there are usually two values you'll come across on the periodic table.
1. the atomic number.
this represents the number of protons an element has. For example, hydrogen (H) has an atomic number of 1, which means that hydrogen has only one proton.
2. the mass number.
this usually represents the number of protons and neutrons an element has. For example, carbon (C) has a mass number of roughly 12, which means that the sum of its protons and neutrons add up to about 12. We know carbon has an atomic number of 6 (6 protons) so the number of neutrons must be 6 aswell.
(the mass numbers can differ depending on which periodic table you are given)
