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Conclusions of a-Scattering Experiment:
On the basis of the above observation, and having realized that the rebounding α-particles had met something even more massive than themselves inside the gold atom, Rutherford proposed an atomic model as follows.
- All the +ve charge and nearly the total mass of an atom is present in a very small region at the centre of the atom. The atom’s central core is called nucleus.
- The size of the nucleus is very small in comparison to the size of the atom. Diameter of the nucleus is about 10–13cm while the atom has a diameter of the order of 10–8 cm. So, the size of atom is 105 times more than that of nucleus.
- Most of the space outside the nucleus is empty.
- The electrons, equal in number to the net nuclear positive charge, revolve around the nucleus with fast speed just like planets around the sun.
- The centrifugal force arising due to the fast speed of an electron balances the coulombic force of attraction of the nucleus and the electron remains stable in its path. Thus according to him atom consists of two parts (a) nucleus and (b) extra nuclear part.
Defects in Rutherford’s Atomic Model:
- Position of electrons: The exact positions of the electrons from the nucleus are not mentioned.
- Stability of the atom: Bohr pointed out that Rutherford’s atom should be highly unstable. According to the law of electro–dynamics, when a charged body moves under the influence of an attractive force, it loses energy continuously in the form of electromagnetic radiation. The electron should therefore, continuously emit radiation and lose energy. As a result of this a moving electron will come closer and closer to the nucleus and after passing through a spiral path, it should ultimately fall into the nucleus.
It was calculated that the electron should fall into the nucleus in less than 10–8 sec. But it is known that electrons keep moving outside the nucleus.
To solve this problem Neils Bohr proposed an improved form of Rutherford’s atomic model.