junior vs jr.

junior is a noun but jr. is not a noun.

Word	Noun	Adjective	Verb	Adverb
junior	Yes	Yes	No	No
jr.	No	Yes	No	No

junior (noun)	jr. (noun)
the younger of two persons
a third-year undergraduate
term of address for a disrespectful and annoying male

junior (adjective)	jr. (adjective)
younger; lower in rank; shorter in length of tenure or service	used of the younger of two persons of the same name especially used to distinguish a son from his father
used of the third or next to final year in United States high school or college
including or intended for youthful persons