sr. vs senior

sr. is not a noun while senior is a noun.

Word	Noun	Adjective	Verb	Adverb
sr.	No	Yes	No	No
senior	Yes	Yes	No	No

sr. (noun)	senior (noun)
	an undergraduate student during the year preceding graduation
	a person who is older than you are

sr. (adjective)	senior (adjective)
used of the older of two persons of the same name especially used to distinguish a father from his son	older; higher in rank; longer in length of tenure or service
	used of the fourth and final year in United States high school or college
	advanced in years; (`aged' is pronounced as two syllables)