Give an evidence for the existence of nucleus in an atom.
                           Or
What important informations are furnished about the nucleus of an atom by α-particle scattering experiment of Rutherford?

When Rutherford bombarded thin sheet of gold foil with α -particles (α-particles are doubly charged helium ions, He²⁺), he found that:
(i) Most of the α-particles passes through the foil without any deflection. He calculated that only one particle in 10⁵ bounced back. This shows that most of the space inside the atom is empty and hollow.

(ii) Some of the α-particles were deflected through various angles while a very small number were actually deflected by as much as 180°.
This shows that:
(a) There is a heavy positively charged centre inside the atom. This centre is known as nucleus.

(b) Since only a very small fraction of α-particles were deflected through large angles, the nucleus is situated in a very small volume of the atom and is positively charged. The nucleus was found to be about 10⁵ times smaller than the total area occupied by the atom as a whole.

(c) Since α-particles deflected by the nucleus have an appreciable mass, it means that the entire mass of the atom lies inside the nucleus.