The hydrogen atom, #""^1H#, is the most abundant atom in the universe....
Explanation:
The so-called #"protium isotope"#, #""^1H#, has ONE nuclear proton in its nucleus, around which a SINGLE electron whizzes around... Hydrogen has several other isotopes, e.g. #"deuterium isotope"#, #""^2H#, and #"tritium isotope"#, #""^3H#. ALL these isotopes contain the ONE proton, but how many neutrons does each isotope contain?