So far we have said nothing about neutrons. The most common type of hydrogen has none in its nucleus (right). Other kinds of hydrogen atoms have either one or two neutrons, in addition to the proton that defines their chemical character. Atoms such as these three, with the same atomic number but with different numbers of neutrons in their nuclei, are called isotopes of the same chemical element. The sum of the number of protons and neutrons is the mass number, and is written as a superscript before the symbol of the element: , , .

The three isotopes of hydrogen have quite different masses: approximately 1, 2, and 3 amu. But because the number of protons is the same, they have the same number of electrons around the nucleus. To an approaching atom, all hydrogen atoms look much the same, and exhibit virtually the same chemical behavior. The differences are important only in properties such as rates of reaction or rates of diffusion of molecules, for which the mass of an atom and its speed are important.