letter of the alphabet vs alphabet

letter of the alphabet and alphabet both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter of the alphabet	Yes	No	No	No
alphabet	Yes	No	No	No

letter of the alphabet (noun)	alphabet (noun)
the conventional characters of the alphabet used to represent speech	the elementary stages of any subject (usually plural)
	a character set that includes letters and is used to write a language