heavy hydrogen vs hydrogen atom

heavy hydrogen and hydrogen atom both are nouns.

Word	Noun	Adjective	Verb	Adverb
heavy hydrogen	Yes	No	No	No
hydrogen atom	Yes	No	No	No

As nouns, hydrogen atom is a hypernym of heavy hydrogen; that is, hydrogen atom is a word with a broader meaning than heavy hydrogen:

heavy hydrogen: an isotope of hydrogen which has one neutron (as opposed to zero neutrons in hydrogen)
hydrogen atom: an atom of hydrogen

Other hypernyms of heavy hydrogen include isotope.

heavy hydrogen (noun)	hydrogen atom (noun)
an isotope of hydrogen which has one neutron (as opposed to zero neutrons in hydrogen)	an atom of hydrogen