chain of mountains vs Atlas Mountains

chain of mountains and Atlas Mountains both are nouns.

Word	Noun	Adjective	Verb	Adverb
chain of mountains	Yes	No	No	No
Atlas Mountains	Yes	No	No	No

chain of mountains (noun)	Atlas Mountains (noun)
a series of hills or mountains	a mountain range in northern Africa between the Mediterranean and the Sahara Desert; extends from southwestern Morocco to northern Tunisia