one vs united

one is a noun but united is not a noun.

Word	Noun	Adjective	Verb	Adverb
one	Yes	Yes	No	No
united	No	Yes	No	No

one (noun)	united (noun)
a single person or thing
the smallest whole number or a numeral representing this number

one (adjective)	united (adjective)
used of a single unit or thing; not two or more	characterized by unity; being or joined into a single entity
being a single entity made by combining separate components	of or relating to two people who are married to each other
(informal) very; used informally as an intensifier	involving the joint activity of two or more
eminent beyond or above comparison
indefinite in time or position
of the same kind or quality
having the indivisible character of a unit