Isotopes of an element have the same number of protons in their atomic nuclei, but different numbers of neutrons.

For example, all hydrogen atoms have a single proton. (The atomic number of hydrogen is therefore one.)

However, the most common isotope of hydrogen atom has no neutrons, while the ‘heavy’ hydrogen isotope called deuterium has one neutron. The ‘heavier’-still hydrogen isotope called tritium has two neutrons in its nucleus. But all three isotopes still have the one proton, which is what makes them hydrogen atoms, and not, say, oxygen atoms with eight protons.