The mass number of an atomic nucleus is

The mass number $A$ of an atomic nucleus is the sum of the number of protons $Z$ (atomic number) and the number of neutrons $N$. This gives the relation $A = Z + N$.

For an ordinary hydrogen nucleus (^{1}H$), there are no neutrons ($N = 0$), so its mass number is equal to its atomic number ($A = Z = 1$).<br><br>For all other atomic nuclei, there is at least one neutron ($N > 0$), making the mass number strictly greater than the atomic number ($A > Z).

Therefore, the mass number is sometimes larger than and sometimes equal to its atomic number.