# Why are all hydrogen atoms not identical?

An hydrogen atom has $Z = 1$....the hydrogen atom can have several isotopes.... the which differ by the number of neutrons, massive nuclear particles of ZERO charge, present in the nucleus...
And so we gots ""^1H, protium isotope; ""^2H, the deuterium isotope; and ""^3H, the tritium isotope...these are the common isotopes of hydrogen, and contain $0$, $1$, and $2$ neutrons respectively.