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.