Rutherford's alpha-scattering experiment recognized that the mass of atom is concentrated with small positively charged region at the centre which is known as 'nucleus'.

Nuclei are made up of neutron and proton. The quantity of protons in a nucleus (known the atomic number or proton number) is shown by the symbol Z. The number of neutrons (neutron number) is represented by N. The total quantity of neutrons and protons in a nucleus is known it's mass number A so A = Z + N.

Neutrons and proton, when explained collectively are known as nucleons.

Nucleus have two types of particles : Protons and neutrons