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; the first known one was this metal’s antimonide. Besides pyrolytic carbon, this metal is the most diamagnetic substance known. In 2003, this metal’s 209 isotope was found to be very weakly radioactive. For 10 points, name this metal that was once thought to be the heaviest stable element. ■END■
ANSWER: bismuth [or Bi]
<Gerhardt Hinkle, Chemistry>
= Average correct buzz position