Yes, RUBIDIUMS is a Scrabble word!

Letter values in Scrabble US: R 1, U 1, B 3, I 1, D 2, I 1, U 1, M 3, S 1.

Is it a valid word?

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

a soft silvery metallic element of the alkali metal group; burns in air and reacts violently in water; occurs in carnallite and lepidolite and pollucite

Also: atomic number 37.

Not playable in Scrabble US: RB.