Yes, POTASSIUM is a Scrabble word!

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

Is it a valid word?

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

a light soft silver-white metallic element of the alkali metal group; oxidizes rapidly in air and reacts violently with water; is abundant in nature in combined forms occurring in sea water and in carnallite and kainite and sylvite

Also: k, atomic number 19.