The sum of the number of protons and neutrons present in the nucleus of an atom is called the mass number. The mass number is denoted by A . Thus, mass number A = No. of protons + No. of neutrons. The mass number of an element is nearly the same as its atomic mass but the latter has certain units of measurements. However, whereas the mass number is always a whole number, the atomic mass of the element may not be a whole number always.

Nucleons are the sub-atomic particles that are present in the nucleus of an atom. The protons and the neutrons are present in the nucleus of an atom and are collectively called as the nucleons. Protons are positively charged particles, and neutrons have no charge. The nucleons make up most of the mass of an atom.