What Is Mass Number (A)?

We know that the nucleus of an atom contains neutrons and protons. The sum of the number of neutrons (n) and protons (p) present in the nucleus of an atom is called the mass number (A) of that atom, i.e.

Mass Number (A) = No. of protons (p) + No. of neutrons (n)

or, A = p+n

In cases where it is not necessary to differentiate between protons and neutrons, these particles are collectively called nucleons. Thus the mass number (A) of an atom is equal to the total number of nucleons in the nucleus of an atom. Obviously the mass number is a whole number.

Each different variety of atom as determined by the composition of its nucleus is called a nucleotide.