Yes, DEADNESS is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, A 1, D 2, N 1, E 1, S 1, S 1.

Is it a valid word?

DEADNESS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the quality of being unresponsive; not reacting; as a quality of people, it is marked by a failure to respond quickly or with emotion to people or events
the physical property of something that has lost its elasticity
the inanimate property of something that has died

Also: unresponsiveness.