What do isotopes of a given element have?

The number of nuclear protons gives #Z#, the atomic number, which defines the identity of the element. For #Z=1#, we have #H#, for #Z=2#, we have #He#, for #Z=3#, we have #Li#, ,,,,,,,,,for #Z=23#, we have #V#, etc.

The nucleus of an atom can also contain various numbers of neutral, massive particles, i.e. neutrons, and different numbers of neutrons gives rise to the phenomenon of isotopes. This is probably best illustrated by the example of isotopic hydrogen.

Most hydrogen nuclei (in the universe!) contain just the one massive particle, a proton, to give the protium isotope, which we would represent as #""^1H#. A smaller number of hydrogen nuclei contain a proton AND a neutron, to give the deuterium isotope, #""^2H#. And still a smaller number of hydrogen nuclei would contain a proton AND TWO neutrons, to give the tritium isotope, #""^3H#.

Note that the isotopes are chemically identical, but because of the different masses the so-called isotopomers (the chemical compounds that are labelled with different isotopes) may have a convenient spectroscopic label, which allows some very elegant experiments.