extra vs additional

extra is a noun but additional is not a noun.

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

As adjectives, extra and additional are synonyms defined as:

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

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

extra (adverb)	additional (adverb)
unusually or exceptionally