Fluorine

Fluorine attacks glass, water, asbestos and most metals, and will make even oxygen react. The nineteenth-century attempts to isolate it were lethal enough that the casualties are still called the fluorine martyrs: Paulin Louhet and Jérôme Nicklès died, and both Knox brothers and Georges Gore were badly injured. Henri Moissan finally succeeded in 1886 by electrolysing potassium bifluoride in anhydrous HF at −50 °C in a platinum-iridium cell, and took the 1906 Nobel Prize for it. Its compounds are far tamer than the element: PTFE, discovered when Roy Plunkett found a cylinder of tetrafluoroethylene had polymerised itself in 1938, is the slipperiest solid known, and uranium hexafluoride is the only practical vehicle for isotope enrichment. Fluoride in drinking water at about 0.7 mg/L cuts tooth decay by hardening enamel into fluorapatite.