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The molecule
Two fluorine atoms held by a weak single bond, eager to let go: the most reactive substance in chemistry.
The most reactive element known. It combines directly with every element except helium and neon, including gold, platinum and even xenon. Glass, sand, water, asbestos and most organic materials burn in it. Hydrogen and fluorine explode on contact, even in the dark and at low temperature. Its weak bond, and the very strong bonds it forms with everything else, are what drive all this.
No chemical can pull the electrons off fluoride, so it is done with electricity: electrolysis of potassium fluoride dissolved in liquid hydrogen fluoride, essentially Moissan's 1886 method, in cells built from metals that survive by growing a protective skin of fluoride.
Most fluorine exists as F₂ only briefly, on its way to becoming other things: uranium hexafluoride for enriching nuclear fuel, sulfur hexafluoride for electrical equipment, and fluorinated compounds that go into refrigerants, non-stick coatings and medicines. The fluoride in toothpaste and drinking water is never made from F₂ gas directly.
Extremely toxic and corrosive at tiny concentrations. It attacks skin, eyes and lungs, and the hydrogen fluoride it makes on contact with moisture causes deep burns that keep going. It is handled only in specialized plants.