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

As adjectives, Junior and jr. are synonyms defined as:

Junior and jr.: used of the younger of two persons of the same name especially used to distinguish a son from his father

Other synonyms of Junior include younger.

Junior (noun)	jr. (noun)
a son who has the same first name as his father

Junior (adjective)	jr. (adjective)
used of the younger of two persons of the same name especially used to distinguish a son from his father	used of the younger of two persons of the same name especially used to distinguish a son from his father