atom bomb vs atomic weapon

atom bomb and atomic weapon both are nouns.

Word	Noun	Adjective	Verb	Adverb
atom bomb	Yes	No	No	No
atomic weapon	Yes	No	No	No

As nouns, atomic weapon is a hypernym of atom bomb; that is, atomic weapon is a word with a broader meaning than atom bomb:

atom bomb: a nuclear weapon in which enormous energy is released by nuclear fission (splitting the nuclei of a heavy element like uranium 235 or plutonium 239)
atomic weapon: a weapon of mass destruction whose explosive power derives from a nuclear reaction

Other hypernyms of atom bomb include bomb, nuclear weapon.

atom bomb (noun)	atomic weapon (noun)
a nuclear weapon in which enormous energy is released by nuclear fission (splitting the nuclei of a heavy element like uranium 235 or plutonium 239)	a weapon of mass destruction whose explosive power derives from a nuclear reaction