STRUCTURE OF THE ATOM
Charged Particles in Matter
Thomson’s Model of an Atom
Rutherford’s Model of an Atom
Bohr’s Model of Atom
Neutrons
Electrons
Valency
Atomic Number
Mass Number
The existence of different kinds of matter is due to different atoms constituting them. Two
questions arise:
(i) What makes the atom of one element different from the atom of another element?
English: Model of the atom by Ernest Rutherford. Français : Modèle d’atome de Ernest Rutherford. (Photo credit: Wikipedia)
and
(ii) Are atoms really indivisible, or are there smaller constituents inside the atom?
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.
Charged Particles in Matter
Many scientists contributed in revealing the presence of charged particles in an atom.
It was known by 1900 that the atom was not a simple, indivisible particle but contained
at least one sub-atomic particle – the electron identified by J.J. Thomson.
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 i ts charge as plus one. The mass of an electron is considered
to be negligible and its charge is minus one.
The Structure of an Atom
J.J. Thomson was the first one to propose a model for the structure of an atom.
THOMSON’S MODEL OF AN ATOM
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.
Thomsan’s model of and atom.
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 ( )- 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.
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.
Positive
Sphere
Electron
• α-particles are doubly-charged helium ions. Since they have a mass of 4 u, the fastmoving 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.
Scattering of α particles by a Gold foil
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.
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 1800, indicating that all the
positive charge and mass of the gold atom were concentrated in a very small volume within
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 well-defined orbits.
(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 orbital revolution of the electron 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 i nto 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.
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. These orbits or
shells are called energy levels.
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,….
NEUTRONS
In 1932, J. Chadwick discovered another subatomic 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.
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 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, 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.
Note Blast 