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釷 1. See "钍" (Thorium). Noun (thorium, Th) A chemical element. Atomic number 90. It is a metallic element belonging to the actinide series, found in thorium ore or obtained by reducing thorium fluoride and thorium dioxide, or by electrolysis of thorium chloride, which can also yield metallic thorium. It has ductility, radioactivity, and is insoluble in water and alkalis but soluble in acids. In the air, it changes from silvery-white to dark gray or black and is a raw material for nuclear energy. It glows at high temperatures in the air and can be used to manufacture sunlight lamps.