The key difference between protium and deuterium is that protium has no neutrons in its atomic nucleus, whereas deuterium has one neutron.
Protium and deuterium are isotopes of hydrogen. Therefore, they differ from each other according to the number of neutrons present in their atomic nuclei. Hydrogen contains one proton in the atomic nucleus: thus, the atomic number of hydrogen is 1. There are three isotopes of hydrogen. All three isotopes also contain one proton. We can denote the three isotopes as 1H, 2H and 3H. The values in superscript are the atomic masses of these elements.
