Yes, MENTUM is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, N 1, T 1, U 1, M 3.

Is it a valid word?

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

a projection like a chin formed by the sepals and base of the column in some orchids
the protruding part of the lower jaw
a projection below the mouth of certain mollusks that resembles a chin