Via an unusual, poorly understood mechanism, this metal explodes on contact with perchloric acid. A compound of this metal with an unusual crystal structure includes tubes made of rhombic dodecahedra of this element and oxygen that are partially filled by more oxygen; that substance, formed by heating this metal’s oxychloride, is Arppe’s compound. A compound of sodium and this metal is a “3D graphene” that is the first known three-dimensional topological Dirac semimetal. This metal’s vanadate is the most powerful known catalyst for the (*) photooxidation of water. Most topological insulators are based on this metal; (-5[1])the first known one was this metal’s antimonide. (10[2]-5[1])Besides pyrolytic carbon, this metal is the most diamagnetic substance known. (10[2])In 2003, this metal’s 209 isotope was found (10[1])to be very weakly radioactive. (10[1])For 10 points, name this metal that was once thought to be the heaviest (10[1])stable element. (10[1])■END■ (10[2])