additional vs extra

additional is not a noun while extra is a noun.

Word	Noun	Adjective	Verb	Adverb
additional	No	Yes	No	No
extra	Yes	Yes	No	Yes

As adjectives, additional and extra are synonyms defined as:

additional (noun)	extra (noun)
	something additional of the same kind
	an additional edition of a newspaper (usually to report a crisis)
	a minor actor in crowd scenes

additional (adjective)	extra (adjective)
further or added	further or added
	more than is needed, desired, or required
	added to a regular schedule

additional (adverb)	extra (adverb)
	unusually or exceptionally