The mass of an atom depends only on the mass of the nucleon, since the mass of the electrons is negligibly small. Protons and neutrons have approximately the same mass, which again is equal to the unit mass , and their total number is the mass number of atoms and is denoted by the letter A.
Explanation:
A proton (p +) is a positively charged particle whose mass is 1u, and its relative charge is q (p) = + 1.
A neutron (n0) is a neutral particle without charge, of mass approximately equal to that of protons. The relative charge q (n) = 0.
