Yes, RADICAL is a Scrabble word!

Letter values in Scrabble US: R 1, A 1, D 2, I 1, C 3, A 1, L 1.

Is it a valid word?

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

(chemistry) two or more atoms bound together as a single unit and forming part of a molecule
an atom or group of atoms with at least one unpaired electron; in the body it is usually an oxygen molecule that has lost an electron and will stabilize itself by stealing an electron from a nearby molecule
a person who has radical ideas or opinions
(mathematics) a quantity expressed as the root of another quantity
a character conveying the lexical meaning of a logogram
(linguistics) the form of a word after all affixes are removed

(used of opinions and actions) far beyond the norm
markedly new or introducing radical change
arising from or going to the root or source
of or relating to or constituting a linguistic root
especially of leaves; located at the base of a plant or stem; especially arising directly from the root or rootstock or a root-like stem

Also: chemical group, free radical, root word.