jr. vs Junior

jr. is not a noun while Junior is a noun.

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

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

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

Other synonyms of jr. include younger.

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

jr. (adjective)	Junior (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