Fluorine
Fluorine is a chemical element with symbol F and atomic number 9. It is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. As the most electronegative element, it is extremely reactive:almost all other elements, including some noble gases, form compounds with fluorine.
Identity
- Atomic mass
- 18.998 u
- Category
- Halogens
- Group
- 17
- Period
- 2
- Block
- p
- Oxidation States
- −1
- Phase (25 °C)
- Gas
Electronic Structure
- Electron configuration
- 1s2 2s2 2p5
- Short configuration
- [He] 2s2 2p5
- Electrons per shell
- 2, 7
- Electronegativity (Pauling)
- 3.98
- Electron affinity
- 328 kJ/mol
- 1st ionization energy
- 1681 kJ/mol
- Ionization energies
- 1681 · 3374 · 6050 · 8408 · 11023… kJ/mol
Physical properties
- Melting point
- -219.7 °C (53 K)
- Boiling point
- -188.1 °C (85 K)
- Density
- 0.001696 g/cm³
- Molar heat capacity
- —
Chemical behavior of fluorine
Fluorine has seven valence electrons ([He] 2s² 2p⁵) and is the most electronegative of all elements (3.98): it has one electron in its octet, which it steals from almost any other atom. It invariably acts as -1 in very polar or ionic bonds, and even oxidises oxygen (OF₂) and some noble gases (XeF₂, KrF₂). Its F₂ molecule has a weak F–F bond (159 kJ/mol) and dissociates easily, which explains why it is the most violent oxidant that exists.
Reactivity
H₂ + F₂ → 2 HF
It reacts with hydrogen explosively even in the dark and at -250 °C, giving hydrogen fluoride. It is the fastest reaction in chemistry: it needs no spark or light to start.
2 F₂ + O₂ → 2 OF₂
It is the only element that oxidises oxygen in direct combustion, acting with oxygen as +2. The product, oxygen difluoride, is an even more aggressive and toxic oxidant.
Xe + F₂ → XeF₂
With xenon, by heating or light, it forms xenon difluoride: proof that not even noble gases resist fluorine. It was the first noble gas compound, synthesised in 1962.
SiO₂ + 4 HF → SiF₄ + 2 H₂O
Hydrogen fluoride acid attacks glass (silicon dioxide), forming gaseous silicon tetrafluoride. That is why it is stored in plastic and used for glass etching and circuits.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Hydrogen fluoride | HF | Glass etching, semiconductors and cryolite synthesis |
| Sodium fluoride | NaF | Toothpaste and fluoridation of drinking water |
| Teflon (PTFE) | (C₂F₄)ₙ | Non-stick, chemical seals and medical implants |
| Uranium hexafluoride | UF₆ | Uranium enrichment for nuclear fuel |
| Cryolite | Na₃AlF₆ | Flux for obtaining aluminium by electrolysis |
Electron shell distribution
View in the atom builderIsotopes
All natural fluorine is fluorine-19 (¹⁹F), with ten neutrons: it is monoisotopic and moreover the only stable isotope. Fluorine-18 (¹⁸F), with nine neutrons and a half-life of 110 minutes, is the star tracer in positron emission tomography (PET) in oncology and neurology.
Occurrence and production
It is the most abundant halogen in the crust (about 585 ppm) but never appears free due to its extreme reactivity. Its ores are fluorite (CaF₂), cryolite and fluorapatite of bones and teeth. It is obtained by electrolysis of anhydrous hydrogen fluoride with potassium bifluoride (Moissan process) and as a byproduct of phosphoric acid.
History and origin of the name
Nineteenth-century chemists pursued fluorine for decades suffering burns and poisonings; their victims called it 'the devouring element'. Henri Moissan isolated it in 1886 by electrolysis at low temperature after years of attempts, an achievement that earned him the Nobel Prize in 1906. Ampère had baptised it fluorine, from Latin fluere (to flow), because of the fusible fluorite.
Applications and relevance of fluorine
Uses of the element
Pure F₂ is used in the manufacture of UF₆ for uranium enrichment, in plasma cleaning of the Semiconductor industry and in the synthesis of fluorinated anaesthetics and drugs. Its handling requires nickel or passivated Monel equipment.
Uses of its compounds
Fluorides protect teeth in toothpastes and waters; Teflon coats frying pans and prostheses; cryolite allows aluminium to melt; fluorinated gases refrigerate and insulate electrical equipment; ¹⁸F makes tumours visible in PET.
Health, safety and environment
Biological role
In traces it strengthens tooth enamel and bones: fluoride converts hydroxyapatite into fluorapatite, more resistant to acids. In excess it causes dental and skeletal fluorosis, with spots and bone stiffness.
Safety
F₂ and HF are extremely corrosive and toxic: HF penetrates the skin without initial pain and precipitates calcium in blood and bones, and can be fatal. Its handling requires showers, calcium gluconate and specific training.
Environment
CFCs destroyed the ozone layer (prohibited by Montreal 1987) and their substitutes HFCs warm the climate thousands of times more than CO₂. PFAS ('forever pollutants' of Teflon) persist in waters and organisms; the EU is preparing their general restriction.
Frequently asked questions about Fluorine
Why is fluorine the most reactive element?
Because it combines maximum electronegativity (3.98) with a tiny atom that is missing only one electron for the octet, and its F₂ molecule has a weak bond that breaks easily. Everything in it pushes to react: it even steals electrons from oxygen and xenon, and starts reactions without a spark at -250 °C. It is the definitive oxidant: no other element oxidises so many companions. Its reactivity is at the same time its usefulness and its danger.
How was fluorine isolated if it attacked everything?
Henri Moissan achieved it in 1886 after years of burns and rotten apparatus: electrolysed anhydrous hydrogen fluoride dissolved in potassium bifluoride at -23 °C, in platinum cells that were passivated. Several chemists died or went blind during the 19th-century attempts. The 1906 Nobel Prize rewarded his tenacity: mastering the most untamable element required cooling it, disguising it as fused salt and enclosing it in platinum.
Why does Teflon not stick to anything?
Because its carbon chain is shielded by strongly bonded fluorine atoms, forming a smooth surface with no chemical 'hooks' to grab. Neither eggs nor almost any solvent can adhere to or dissolve it: it resists 260 °C and almost all acids. Accidentally discovered by Roy Plunkett in 1938, it coats frying pans, vascular prostheses and chemical industry joints. It only degrades above 350 °C, releasing toxic fumes.
Is fluorine in drinking water dangerous?
At the fluoridation doses (0.7-1.2 mg per litre) decades of evidence show less caries without general damage: it is one of the most studied public health measures. The real risk is excess natural: aquifers with several mg per litre cause dental and bone fluorosis in wide areas of India, China and Africa. The key, as always with fluorine, is the exact dose.
Is the fluorine in toothpaste the same as that which burns glass?
The same element, opposite forms: in toothpaste there is sodium fluoride in parts per million of doses, which hardens enamel without risk; the one that etches glass is concentrated hydrofluoric acid, extremely corrosive. As chlorine from salt versus war gas chlorine: dose and form are everything. Swallowing usual toothpaste does not intoxicate, but children's toothpastes contain less fluorine for precaution.
What are PFAS or 'forever pollutants'?
Thousands of compounds with carbon-fluorine bonds, almost indestructible, used in Teflon, waterproofing and fire foams. Their strength is their condemnation: they do not degrade and accumulate in waters, soils, blood and breast milk worldwide. They are associated with hormonal alterations, cholesterol and certain cancers. The EU is preparing their almost total prohibition: the greatest chemical veto in history.
What does a doctor see in a PET with fluorine-18?
The metabolism of the tumour: the patient receives glucose marked with ¹⁸F, which cancer cells, voracious for sugar, accumulate more than healthy ones. As it decays, ¹⁸F emits positrons which the scanner converts into a bright map of the disease. With a 110-minute half-life, it must be manufactured in a cyclotron near each morning. It is the ruling test to search for metastases and check if chemotherapy works.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.