F · Element 9 · Halogen
The most electronegative element known: fluorine takes electrons from almost anything, including glass and water.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where fluorine sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All fluorine atoms have the same number of protons, 9, and differ only in their number of neutrons: those variants are its isotopes. The most common, F-19, has 10 neutrons and makes up 100% of natural fluorine.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| F-18 | 9 | — | 109.77 min | ε/β⁺ |
| F-19 most common | 10 | 100% | Stable | — |
Here the classroom shortcut, rounded atomic mass (18.998) minus atomic number, gives the same answer: 10.
Decay modes
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 50 pm — that is 0.05 nm, so about 10 million of them side by side would span a millimetre.
Thermal range
Fluorine is liquid over a 32 K window, from 53 K to 85 K.
Where it sits
Fluorine sits in period 2, group 17. Everything in group 17 shares the same outer-electron count, which is why they behave so similarly.
Other halogens
All halogensThe story
Fluorine is the most electronegative and reactive element on the periodic table, earning its reputation as the "king of reactivity." This pale yellow, highly toxic gas attacks virtually every other element, including noble gases like xenon and krypton, which were once thought to be very low reactivity.
Discovered by Henri Moissan in 1886 after decades of dangerous attempts by chemists, fluorine defied isolation for so long that it was nicknamed "the element that refuses to be discovered." Its high reactivity makes pure fluorine very dangerous to handle, yet its compounds have transformed modern life.
Fun Fact: Fluorine is so reactive that it can make water burn. When fluorine gas meets water vapor, the water itself burns, yielding hydrogen fluoride and oxygen, creating an eerie flame.Despite its fearsome reputation in elemental form, fluorine compounds are essential to modern society. From the non-stick coating on cookware (Teflon) to the fluoride in toothpaste that prevents tooth decay, controlled fluorine chemistry has provided countless benefits while requiring strict safety precautions.
Fluorine's unique properties stem from its small atomic size and highest electronegativity (4.0 on the Pauling scale). This makes it the most electronegative element and the strongest oxidizing agent among the elements, forming very strong bonds with other elements and creating some of the most stable compounds known to chemistry, including the greenhouse gas sulfur hexafluoride (SF₆).
Applications
The largest industrial use of fluorine is in uranium hexafluoride (UF₆) production for nuclear fuel enrichment:
Fluorine creates exceptionally durable and chemically resistant polymers:
Fluorinated compounds transformed cooling technology:
Fluorine compounds are essential for modern electronics manufacturing:
Fluorine dramatically enhances drug and pesticide properties:
Fluorine compounds provide superior surface protection:
Fluorine compounds enable extreme-environment technologies:
Where it comes from
585 mg/kg of Earth's crust · more abundant than 89% of elements
Fluorine never occurs as a free element in nature due to its high reactivity, but forms abundant minerals in Earth's crust. With an average crustal abundance of about 585 parts per million, fluorine ranks as the 13th most abundant element in the lithosphere.
World fluorite production centers around key geological formations:
Fluorite deposits form through hydrothermal processes in specific geological environments:
Oceans contain the largest reservoir of dissolved fluoride, with an average concentration of 1.3 mg/L (1.3 ppm):
Natural fluoride occurs in freshwater systems through rock-water interactions:
Living organisms interact with environmental fluoride in complex ways:
Active geological processes continuously release fluorine compounds:
Fluorine's cosmic abundance and distribution in space:
Handling
Fluorine and hydrogen fluoride cause severe health effects:
Extremely low exposure limits reflect fluorine's toxicity:
Critical emergency procedures:
Fluorine creates serious fire and explosion risks:
Full protection required for fluorine handling:
Specialized safety measures for fluorine work:
Fluorine compounds require careful environmental management:
Strict regulations govern fluorine transport and storage:
Long-term exposure can cause permanent health damage:
Quick answers
Fluorine (symbol F) is element 9 on the periodic table, a halogen in period 2, group 17. The most electronegative element known: fluorine takes electrons from almost anything, including glass and water. At room temperature it is a gas.
[He] 2s² 2p⁵, giving 2 occupied shells with the distribution 2, 7. Its outer shell holds 7 electrons, which is what sets its bonding behaviour.Fluorine melts at 53.5 K (-219.7 °C) and boils at 85 K (-188.1 °C).
The standard atomic weight of fluorine is 18.998 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
The most common fluorine atom, F-19, has 10 neutrons: its mass number, 19, minus its atomic number, 9. It makes up 100% of natural fluorine. The classroom shortcut, rounded atomic mass (18.998) minus atomic number, gives the same 10.
Fluorine is a gas at room temperature; at 0 °C and 1 atm its density is about 1.696 g/L, so it is heavier than air.
Fluorine has a Pauling electronegativity of 3.98. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). A value this high means it tends to pull electrons away from almost anything it bonds with.
Fluorine was discovered in 1886 by Henri Moissan. Origin of the name: Latin fluere, "to flow".
Fluorine makes up about 585 mg/kg of the Earth's crust — common enough to be mined at scale.