Discovery of Neutron – discover by James Chadwick that fundamental particle which carries on charge but has a mass nearly equal to that of hydrogen atom or proton

Proof- when lighter element (like Be, B) bombarded with fast moving X-particles (2P+2N+2e)⁴₂He

                ₄⁹Be+⁴₂He→¹²₆C+¹₀n

                ₅¹¹B+⁴₂He→¹⁴₇N+¹₀n

No change

M_n=1.6749 x 10^-24g

M_p=1.6726 x 10^-24g

Nearly same or supply more than p.

Note:- Double charge helium nucleus called X-particle