Atomic number 9

Fluorine

F
+ phase
− phase

Shells Orbitals 2s 2p

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9 electrons in 2 shells: 2, 7.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Halogen
Period: 2
Group: 17
Block: p
Standard atomic weight: 18.998(1) Da
Electron configuration: [He] 2s² 2p⁵
Phase at 25 °C: gas
Melting point: −219.67 °C
Boiling point: −188.11 °C
Density: 1.696 g/L
Electronegativity (Pauling): 3.98
Discovered: 1886, Henri Moissan

Neighbours in the table

O Ne

Fluorine’s nine electrons sit in two shells, two inside and seven in the outer one. That leaves the 2p subshell, the part of the outer shell that fills last, one electron short of complete. Only the inner pair stands between those seven and a nucleus of nine protons. Nothing else pulls as hard on the electrons of a bond it shares, the property chemists call electronegativity. Fluorine’s 3.98 on the Pauling scale is the highest value in this data set.

The free element is diatomic: two atoms share a pair of electrons as F₂. At room temperature that is a pale yellow gas, the palest of the four common halogens, and NIOSH adds a pungent, irritating odour. It melts at −219.67 °C and boils at −188.11 °C, so ordinary temperatures leave it a gas.

OpenStax ranks fluorine first among known elements as an oxidising agent, a substance that takes electrons from whatever it meets. It shows one oxidation state, minus one: it takes an electron and never gives one up, and it does that even to oxygen. It forms a fluoride with every element except helium, neon and argon, and no other element reacts directly with xenon. It sets fire to powdered metal, carbon, glass, ceramics and even water, each burning with a bright flame.

Where it is found

No halogen occurs free in nature: the group is too reactive, though its compounds are spread everywhere. Fluorine barely survives in the open, because it reacts with whatever it meets, water included. It has been detected outdoors only as a trace, in volcanic emissions and in those of coal-fired power stations.

What the rocks hold is fluoride, locked into fluorapatite, cryolite and fluorite (CaF₂), which miners call fluorspar. PubChem’s element record gives the crust 585 milligrams of fluorine per kilogram of rock and seawater 1.3 milligrams per litre, where it is present as fluoride ions.

Fluorine takes electrons so readily that pulling one back off a fluoride ion is very hard, and no chemical reagent frees the element. Electrolysis is the only route, in the laboratory and in industry alike. Electrolysing a molten mixture of potassium hydrogen fluoride and anhydrous hydrogen fluoride releases fluorine at one electrode and hydrogen at the other. Copper, iron and nickel vessels can hold the product, because the first attack leaves a fluoride film that seals the metal underneath.

Why it matters

The story begins in 1670 with a recipe for etching glass with acid-treated fluorspar, the earliest recorded use of any fluorine compound. Carl Wilhelm Scheele found the new element inside hydrofluoric acid in 1771 but could not free it, and neither could Davy, Gay-Lussac, Lavoisier or Thenard. Los Alamos records that some of those experiments ended tragically.

George Gore obtained a small amount electrolytically in 1869, and the fluorine met the hydrogen from the other electrode; his apparatus exploded. Henri Moissan succeeded in 1886, electrolysing potassium fluoride in hydrofluoric acid in a device built entirely from platinum, with the two gases kept apart. The Nobel Prize in Chemistry followed in 1906, and industry still makes fluorine his way.

Nobody produced it commercially until the Second World War, when the atomic bomb project created the demand. PubChem’s compound record puts 70 to 80 per cent of the gas into one product, uranium hexafluoride. That compound is used to separate the isotopes of uranium. The same record names another destination, sulfur hexafluoride, made for dielectrics: materials that block an electric current. US industry reported making 10 to 25 million pounds of the element in each year from 2020 to 2023.

Properties

Properties of fluorine Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	9
Standard atomic weight	18.998(1) Da	Da	IUPAC/CIAAW 2024
Melting point	−219.67 °C	°C	Los Alamos: −219.67 °C
Boiling point	−188.11 °C	°C	Los Alamos: −188.11 °C
Density	1.696 g/L	g/L
Electronegativity (Pauling)	3.98
Electron configuration	1s² 2s² 2p⁵
Phase at 25 °C	gas

Safety

Fluorine gas is hazardous, and the hazard belongs to the industry that makes and uses it. PubChem’s Laboratory Chemical Safety Summary gives the signal word Danger: the gas is an oxidiser, it is fatal to breathe, and it corrodes skin and eyes. The NIOSH pocket guide names the eyes, skin, respiratory system, liver and kidneys as the organs affected, and inhalation and skin or eye contact as the routes.

Questions

Why is fluoride in toothpaste?

Fluoride swaps into the calcium phosphate mineral of teeth, converting part of it into a fluoride-bearing form of the same mineral, which resists decay better. Three fluorine compounds do that job in toothpaste: sodium fluoride, tin(II) fluoride and sodium monofluorophosphate. Public water supplies are treated for the same reason.

Is Teflon made from fluorine?

Yes. Teflon is a polymer whose repeating unit is two carbon atoms carrying four fluorine atoms, so a non-stick surface is a carbon chain wrapped in fluorine. Fluorine and its compounds go into more than 100 commercial fluorochemicals, and Los Alamos counts many high-temperature plastics among them. Fluorine gas is also used to treat the surface of polyethylene bottles.

Why does hydrofluoric acid etch glass?

Glass is a mixture of silicates: silicon bonded to oxygen. OpenStax explains why, among common acids, only hydrofluoric acid attacks them. Of all the bonds silicon makes, only the one to fluorine is stronger than its bond to oxygen, so the silicon leaves as gaseous silicon tetrafluoride. That is why the acid frosts glass and etches the markings on thermometers and burets.

Why were Freon refrigerants replaced?

Freons are compounds of carbon, chlorine and fluorine. Dichlorodifluoromethane, one of them, was widely used in air conditioning, refrigeration and aerosol cans, then phased out because of the damage it was doing to the Earth's ozone layer. The replacements keep the fluorine and drop the chlorine: compounds of carbon, hydrogen and fluorine now do the same job.

Compounds containing fluorine

HF Hydrogen fluoride 2 atoms, 1 single bond.
CaF₂ Calcium fluoride 3 atoms in one formula unit of an ionic lattice.
CCl₂F₂ Dichlorodifluoromethane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂F₄ 1,1,1,2-Tetrafluoroethane 8 atoms, 7 single bonds.
C₁₇H₁₈F₃NO Fluoxetine 40 atoms, 41 bonds.
C₃₃H₃₅FN₂O₅ Atorvastatin 76 atoms, 79 bonds.

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