Thomson's Model of atom
In 1904 Sir J. J. Thomson proposed the first definite theory as to the
internal structure of the atom. According to this theory the atom was assumed
to consist of a sphere of uniform distribution of about 10-10m
positive charge with electrons embedded in it such that the number of electrons
equal to the number of positive charges and the atom as a whole is electrically
neutral.
This model of atom could account the electrical neutrality of atom, but
it could not explain the results of gold foil scattering experiment carried out
by Rutherford.
Rutherford's Scattering Experiment
Rutherford conducted a scattering experiment in 1911 to find out the
arrangement of electrons and protons. He bombarded a thin gold foil with
a stream of fast moving positively charged alpha.-particles emanating from radium.
Rutherford's Nuclear
model of a atom
This model resulted from conclusion drawn from experiments on the
scattering of alpha particles from a radio active source when the particles
were passed through thin sheets of metal foil. According to him
(i) Most of the space in the atom is empty as most of the alpha.-particles passed
through the foil.
(ii) A few positively charged alpha-particles
are deflected. The deflection must be due to enormous repulsive force showing that the positive charge
of the atom is not spread throughout the atom as Thomson had thought. The
positive charge has to be concentrated in a very small volume that repelled and
deflected the positively charged alpha-particles. This very small portion of the atom was called nucleus by Rutherford.
(iii) Calculations by Rutherford showed that the volume occupied by the
nucleus is negligibly small as compared to the total volume of the atom. The
diameter of the atom is about 10-10 m while that of nucleus is 10-15m.
One can appreciate this difference in size by realizing that if a cricket ball
represents a nucleus, then radius of the atom would be about 5 km.
On the basis of above observations and conclusions, Rutherford proposed
the nuclear model of atom. According to this model:
1.
An atom consists of a tiny positively charged
nucleus at its centre.
2.
The positive charge of the nucleus is due to
protons. The mass of the nucleus, on the other hand, is due to protons and some
neutral particles each having mass nearly equal to the mass of proton. This
neutral particle, called neutron, was discovered later on by Chadwick in 1932.
Protons and neutrons present in the nucleus are collectively also known as
nucleons. The total number of nucleons is termed as mass number(A) of the atom.
3.
The nucleus is surrounded by electrons that move
around the nucleus with very high speed in circular paths called orbits. Thus, Rutherford's model of
atom resembles the solar system in which the sun plays the role of the nucleus
and the planets that of revolving electrons.
4.
The number of electrons in an atom is equal to
the number of protons in it. Thus, the total positive charge of the nucleus
exactly balances the total negative charge in the atom making it electrically
neutral. The number of protons in an atom is called its atomic number(Z).
5.
Electrons and the nucleus are held together by
electrostatic forces of attraction.
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