Northern United States vs U.S.

Northern United States and U.S. both are nouns.

Word	Noun	Adjective	Verb	Adverb
Northern United States	Yes	No	No	No
U.S.	Yes	No	No	No

Northern United States (noun)	U.S. (noun)
the region of the United States lying to the north of the Mason-Dixon line	the executive and legislative and judicial branches of the federal government of the United States
	North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776