Fluorine

Fluorine is the element of atomic number 9, the lightest of the halogens. Fluorine is an interesting element - it's extremely reactive, and it explodes a lot (in the right circumstances). It could be in your home, too! But in the slightly less dangerous form Teflon.

A fart in a bottle Fluorine in gaseous form.
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Properties

The properties of fluorine depend very much on which kind of compound it is a part of.

Pure fluorine is a greenish-yellow, extremely reactive, toxic gas.[1] Fluorine is incredibly reactive because its electronegativity is highest of all elements: its outermost electron shell has only a single hole in it, and it loves to pull any other electrons it can lay its hands on into said hole.[2]

In covalent compounds, fluorine clings like a mofo to practically any molecule you bond it to. Its atomic radius is rather small, and its bond to carbonFile:Wikipedia's W.svg is "the strongest in organic chemistry". As a result, it can substitute hydrogen in organic compounds to form covalent organofluorine compounds, which are less reactive. This feature is what makes Teflon (polytetrafluoroethylene) nicely slippery and unreactive. Higher halogens, such as chlorine, are much less well-behaved. Light fluorocarbons are inert liquids and gases, useful as refrigerants. Fully fluorinated covalent fluorides, such as sulfur hexafluoride (SF6), are also very unreactive, and SF6 is used for example to fill electrical transformers. The chemical industry likes fluorine because it makes nasty things like uranium much easier to work with: uranium hexafluoride is not a salt, like one would expect from a simple ionic compound, but a volatile, easily sublimed solid.

In pharmaceuticals, substitution of hydrogen with fluorine is particularly useful because it can block enzymes from digesting the molecule, which slows down its elimination from the body and increases potency. Furthermore, its electronegativity can be useful in its own right.

Fluorine can do neat tricks like making compounds with the heavier noble gases argon, krypton and xenon; possibly radon, but it doesn't seem like anyone would want to try that one because radon is radioactive.

Fluorine readily forms ionic fluoride salts, which contain the fluoride ion (F-), for example sodium fluoride (NaF). They are moderately toxic if ingested in sufficiently large quantities, but are otherwise as boring as regular ionic salts. Fluoride salts are used in water fluoridation.

Crazy cool facts
  • Elemental fluorine is a yellow gas that killed a lot of the people who tried to isolate it.
  • Hydrofluoric acid (HF) is corrosive, toxic and a contact poison, being much more dangerous than hydrochloric acid. If it gets into your blood, it will suck the calcium out of your bones into your bloodstream, screwing up your electrolyte balance, which can cause heart attack and death. People rarely encounter it, however, unless they're involved with e.g. semiconductor manufacturing.
  • Fluorine, either free or as a fluoride ion in hydrofluoric acid, can dissolve glass – it is usually stored in flasks made of Teflon, which, by the way, is also made with fluorine.
  • Then there is the nasty darling of mad scientists everywhere, chlorine trifluoride. Three atoms of fluorine will combine with a single atom of chlorine (chlorine trifluoride, ClF3) to demonstrate emphatically that there are things more entertainingly hazardous than fluorine gas.[3][4] Does a whole frozen chicken usually catch fire spontaneously? Yes - if doused in chlorine trifluoride. It'd be an excellent oxidizer for rocketry, except for the fact that it sets everything on fire - including things like aluminum plate, people, water, concrete and sand.
  • To top the one above, there is dioxygen difluoride, O2F2 or more evocatively FOOF,[5][6] an apt description of its reaction with any combustible material.

Fluorine and conspiracy nuts

Fluorine, in the form of salts such as sodium fluoride, is an essential component in water fluoridation, which some conspiracy theorists say is a plot to poison us all. In pharmaceuticals, organofluorines are used because they're less reactive. But, the same nutters nevertheless bring them up, for example fluoxetine - which contains no free fluoride.

gollark: Honestly I should probably just have some sort of message passing approach to this.
gollark: Oh, another one was PS#2DAA86DC. That was when you could run a privileged function in a coroutine and... also feed it fake events.
gollark: <@263493613860814848> How DARE YOU.
gollark: No, 'twas self-starred.
gollark: Another one was that for no apparent reason `getfenv` would sometimes return out of sandbox stuff despite it being explicitly programmed to prevent this.

References
  1. Fluorine PubChem: Open Chemistry Database
  2. Chemistry of Fluorine LibreTexts: Chemistry
  3. Things I Won't Work With: Sand Won't Save You This Time
  4. Things I Won't Work With: Chlorine Trifluoride: Some Empirical Findings ClF3 versus other matter
  5. For more horrifying details, visit http://what-if.xkcd.com/40/.
  6. Things I Won't Work With: Dioxygen Difluoride
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