An atom has a nucleus. The nucleus is positively charged. The radius of the nucleus is smaller than the radius of an atom by a factor of 104. More than 99.9% mass of the atom is concentrated in the nucleus. 

A nucleus contains a neutral particle called neutron. Its mass is almost the same as that of proton. 

The atomic number Z is the number of protons in the atomic nucleus of an element. The mass number A is the total number of protons and neutrons in the atomic nucleus; A = Z+N; Here N denotes the number of neutrons in the nucleus. 

A nuclear species or a nuclide is represented as ^A_ZX, where X is the chemical symbol of the species. A nucleus can be considered to be spherical in shape and assigned a radius. Electron scattering experiments allow determination of the nuclear radius; it is found that radii of nuclei fit the formula R = R₀ A^1/3, 

where R₀ = a constant = 1.2 fm. This implies that the nuclear density is independent of A. It is of the order of 10¹⁷ kg/m³. 

Neutrons and protons are bound in a nucleus by the short-range strong nuclear force. The nuclear force does not distinguish between neutron and proton.