Yes, ATOMISM is a Scrabble word!

Letter values in Scrabble US: A 1, T 1, O 1, M 3, I 1, S 1, M 3.

Is it a valid word?

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

(psychology) a theory that reduces all mental phenomena to simple elements (sensations and feelings) that form complex ideas by association
(chemistry) any theory in which all matter is composed of tiny discrete finite indivisible indestructible particles

Also: atomic theory, atomist theory, atomistic theory.