Yes, RADON is a Scrabble word!

Letter values in Scrabble US: R 1, A 1, D 2, O 1, N 1.

Is it a valid word?

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

a radioactive gaseous element formed by the disintegration of radium; the heaviest of the inert gasses; occurs naturally (especially in areas over granite) and is considered a hazard to health

Also: atomic number 86.

Not playable in Scrabble US: RN.