A rare metallic element that is liquid near room temperature, expands on solidifying, and is found as a trace element in coal, bauxite, and other minerals. It is used in semiconductor technology, as a component of various low-melting alloys, and in producing blue light-emitting diodes. Atomic number 31; atomic weight 69.72; melting point 29.78°C; boiling point 2,403°C; specific gravity 5.907; valence 2, 3. See Periodic Table.