Fluorine
Atomic Data
| Atomic Number | 9 |
| Symbol | F |
| Atomic Weight | 18.998 u |
| Density (STP) | 1.696 g/L |
| Melting Point | −219.67 °C (53.48 K) |
| Boiling Point | −188.12 °C (85.03 K) |
| Electronegativity | 3.98 (Pauling) |
| Electron Config. | 1s2 2s2 2p5 |
| Oxidation States | −1 |
| Phase at STP | Gas |
| Category | Halogen |
| Period / Group | 2 / 17 |
| CAS Number | 7782-41-4 |
Electron Configuration
[He] 2s2 2p5
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 5 | 9 |
| Total | 9 | 9 | ||
Isotopes of Fluorine
Fluorine is monoisotopic — ¹⁹F is its only naturally occurring stable isotope, accounting for 100% of all natural Fluorine.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Fluorine-19 | ¹⁹F | 9 | 10 | 100 | Stable |
Abundance & Occurrence
Fluorine is present in Earth's crust at approximately 585 ppm by mass and at approximately 400 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Extreme corrosivity: Fluorine gas is the most reactive element; it reacts violently with almost every substance, including water, glass, ceramics, and many metals; contact causes immediate, severe chemical burns to all tissues.
- Inhalation toxicity: Even brief inhalation of fluorine gas at low concentrations causes severe pulmonary oedema and chemical pneumonia; the threshold limit value (TLV-TWA) is 1 ppm — extremely low.
- Hydrofluoric acid (HF): Fluorine compounds, particularly HF, are insidiously dangerous; HF penetrates skin without immediate pain, then causes deep tissue destruction and systemic hypocalcaemia that can cause fatal cardiac arrhythmia hours after exposure.
- Handling requirements: Fluorine gas requires specialised inert-metal equipment (nickel, Monel, copper), fluoropolymer seals, and full respiratory protection; reactions must be performed behind blast shields in dedicated fume hoods.
- First aid for HF contact: Apply calcium gluconate gel immediately to any skin contact and seek emergency medical attention; standard burn first aid is insufficient — systemic calcium treatment may be required.
Fluorine in the Real World
Real-World Uses
- PTFE and fluoropolymer coatings — Polytetrafluoroethylene (PTFE/Teflon) is synthesised from fluorine-derived feedstocks and used for non-stick cookware coatings, chemical-resistant linings, medical devices, and low-friction cable insulation.
- Refrigerants — Hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs), all containing fluorine, are the working fluids in air conditioners, heat pumps, and refrigerators following the phase-out of ozone-depleting CFCs.
- Dental and water fluoridation — Fluoride ions incorporated into tooth enamel as fluorapatite increase resistance to acid demineralisation; fluoridated toothpaste and controlled water fluoridation significantly reduce dental caries rates.
- Uranium hexafluoride enrichment — Uranium is converted to UF₆ gas and processed through gas centrifuges to enrich the fissile U-235 isotope for use in nuclear reactor fuel and weapons.
- Pharmaceuticals — Fluorine atoms are incorporated into approximately 20% of all pharmaceuticals, including antidepressants (fluoxetine/Prozac), anti-inflammatories, and antibiotics, where fluorination improves metabolic stability and bioavailability.
- Semiconductor etching — Fluorine-based plasma gases (SF₆, NF₃, CF₄) are used in reactive-ion etching to pattern nanometre-scale features on silicon wafers during microchip fabrication.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is fluorine used for?
Fluorine is used to make PTFE (Teflon) non-stick coatings and fluoropolymer insulation, in refrigerants (HFCs and HFOs), in toothpaste and water fluoridation to prevent tooth decay, in the uranium enrichment process (as UF₆ gas), in pharmaceutical drugs (approximately 20% of all drugs contain fluorine), and as a plasma etchant in semiconductor fabrication.
Is fluorine the most reactive element?
Yes, fluorine is the most chemically reactive of all elements. It reacts vigorously or explosively with almost every other element and compound, including noble gases like xenon and krypton, glass, and even water under certain conditions. This extreme reactivity stems from fluorine’s very high electronegativity (3.98 Pauling), small atomic radius, and a relatively weak F–F bond that is easily broken to initiate reactions.
Is fluoride in drinking water safe?
At the levels used in water fluoridation (typically 0.7 mg/L, as recommended by the WHO and many national health authorities), fluoride is considered safe and effective at reducing dental caries rates. The concentration is far below the level that would cause dental fluorosis (mottling of teeth, at levels above ~1.5 mg/L) or skeletal fluorosis (at very high chronic exposures). Fluoridation has been endorsed by major public health organisations worldwide as one of the most cost-effective public health measures in history.
Why is fluorine so electronegative?
Fluorine’s extreme electronegativity (the highest of any element) arises from the combination of its very small atomic radius and its seven valence electrons, which are held tightly by a nuclear charge of only 9 protons. This means the nucleus attracts electrons very strongly relative to the atom's size. When fluorine bonds with other atoms, it pulls electron density toward itself more effectively than any other element, making its bonds the most polarised in chemistry.