What is a mass number (A) ?

Mass number of an element is denoted by A. The mass number of an element is equal to the sum of the number of protons (P) and number of neutrons (N). Thus,

Mass number (A) = No. of protons (P) + No. of neutrons (N)

Or, A = P + N

The number of protons (P) inside the nucleus of an atom is equal to the atomic number (Z) of that element. So, Mass number (A) = atomic number (Z) + No. of neutrons (N)

Or, A = Z + N

The mass number (A) and atomic number (Z) of an element are shown as superscript and subscript respectively, only the left side of the symbol of that element. For example, if A is the mass number, and with is the atomic number of an element X.