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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.

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They are both nouns. Hydrogen atom is a compound noun. The word hydrogen alone doesn't refer to a single atom but the element hydrogen in general. –  z7sg Ѫ Jan 10 '12 at 14:09
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Please do not vote to close this. It is a legitimate question. See the discussion Kosmonaut and I had on this very topic last year, referenced in my answer. The OP is not expressing the idea well, but I believe this is the point of the question. –  Robusto Jan 10 '12 at 14:54
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+1 : This is an excellent question, the down/close votes are disheartening –  cindi Jan 10 '12 at 15:10
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You've got it backwards. In hydrogen atom, it's hydrogen that qualifies atom, not the other way round. We are not speaking French. –  RegDwigнt Jan 10 '12 at 15:38
    
For scientific purposes it is sometimes necessary to distinguish between individual hydrogen atoms and the usual free form which is a diatmoic molecule. –  dmckee Jan 10 '12 at 15:47
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2 Answers

Atom is a countable noun. Hydrogen is a mass noun. Hydrogen does not have the same meaning as hydrogen atom.

Hydrogen atom is another way to say atom of hydrogen (just like ice cube is another way to say cube of ice). You need to say atom of hydrogen (or 100 ml of hydrogen, or something similar) because hydrogen is a mass noun.

So atom (of) does qualify hydrogen.

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Atom doesn't qualify hydrogen, hydrogen qualifies atom. It tells what kind of atom it is. –  Phoenix Jan 10 '12 at 15:19
    
Besides not addressing the OP question, it is wrong. Hydrogen qualifies atom, and not the other way round. –  Eduardo Jan 10 '12 at 15:27
    
I have used the term qualify in the sense that the OP used it. In the OP's sense, atom certainly does qualify hydrogen, as in atom of hydrogen or hydrogen atom. The OP's question was: what are the word classes of atom and hydrogen, and why can't I just say a hydrogen? I have answered both those points. However, I didn't clearly state the connection between hydrogen atom and an atom of hydrogen, so I've amended that now. –  Pitarou Jan 10 '12 at 21:39
    
I see a difference between fruit basket, and a basket of fruit. –  kiamlaluno Jan 11 '12 at 19:35
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A fruit basket is a basket whose purpose is to hold fruit. Remove the fruit and it is still a fruit basket. The closest equivalent would be a "basket for fruit." A basket of fruit would refer to the quantity of fruit. –  dave Jan 11 '12 at 19:50
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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.

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+1 : it would be interesting if you could elaborate on that debate. –  cindi Jan 10 '12 at 15:05
    
Well for me this is a noun enclosed in 2 words, and in German that is definitely going to be one word. And I don't like Noun pair, if there are three nouns what is it going to be - noun triplet? :D –  speedyGonzales Jan 10 '12 at 15:37
    
@speedyGonzales Would adj+noun be one word in German? –  Kris Jan 12 '12 at 9:05
    
@Kris: If they make it so. Selbstbedienungsrestaurant would be such a combo, I believe. –  Robusto Jan 12 '12 at 12:21
    
@Robusto初夢 Selbstbedienungsrestaurant is a word. Don't if 'they made it so'. :) –  Kris Jan 12 '12 at 12:25
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