plural of thorium

A chemical element (symbol Th) with atomic number 90: a weakly radioactive, malleable, moderately hard silvery metal that tarnishes black when exposed to air.

(physics) A radioactive isotope of thorium, ²²⁸₉₀Th, having a half-life of 1.4 x 10¹⁰ years; it occurs naturally, and is a source of nuclear energy