A neutral atom's mass number is equal to the total number of protons and neutrons in its nucleus. The mass number is often represented by the symbol 'A'. In a neutral atom, the number of protons (which determines the element) is balanced by the number of electrons, resulting in a net charge of zero.
For example, a neutral carbon atom has a mass number of 12. This means it has 6 protons (since carbon's atomic number is 6) and 6 neutrons in its nucleus. The sum of protons and neutrons gives the mass number.
It's important to note that the mass number specifically refers to an atom of a particular isotope, as different isotopes of an element have different numbers of neutrons and, therefore, different mass numbers.