This is the smallest anion that can form multi-dimensional chain structures in a phenomenon called “dimensional caging.” A solution containing this anion can test for explosive peroxides in stocks of diethyl ether exposed to air. Sulfuric acid and a compound with this anion are the products of the Bunsen reaction used to produce hydrogen gas. Thiosulfate reacts with a mixture of manganese and this anion to quantify dissolved oxygen in the (*) Winkler method. (10[1])Since this weakest ligand on the spectrochemical series resembles ice when bonded to silver, it is used to induce nucleation during cloud seeding. (10[2]-5[2])The blue charge-transfer complexes formed by this monatomic anion make it a useful starch indicator. For 10 points, name this anion that is the common form of an element found in the thyroid. ■END■ (10[1]0[3])