This element attacks a carbon singly-bonded to this element in the first step of native chemical ligation. This element is the central nonmetal in a compound that reacts with oxalyl chloride in the beginning of the Swern oxidation. This element substituted with two methyl groups is doubly-bonded to an oxygen in a common organic solvent. Clusters of iron and this element are widely present in enzymes that mediate (10[1])biochemical electron transfers. Cross-links with this element are created in vulcanization. This element bonded to itself creates a namesake “bridge,” and this element bonded to hydrogen forms a thiol group, such as that in cysteine. For 10 points, name this chalcogen found under oxygen in the periodic table, which has a characteristic odor of rotten eggs. ■END■