The highest-oxidation-state stable oxyanion of this element was originally made by radioactive decay of a different element’s oxyanion. Adding this element to the 6 and 6-prime positions of indigo yields Tyrian purple. In mass spectrometry, this element is identified by two peaks of the same height two mass units apart. (15[3]-5[1])The carboxyl group of a silver carboxylate is replaced with this element in the (-5[1])(*) Hunsdiecker reaction. The presence of peroxides makes the addition (10[1])of this element’s binary acid to alkenes anti-Markovnikov. Because organic compounds containing this (-5[1])element release this element’s radicals when heated, they are common flame retardants. Double bonds can be detected by using a solution of this element, which turns colorless in their presence. For 10 points, name this red-brown halogen that is a liquid at STP. ■END■ (10[1])