The number of protons in the nucleus of an atom determines
what element the atom is an atom of.
If there's one proton in the nucleus of an atom, then the atom
is an atom of Hydrogen.
If there's any other number of protons other than one in the nucleus
of an atom, then the atom is an atom of some other element. It's not
an atom of Hydrogen.
If you say you have an atom of Hydrogen, then I know there's exactly
one and only one proton in its nucleus. If you call it Hydrogen-2, I know
you must be talking about the Atomic Weight of the nucleus, and there
must be a neutron in there too.
If you called it Hydrogen-37, I would know that there would still be exactly
one proton there, lost in a crowd of 36 neutrons.
