grieving vs mourning

grieving is not a noun while mourning is a noun.

Word	Noun	Adjective	Verb	Adverb
grieving	No	Yes	No	No
mourning	Yes	Yes	No	No

As adjectives, grieving and mourning are synonyms defined as:

Other synonyms of grieving include bereaved, bereft, grief-stricken, sorrowing.

grieving (noun)	mourning (noun)
	the passionate and demonstrative activity of expressing grief
	state of sorrow over the death or departure of a loved one

grieving (adjective)	mourning (adjective)
sorrowful through loss or deprivation	sorrowful through loss or deprivation