Atom is a noun. Hydrogen atom is a noun pair. It is a matter of debate whether the first word in a noun pair is an actual adjective, but it functions like one. Cf. coat rack, etc.
See the discussion about this very matter, which @Kosmonaut and I had in chat last year.
Here is the discussion, minus the interwoven chat threads that are not germane to the issue:
Hey, @Kosmonaut, been meaning to ask you a question. Do noun pairs always subordinate one noun into adjectival status, or do they ever coexist as equal, independent nouns. In The Marble Faun it feels to me that marble serves as an adjective, but I don't quite feel that princess is completely adjectival in The Princess Bride. Any thoughts?
In fact, I would say that those two words [marble and faun] are functioning as adjectives.
I know what you're saying, but if two states of being are coequal, or the one that comes first actually forms a more applicable noun (whatever that means; work with me here), can't noun pairs simply be appositive and not in an adjectival-nominal relationship?
Noun pairs can certainly be in a noun-noun relationship, just not these two cases.
One way that often sheds light on this is the prosody of the phrase.
Can you give me some examples of legitimate noun-noun combos?
But doesn't "hat" get subordinated to "rack" in that combo?
A "hat rack" is a type of rack for holding hats.
In adj+noun pairs, usually the primary stress falls on the noun. But in noun+noun pairs, the stress falls on the first noun.
I'm actually going by pure phonology/syntax to decide this — how they behave in the phrase, and not semantics.
Ahh, interesting distinction. Hence my confusion with "princess bride".
The stress is on the princess.
[Note: I disagreed with @Reg on this, and still feel the stress is on "bride" in The Princess Bride, but that did not get entered into the text.]
But, your followup question about which thing is central to the meaning, that's an interesting question unto itself.
I just wouldn't use that to determine the lexical category.
Ah, you've seen The Adjective Noun!
No, but I've seen The Noun Noun.