Yes, PHOSPHORUS is a Scrabble word!

Letter values in Scrabble US: P 3, H 4, O 1, S 1, P 3, H 4, O 1, R 1, U 1, S 1.

Is it a valid word?

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

a multivalent nonmetallic element of the nitrogen family that occurs commonly in inorganic phosphate rocks and as organic phosphates in all living cells; is highly reactive and occurs in several allotropic forms
a planet (usually Venus) seen just before sunrise in the eastern sky

Also: p, atomic number 15, morning star.