seriousness vs serious

seriousness is a noun but serious is not a noun.

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

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

seriousness (adjective)	serious (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