Yes, LITHIUM is a Scrabble word!

Letter values in Scrabble US: L 1, I 1, T 1, H 4, I 1, U 1, M 3.

Is it a valid word?

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

a soft silver-white univalent element of the alkali metal group; the lightest metal known; occurs in several minerals

Also: atomic number 3.