The atomic nucleus is composed of protons and neutrons.

The atomic number, Z give the number of protons in the nucleus.

The number of neutrons in the nucleus is the neutron number, N. The number of nucleons is mass number, A.

Atomic nuclei with the same atomic number, Z, but different neutron numbers, N, are isotopes of the same element.

The atomic mass of an element is the weighted average of the masses of its isotopes.