In 1920, Rutherford found that except hydrogen atom, the atomic masses of other atoms could not be explained only the basis of only electrons and protons. For example, helium atom contains two protons in it. As a result, the atomic mass of helium should be double than that of a hydrogen atom. From the e/make values, it was discovered that the mass of helium atom (to be precise, that of a helium ion), is four times the mass of a hydrogen atom. To explain this, Rutherford in 1920, predicted the presence of another kind or particle in atoms. He further predicted that such a particle should be electrically neutral, and have a mass equal to that of a proton.

Characteristics of a neutron

(i) Mass of a neutron: Mass of a neutron is equal to that of a proton. Thus , mass of a neutron = 1.676 ×10-27 kg= 1.676 ×10-24g

(ii) Charge on a neutrons = Neutrons do not carry any electrician charge. So, neutrons are neutral particles.