Trace levels of this element are detected using particle-induced gamma ray emission or an electrode that uses a “total ionic strength adjustment buffer.” Telomer (“TEE-loh-mer”) alcohols containing this element oligo·merize to form a short-chain propylene oxide dimer acid called GenX. The last digit of a molecule’s R-number indicates its number of atoms of this element, in line with empirical nomenclature. In March 2023, the EPA set a four-parts-per-trillion (10[1])concentration (-5[1])limit on a carcinogenic octane·sulfonic acid substituted (10[1])with (10[1])this (10[1])element (10[1])that is still used in firefighting foam. Aluminum (10[1])smelting is the main source of water pollution by this nonmetal found in cryolite. (10[1]-5[1])Because (10[1])this element (10[1])forms the strongest single bond with carbon, inert alkyl (10[1]-5[1])surfactants (10[1]-5[1])containing (10[1])it are colloquially (10[1])called “forever (10[1])chemicals.” For 10 points, (10[1])name this element (10[1])found with carbon and chlorine (10[3]-5[1])in freons and other ozone-destroying molecules. ■END■ (10[5])