After the discovery of nucleus in an atom, Rutherford found that the fast moving α-particles transmuted nitrogen into oxygen with simultaneous liberation of hydrogen. \(_7^{14}N +_2^4\alpha \longrightarrow ^{17}_8O + _1^1H\)
He further showed that other elements could also be transmuted similarly and hydrogen was always emitted in the process.
Based on these observations, he proposed that the hydrogen nucleus must be contained inside nuclei of all the elements. Hence, he renamed hydrogen nucleus as proton.