mental vs rational

mental is not a noun while rational is a noun.

Word	Noun	Adjective	Verb	Adverb
mental	No	Yes	No	No
rational	Yes	Yes	No	No

mental (noun)	rational (noun)
	an integer or a fraction

mental (adjective)	rational (adjective)
involving the mind or an intellectual process	consistent with or based on or using reason
of or relating to the mind	capable of being expressed as a quotient of integers
of or relating to the chin- or liplike structure in insects and certain mollusks	having its source in or being guided by the intellect (as distinguished from experience or emotion)
of or relating to the chin or median part of the lower jaw	of or associated with or requiring the use of the mind
affected by a disorder of the mind