seventh vs 7th

seventh is a noun but 7th is not a noun.

Word	Noun	Adjective	Verb	Adverb
seventh	Yes	Yes	No	No
7th	No	Yes	No	No

As adjectives, seventh and 7th are synonyms defined as:

seventh and 7th: coming next after the sixth and just before the eighth in position

seventh (noun)	7th (noun)
the musical interval between one note and another seven notes away from it
one part in seven equal parts
position seven in a countable series of things

seventh (adjective)	7th (adjective)
coming next after the sixth and just before the eighth in position	coming next after the sixth and just before the eighth in position