Question

Why are all hydrogen atoms not identical?

Answer 1

Because of the existence of isotopes....

Explanation:

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.

You will have to look up the isotopic abundances on the web....