Yes, TERBIUMS is a Scrabble word!

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

Is it a valid word?

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

a metallic element of the rare earth group; used in lasers; occurs in apatite and monazite and xenotime and ytterbite

Also: atomic number 65.

Not playable in Scrabble US: TB.