The highest-oxidation-state stable oxyanion of this element was originally made by radioactive decay of a different element’s oxyanion.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 (15[1])height (15[2])two (15[1])mass (15[1])units (15[1])apart. (15[5]-5[1])The carboxyl group (15[1])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 (10[1])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 (10[2])of this element, which turns colorless in their presence. For 10 points, name this red-brown halogen (10[2]-5[1])that is a liquid (10[1]-5[1])at STP. (10[3])■END■ (10[2])