Yes, NEGATIVITY is a Scrabble word!

Letter values in Scrabble US: N 1, E 1, G 2, A 1, T 1, I 1, V 4, I 1, T 1, Y 4.

Is it a valid word?

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

the character of the negative electric pole
characterized by habitual skepticism and a disagreeable tendency to deny or oppose or resist suggestions or commands
an amount less than zero
(chemistry) the tendency of an atom or radical to attract electrons in the formation of an ionic bond

Also: electronegativity.