Fluorine
[He] 2s² 2p⁵ · 2 · 7
The most electronegative and most reactive element in the table. Fluorine takes electrons from anything: water, glass and asbestos burn in a stream of it, and several chemists died trying to isolate it.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 9
- Atomic mass
- 18.998 а.е.м.
- Electron configuration
- [He] 2s² 2p⁵
- Electrons per shell
- 2 · 7
- Block
- p-block
- Group
- Halogens
- Period
- 2
- Electronegativity (Pauling)
- 3.98
- Electronegativity (Allen)
- 4.19
- Atomic radius
- 42 pm
- Covalent radius
- 57 pm
- Van der Waals radius
- 147 pm
- Ionisation energy 1
- 1 681 kJ/mol
- Ionisation energy 2
- 3 374.2 kJ/mol
- Ionisation energy 3
- 6 050.4 kJ/mol
- Ionisation energy 4
- 8 407.7 kJ/mol
- Ionisation energy 5
- 11 023 kJ/mol
- Electron affinity
- 328 kJ/mol
- Common oxidation states
- -1
- Oxidation states
- -1
Physical properties
- State at 25 °C
- gas
- Density
- 0.0017 g/cm³
- Melting point
- 53.48 K · -219.7 °C
- Boiling point
- 85.03 K · -188.1 °C
- Speed of sound
- no data
Thermal properties
- Heat of fusion
- 0.51 kJ/mol
- Heat of vaporisation
- 6.62 kJ/mol
- Specific heat capacity
- 0.824 J/(g·K)
- Thermal conductivity
- 0.0277 W/(m·K)
Mechanical properties
- Young's modulus
- no data
- Shear modulus
- no data
- Bulk modulus
- no data
- Poisson ratio
- no data
- Mohs hardness
- no data
- Brinell hardness
- no data
Electrical and magnetic properties
- Electrical resistivity
- no data
- Magnetic ordering
- diamagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- monoclinic
- Lattice constants
- a = 550 pm · b = 328 pm · c = 728 pm
Abundance
- In the crust
- 585 mg/kg
- In the ocean
- 1.3 mg/L
- In the universe
- 400 mg/kg
- In the human body
- 37 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 19F | 100 % | stable | — |
Fluorine-19 is the only stable isotope, which makes ¹⁹F NMR an exceptionally clean technique. Fluorine-18, with a 110-minute half-life, is the workhorse label in PET imaging.
Discovery
- Year of discovery
- 1886
- Discovered by
- Henri Moissan
- Where
- France
- Origin of the name
- from Latin fluere (to flow): fluorspar was used as a smelting flux
History
Fluorspar had been used as a smelting flux since the sixteenth century, but the element itself yielded only to Henri Moissan in 1886, by electrolysis of anhydrous HF at −50 °C. It won him the 1906 Nobel Prize.
Where it occurs
The thirteenth most abundant element in the crust at 585 mg/kg. The main minerals are fluorite, cryolite and fluorapatite.
How it is produced
Only by electrolysis of molten KF·2HF: no chemical oxidiser is stronger than fluorine, so no displacement reaction can make it.
Role in living things
The fluoride ion builds into tooth enamel, turning hydroxyapatite into the markedly more acid-resistant fluorapatite. Beyond that, whether humans need fluorine at all is disputed.
Safety
Fluorine gas is lethal at tens of parts per million. Hydrofluoric acid is treacherous in a different way: it causes little immediate pain but soaks through skin and strips calcium from the blood, stopping the heart.
Uses
- Uranium hexafluoride for isotope separation in the nuclear industry
- Fluoropolymers, above all PTFE: chemically inert and low-friction
- Fluorocarbon refrigerants and blowing agents
- Fluoridation of drinking water and toothpaste against tooth decay
- Plasma etching of silicon in chip manufacture
Curiosities
- Fluorine tops the Pauling electronegativity scale at exactly 3.98 — nothing goes higher.
- PTFE was discovered by accident in 1938: gas in a cylinder had polymerised itself into a white powder.
- Fluorine is the only element that reacts with xenon under ordinary conditions.
- Early attempts to isolate fluorine cost several researchers their health or their lives; they are remembered as the fluorine martyrs.