serious vs seriousness

serious is not a noun while seriousness is a noun.

Word	Noun	Adjective	Verb	Adverb
serious	No	Yes	No	No
seriousness	Yes	No	No	No

serious (noun)	seriousness (noun)
	the trait of being serious
	the quality of arousing fear or distress
	an earnest and sincere feeling

serious (adjective)	seriousness (adjective)
concerned with work or important matters rather than play or trivialities
completely lacking in playfulness
causing fear or anxiety by threatening great harm
requiring effort or concentration; complex and not easy to answer or solve
of great consequence
appealing to the mind