We
have been that, according to Thomson's atomic model, an atom consists of electrons and protons. In order to test Thomson's model and to know as to how and where the electrons are located in the atom, Rutherford (1911), a British scientist, performed some experiments on the scattering of a-particles by a thin metal foil. Here it may be mentioned that a-particle is a helium nucleus which consists of two units of positive charge and its mass number is equal to 4. Hence it is represented by the symbol, +204 or +2He4.Rutherfords Nuclear Atomic Model । Discovery of Nucleus
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The pictorial representation, of Rutherford's experiment is given. In his experiment he produced a narrow beam of a-particles from a radioactive substance like radium placed in a lead block and directed it at a thin sheet of gold foil of thickness of 0.00004 cm. A movable circular screen coated with ZnS was placed on the other side of the foil. This screen helped in detecting the direction in which the a-particles moved. When these particles struck the screen, a flash of light was observed.
Rutherford observed that most of the a-particles passed through the foil without any deflection from their path and struck the screen at its centre, causing illuminations. A few of them were deflected at some angles (90° or wider angles) after passing through the foil. Very few (not more than one in 10,000) turned back on their original path. All the three types of deflections.
On the basis of the observations made by Rutherford in his experiments, combined with the discovery of neutron made by Chadwick in 1932, he proposed a complete model of the atom which is named as Rutherford's nuclear atomic model after his name. This is also called nuclear atom. According to this model:
Postulates of Rutherford's Model
1. Atom contains a massive (heavy) and positively charged part at its centre. This central part of the atom is called nucleus. The presence of the nucleus at the central part of the atom is based on the observation that a few a-particles are deflected back. It is this positively charged nucleus which repells the positively charged a- particles to go back. All the protons and neutrons are present in the nucleus of the atom. Although the nucleus is small in size, it is heavy due to the presence of all protons and neutrons in it. Thus the entire mass of the atom resides in its nucleus and is equal to the sum of the masses of protons and neutrons, since the mass of the electrons is negligible. The remaining part of the atom round the nucleus is left empty. Thus the atom contains a lot of empty space round the nucleus. The nucleus has positive charge which is different in magnitude for different elements.
2. The volume occupied by the nucleus is only a minute fraction of the total volume of the atom, i.e. the size of the nucleus is very small as compared to that of the whole atom. This fact is based on the observation that a very small fraction of a-particles turned back on their original parts without undergoing any deflection.
3. Atom is not all solid, as was earlier suggested by Dalton, but is extraordinarily hollow, since it consists of a lot of empty space round the nucleus. Most of the space round the nucleus is empty, except for the presence of electrons which themselves are extremely minute. This conclusion has been drawn from the observation that most of a-particles passed straight unrough the foil undeflected. It is this empty space around the nucleus which allows the a-particles to pass undeflected through the foil.
4. Electrons are revolving round the nucleus in closed orbits with a fast speed and hence almost all the space round the nucleus is occupied by the revolving electrons. The electrons keep revolving in orbits round the uncleus as the planets revolve round the sun. Thus Rutherford's model of atom bears a close resemblance with the solar system in which the massive sun plays the role of the massive nucleus and the planets play the role of the revolving electrons. It is because of the close resemblance of Rutherford's model with solar system that the electrons revolving round the nucleus are generally referred to as planetary electrons.
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