average mass of all the naturally
occuring atoms of that element. If an element
has no isotopes, then the mass of its atom
would be the same as the sum of protons and
neutrons in it. But if an element occurs in
isotopic forms, then we have to know the
percentage of each isotopic form and then the
average mass is calculated.
This does not mean that any one atom of
chlorine has a fractional mass of 35.5 u. It
means that if you take a certain amount of
chlorine, it will contain both isotopes of
chlorine and the average mass is 35.5 u.
Applications
Since the chemical properties of all the
isotopes of an element are the same,
normally we are not concerned about
taking a mixture. But some isotopes have
special properties which find them useful
in various fields. Some of them are :
(i) An isotope of uranium is used as a fuel
in nuclear reactors.
(ii) An isotope of cobalt is used in the
treatment of cancer.
(iii) An isotope of iodine is used in the
treatment of goitre.
4.6.1 ISOBARS
Let us consider two elements — calcium,
atomic number 20, and argon, atomic
number 18. The number of protons in these
atoms is different, but the mass number of
both these elements is 40. That is, the total
number of nucleons is the same in the atoms
of this pair of elements. Atoms of different
elements with different atomic numbers, which
have the same mass number, ar
e known as
isobars.
uestions
1. For the symbol H,D and T
tabulate three sub-atomic
particles found in each of them.
2. Write the electronic
configuration of any one pair of
isotopes and isobars.
What
you have
learnt
• Credit for the discovery of electron and proton goes to J.J.
Thomson and E.Goldstein, respectively.
• J.J. Thomson proposed that electrons are embedded in a
positive sphere.
Rationalised 2023-24
SCIENCE46
• Rutherford’s alpha-particle scattering experiment led to the
discovery of the atomic nucleus.
• Rutherford’s model of the atom proposed that a very tiny
nucleus is present inside the atom and electrons revolve
around this nucleus. The stability of the atom could not be
explained by this model.
• Neils Bohr’s model of the atom was more successful. He
proposed that electrons are distributed in different shells with
discrete energy around the nucleus. If the atomic shells are
complete, then the atom will be stable and less reactive.
• J. Chadwick discovered presence of neutrons in the nucleus of
an atom. So, the three sub-atomic particles of an atom are:
(i) electrons, (ii) protons and (iii) neutrons. Electrons are
negatively charged, protons are positively charged and neutrons
have no charges. The mass of an electron is about
1
2000
times
the mass of an hydrogen atom. The mass of a proton and a
neutron is taken as one unit each.
• Shells of an atom are designated as K,L,M,N,….
• Valency is the combining capacity of an atom.
• The atomic number of an element is the same as the number
of protons in the nucleus of its atom.
• The mass number of an atom is equal to the number of nucleons
in its nucleus.
• Isotopes are atoms of the same element, which have different
mass numbers.
• Isobars are atoms having the same mass number but different
atomic numbers.
• Elements are defined by the number of protons they possess.
Exercises
1. Compare the properties of electrons, protons and neutrons.
2. What are the limitations of J.J. Thomson’s model of the atom?
3. What are the limitations of Rutherford’s model of the atom?
4. Describe Bohr’s model of the atom.
5. Compare all the proposed models of an atom given in this
chapter.
6. Summarise the rules for writing of distribution of electrons in
various shells for the first eighteen elements.
7. Define valency by taking examples of silicon and oxygen.
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STRUCTURE OF THE ATOM 47
8. Explain with examples (i) Atomic number, (ii) Mass number,
(iii) Isotopes and iv) Isobars. Give any two uses of isotopes.
9. Na
+
has completely filled K and L shells. Explain.
10. If bromine atom is available in the form of, say, two isotopes
79
35
Br (49.7%) and
81
35
Br (50.3%), calculate the average atomic
mass of bromine atom.
11. The average atomic mass of a sample of an element X is 16.2 u.
What are the percentages of isotopes
16
8
X and
18
8
X in the
sample?
12. If Z = 3, what would be the valency of the element? Also, name
the element.
13. Composition of the nuclei of two atomic species X and Y are
given as under
X Y
Protons =
6 6
Neutrons = 6 8
Give the mass numbers of X and Y. What is the relation between
the two species?
14. For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom
contains only nucleons.
(b) A neutron is formed by an electron and a proton
combining together. Therefore, it is neutral.
(c) The mass of an electron is about
1
2000
times that of proton.
(d) An isotope of iodine is used for making tincture iodine,
which is used as a medicine.
Put tick (ü
) against correct choice and cross (×) against
wrong choice in questions 15, 16 and 17
15. Rutherford’s alpha-particle scattering experiment was
responsible for the discovery of
(a) Atomic Nucleus (b) Electron
(c) Proton (d) Neutron
16. Isotopes of an element have
(a) the same physical properties
(b) different chemical properties
(c) different number of neutrons
(d) different atomic numbers.
17. Number of valence electrons in Cl
–
ion are:
(a) 16 (b) 8 (c) 17 (d) 18
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SCIENCE48
18. Which one of the following is a correct electronic configuration
of sodium?
(a) 2,8 (b) 8,2,1 (c) 2,1,8 (d) 2,8,1
19. Complete the following table.
Atomic Mass Number
Number Number Name of
Number
Number of of of the Atomic
Neutrons Protons Electrons Species
9 - 10 - - -
16 32 -
- - Sulphur
- 24 - 12 - -
- 2 - 1 - -
- 1 0 1 0 -
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