What do isotopes of a given element have?
The same number of nuclear protons.
The number of nuclear protons gives
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
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.