Fluorine

Fluorine is the most reactive element in the periodic table and the most electronegative, it grabs electrons from other elements more powerfully than anything else. It is a pale yellow gas that attacks nearly every substance it touches, including most metals and even glass.

  • Atomic Number99 protons, 9 electrons
  • Atomic Mass18.99840316 uAbout 19× heavier than hydrogen
  • State at Room TempGaspale yellow gas
  • Density0.001696 g/cm³About 1.3× heavier than air
  • Melting / Boiling-219.6°C / -188.1°CLiquefies at −188°C
  • Discovered1670Henri Moissan, 1886

How does fluorine compare to the other halogens in mass?

Fluorine is the lightest halogen. Each step down Group 17 roughly doubles the atomic mass.

Atomic Mass Comparison
Fluorine19 u
Chlorine35.5 u
Bromine79.9 u
Iodine127 u
Iron56 u

At 19 atomic mass units, fluorine is the lightest halogen. Chlorine (35.5 u) is nearly twice as heavy, bromine (80 u) is about four times heavier. Despite these differences, all halogens are eager to gain one more electron to complete their outer shell.

What is fluorine?

Fluorine is a halogen in Group 17 of the periodic table. It has nine protons and nine electrons, with seven electrons in its outer shell, just one short of a full set. That near-full shell drives fluorine to steal an electron from almost any other atom it meets, making it extraordinarily reactive. It is so aggressive that it must be stored in containers made of the few materials that have already reacted with fluorine and formed a stable fluoride layer that protects the metal beneath.

Fluorine gets its name from the mineral fluorspar (calcium fluoride, CaF₂), which was used as a flux in metallurgy, a substance added to help metals melt and flow more easily. The Latin word fluere means to flow. The element was named after this mineral long before it was actually isolated, because chemists had deduced it must be present inside fluorspar.

Fact Isolating pure fluorine was so dangerous that several chemists in the 19th century were killed or seriously injured trying. The scientists who suffered injuries became known as the "fluorine martyrs". Henri Moissan finally succeeded in 1886 and won the Nobel Prize in Chemistry for it in 1906.

Where you find fluorine

On Earth

Fluorine is never found as a free gas in nature, it is always bonded to other elements, because it reacts so violently with almost everything.

  • Fluorite (fluorspar). Calcium fluoride (CaF₂) is the most important fluorine mineral. China produces more than half the world's fluorite, with Mexico, South Africa and Mongolia also being significant producers.
  • Fluorapatite. This fluorine-containing mineral is found in phosphate rock deposits. The fluoride in toothpaste ultimately traces back to compounds related to this mineral.
  • Seawater and groundwater. Fluoride ions are dissolved in small amounts in water throughout the world. Fluoride in drinking water is what first prompted adding it to public water supplies.

How we use fluorine

  • Toothpaste and water fluoridation. Fluoride compounds added to toothpaste and drinking water strengthen tooth enamel and dramatically reduce tooth decay. This is one of the most cost-effective public health measures ever implemented.
  • Teflon (PTFE) non-stick coatings. Polytetrafluoroethylene is a plastic made from carbon and fluorine. It is almost completely unreactive, which is why food slides off non-stick pans and why it is used in pipes carrying corrosive chemicals.
  • Refrigerants. Hydrofluorocarbons (HFCs) are used as refrigerants in fridges and air conditioners. They replaced the older CFCs that were damaging the ozone layer.
  • Uranium processing. Uranium hexafluoride (UF₆) is used to separate uranium-235 from uranium-238 in the process that produces enriched fuel for nuclear power stations.
Did you know? Fluorine reacts with so many things that it attacks glass, which chemists normally consider almost inert. It must be stored in containers made of copper, nickel or special PTFE plastics that have already reacted with fluorine and formed a stable protective layer. Even noble gases like xenon and krypton can be forced to react with fluorine under the right conditions.

How it was discovered

Fluorine was known to be present in minerals from the 18th century, but isolating the pure element proved extraordinarily difficult and dangerous. Many chemists were injured or killed trying, by explosions, poisoning and burns from hydrogen fluoride. Henri Moissan in Paris finally succeeded in 1886 by using electrolysis of liquid hydrogen fluoride at −23°C in a platinum container, with fluorite stoppers to resist the fluorine produced. He received the 1906 Nobel Prize in Chemistry for this achievement, 20 years after his discovery.

Deeper dive: halogens, fluorine's extreme reactivity and the fluorine-carbon bond

Fluorine is the first member of Group 17, the halogens. All halogens have seven electrons in their outer shell and are eager to gain one more. Fluorine is the most extreme because its atoms are so small that the electrons are very close to the nucleus and held very tightly. This makes fluorine both the best at accepting electrons and the most aggressive in doing so.

The fluorine-carbon bond is one of the strongest single bonds in organic chemistry, which is why fluorine-containing compounds are so chemically stable and resistant to breakdown. PTFE (Teflon) is a carbon backbone with fluorine atoms completely surrounding every side: the fluorine sheath is almost impenetrable. This makes PTFE useful not just in cooking but in medical implants, fuel lines and industrial seals.

Hydrogen fluoride (HF) is unusual: despite fluorine being so electronegative, HF is a weak acid in water because the HF bond is so strong that the molecule does not fully dissociate. It burns skin badly but does not show immediate pain because it penetrates deeply before causing damage, making it one of the most treacherous laboratory hazards.

Fluorine is the most reactive of all the halogens. Moving one step further along brings us to neon, a noble gas that sits at the opposite extreme, it barely reacts with anything at all.