burden vs unburden

burden is a noun but unburden is not a noun.

Word	Noun	Adjective	Verb	Adverb
burden	Yes	No	Yes	No
unburden	No	No	Yes	No

burden (noun)	unburden (noun)
weight to be borne or conveyed
the central idea that is expanded in a document or discourse
an onerous or difficult concern
the central meaning or theme of a speech or literary work

burden (verb)	unburden (verb)
impose a task upon, assign a responsibility to	take the burden off; remove the burden from
weight down with a load	free or relieve (someone) of a burden