Yes, YTTERBIUMS is a Scrabble word!

Letter values in Scrabble US: Y 4, T 1, T 1, E 1, R 1, B 3, I 1, U 1, M 3, S 1.

Is it a valid word?

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

a soft silvery metallic element; a rare earth of the lanthanide series; it occurs in gadolinite and monazite and xenotime

Also: atomic number 70.

Not playable in Scrabble US: YB.