STRUCTURE OF ATOM

STRUCTURE
OF THE
ATOM

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• Are atoms really indivisible,
as proposed by Dalton

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Comb dry hair.

Rub a glass rod with a silk cloth and bring the rod near
an inflated balloon.

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An atom is divisible

and consists of

charged particles.

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J.J. Thomson

The electron was discovered in 1897 by the
English physicist J.J. Thomson

is a subatomic particle with a negative
one elementary electric charge.[

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1886 E Goldstein

A proton is a stable sub atomic particle, symbol

p

positive electric charge of +1 e (elementary charge)

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E. Goldstein in 1886

discovered the

presence of new
radiations in a gas

discharge

and called them canal

rays.

proton as ‘p+

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• 1932, J. Chadwick

• no charge and a mass nearly equal to

that of a proton. It was eventually

named as neutron.

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particle

discovered

Relative

mass

charge

symbol

proton

EE Gold stein

1886

1

+1

p

electron

J J Thomson

1900

1/2000

-1

e

neutron

James

chadwick

1932

1

0

n

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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 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.

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THOMSON’S MODEL OF AN ATOM

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

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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

electricallyneutral.

LIMITATIONS

Thomson’s model explained that

atoms are electrically neutral but they are charged

sometimes

•
•

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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.

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RUTHERFORD’S MODEL OF AN ATOM

He selected a gold foil because he wanted
as thin a layer as possible. This gold foil
was about 1000 atoms thick.
α 𝛼 -particles are doubly-charged helium
ions. Since they have a mass of 4 u, the fast-
moving
α 𝛼 -particles have a considerable amount of
energy.

It was expected that a 𝛼-particles would be
deflected by the sub-atomic particles in
the gold atoms.

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The nuclear model of an atom (RUTHER FORD)

(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

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.

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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.

BOHR’S MODEL OF ATOM
(i) The maximum number of electrons present in a shell is given by the formula 2n2
, 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 × 12 = 2,
second orbit or L-shell will be= 2 × 22 = 8,
third orbit or M-shell will be= 2 × 32 = 18,
fourth orbit or N-shell will be= 2 × 42 = 32,
(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.

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Valency

The electrons present in the outermost shell of an atom are known as the valence

electrons.

The atoms of elements, completely filled with 8 electrons in the outermost shell show little

chemical activity. An outermost-shell, which had eight electrons was said to possess an

octet 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

shared

Therefore, an atom of each element has a definite combining capacity, called its valency.

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Atomic Number

The atomic number is
defined as the total
number of protons

present in the nucleus

of an atom.

It is denoted by ‘Z’.

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MASS NUMBER
The 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.

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’.

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Isotopes
Isotopes are defined as the atoms of the same element, having the
same atomic number but different mass numbers.
protium (11H),

deuterium ( 21Hor D) and

tritium ( 31Hor T).

The atomic number of eachone 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 and37 17 Cl, etc

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Applications

(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.

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ISOBARS
Atoms of different elements with
different atomic numbers, which have
the same mass number, are known as
isobars.
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.

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