What part of speech is “atom” in “hydrogen atom”?

What is the type (adjective, noun, etc.) of the word atom in hydrogen atom?

I think that atom here does not qualify hydrogen in any way and we can use it or not, and the meaning of the word hydrogen will not change. So, hydrogen atom and hydrogen are equivalent.

Answer

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.

ADDENDUM

Here is the discussion, minus the interwoven chat threads that are not germane to the issue:

Robusto
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?

Kosmonaut
In fact, I would say that those two words [marble and faun] are functioning as adjectives.
Even princess.

Robusto
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?

Kosmonaut
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.

Robusto
Can you give me some examples of legitimate noun-noun combos?

Kosmonaut
poster board
hat rack

Robusto
But doesn’t “hat” get subordinated to “rack” in that combo?
A “hat rack” is a type of rack for holding hats.

Kosmonaut
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.

Robusto
Ahh, interesting distinction. Hence my confusion with “princess bride”.

RegDwight
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.]

Kosmonaut
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.

Kosmonaut
Ah, you’ve seen The Adjective Noun!

Robusto
No, but I’ve seen The Noun Noun.

Attribution
Source : Link , Question Author : Zeynel , Answer Author : Robusto

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