Yes, STRONTIUMS is a Scrabble word!

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

Is it a valid word?

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

a soft silver-white or yellowish metallic element of the alkali metal group; turns yellow in air; occurs in celestite and strontianite

Also: atomic number 38.

Not playable in Scrabble US: SR.