atom vs hydrogen atom

atom and hydrogen atom both are nouns.

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

As nouns, hydrogen atom is a hyponym of atom; that is, hydrogen atom is a word with a more specific, narrower meaning than atom:

atom: (physics and chemistry) the smallest component of an element having the chemical properties of the element
hydrogen atom: an atom of hydrogen

Other hyponyms of atom include isotope, monad, carbon atom, free radical, radical.

atom (noun)	hydrogen atom (noun)
(physics and chemistry) the smallest component of an element having the chemical properties of the element	an atom of hydrogen
(nontechnical usage) a tiny piece of anything