bomb vs atom-bomb

bomb is a noun but atom-bomb is not a noun.

Word	Noun	Adjective	Verb	Adverb
bomb	Yes	No	Yes	No
atom-bomb	No	No	Yes	No

As verbs, atom-bomb is a hyponym of bomb; that is, atom-bomb is a word with a more specific, narrower meaning than bomb:

bomb (noun)	atom-bomb (noun)
an explosive device fused to explode under specific conditions
strong sealed vessel for measuring heat of combustion
an event that fails badly or is totally ineffectual

bomb (verb)	atom-bomb (verb)
throw bombs at or attack with bombs	bomb with atomic weapons
fail to get a passing grade