The number of protons in a nucleus decides which element an atom is. Each element has its own characteristic positive charge on its nucleus, because each proton carries a charge of +1. All carbon nuclei have 6 protons and all oxygen nuclei have 8 protons.
Atoms of the same element can still differ. Isotopes are atoms with the same number of protons but different numbers of neutrons. Because neutrons have mass but no charge, isotopes of an element have different masses but the same nuclear charge.
For example, hydrogen has isotopes with 0, 1 or 2 neutrons. They all have one proton, so they are all hydrogen, but the one with 2 neutrons is about three times as heavy as the one with none.
