It has already been stated that mass of an atom is mainly concentrated in the nucleus. In the nucleus, there are protons and neutrons. From this, it follows that mass of an atom is mainly due to protons and neutrons. Protons and neutrons are collectively called nucleons.
The total numbers of protons and neutrons in the nucleus is called mass number of the atom of the element. It is generally represented by the letter A.
Mass Number (A) = Number of protons + Number of neutrons
= Number of nucleons.