- The total number of protons in the nucleus of an atom gives us the atomic number of that atom. It is represented with the letter ‘Z.’ All the atoms of a particular element have the same number of protons, and hence the same atomic number.
- The number of protons and neutrons combined give us the mass number of an atom. It is represented using the letter ‘A.’ As both protons and neutrons are present in the nucleus of an atom, they are together called nucleons.
Mass number = Number of protons + Number of neutrons
31 = Number of protons + Number of neutrons
∴ Number of neutrons = 31 – Number of protons
Number of neutrons = 31 – 15
Number of neutrons = 16