Define “ Mass number”.

1. Mass number is defined as the total number of protons and neutrons in the nucleus of the atom.
2. $\mathrm{Mass}\mathrm{number}=\mathrm{No}.\mathrm{of}\mathrm{protons}+\mathrm{No}.\mathrm{of}\mathrm{neutrons}$
3. It is written at the superscript of the atom and is denoted by A.
4. For example, Carbon has a mass number of 12 which depicts that it has 6 protons and 6 neutrons.