impenetrability vs impenetrable

impenetrability is a noun but impenetrable is not a noun.

Word	Noun	Adjective	Verb	Adverb
impenetrability	Yes	No	No	No
impenetrable	No	Yes	No	No

impenetrability (noun)	impenetrable (noun)
the quality of being impenetrable (by people or light or missiles etc.)
incomprehensibility by virtue of being too dense to understand

impenetrability (adjective)	impenetrable (adjective)
	not admitting of penetration or passage into or through
	impossible to understand
	permitting little if any light to pass through because of denseness of matter