Yes, PLATINUM is a Scrabble word!

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

Is it a valid word?

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

a heavy precious metallic element; grey-white and resistant to corroding; occurs in some nickel and copper ores and is also found native in some deposits

Also: atomic number 78.

Not playable in Scrabble US: PT.