Fluorine (F)
Fluorine (F) is a halogen with atomic number 9 and a standard atomic weight of 18.998. At room temperature it is a gas that melts at -220 °C and boils at -188 °C.
Fluorine key facts
Fluorine at a glance
Identity
| Symbol | F |
|---|---|
| Atomic number (Z) | 9 |
| Atomic weight | 18.998 ± 0.001 (CIAAW 2024) |
| Category | Halogen |
| Group | 17 — halogens |
| Period | 2 |
| Block | p-block |
| CAS number | 7782-41-4 |
Atomic properties
| Electron configuration | [He] 2s2 2p5 Full: 1s2 2s2 2p5 |
|---|---|
| Electrons per shell | 2, 7 |
| Oxidation states | -1 |
| Electronegativity (Pauling) | 3.98 |
| First ionisation energy | 17.423 eV (1,681 kJ/mol) 2nd: 34.971 eV · 3rd: 62.708 eV |
| Electron affinity | 3.401 eV |
| Atomic radius (empirical) | 50 pm |
| Covalent radius | 64 pm |
| Van der Waals radius | 147 pm |
Physical properties
| State at 25 °C | Gas |
|---|---|
| Melting point | -219.7 °C · -363.4 °F · 53.48 K |
| Boiling point | -188.1 °C · -306.6 °F · 85.04 K |
| Density | 1.696 g/L (gas at 0 °C, 1 atm) |
| Specific heat capacity | 0.824 J/(g·K) |
| Thermal conductivity | 0.028 W/(m·K) |
| Crystal structure | Monoclinic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 19F 100% |
| Stable isotopes | 3 |
| Known isotopes | 19 |
| Abundance in Earth's crust | 585 mg/kg |
| Abundance in seawater | 1.3 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Fluorine electron configuration
Fluorine has 9 electrons. Written in short form, its ground-state configuration is [He] 2s2 2p5 — the electrons of the noble gas helium, plus 2s2 2p5.Written out in full: 1s2 2s2 2p5.
As a member of group 17 it has 7 valence electrons in its outer shell (shell 2), which largely decide its chemistry.
Discovery and name
1886 — Henri Moissan (first isolated) · France.
From fluorite (Latin fluere, to flow), a mineral used as a flux.
Known for
The most electronegative and most reactive element. Fluoride protects teeth; fluorine compounds include PTFE (Teflon), refrigerants and uranium hexafluoride for enrichment.
Isotopes of fluorine
Naturally occurring fluorine is a mix of 1 isotope. Each has 9 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 19F | 9 | 10 | 100% | stable |
19 isotopes of fluorine are known in our data set (half-lives as published in nuclear data tables).
Fluorine compared with group 17
The other elements in group 17 (halogens), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Fluorine | 9 | 18.998 | -220 °C | -188 °C | 1.696 g/L | 3.98 |
| Chlorine | 17 | 35.45 | -101 °C | -34 °C | 3.2 g/L | 3.16 |
| Bromine | 35 | 79.904 | -7.2 °C | 59 °C | 3.103 g/cm³ | 2.96 |
| Iodine | 53 | 126.90 | 114 °C | 184 °C | 4.933 g/cm³ | 2.66 |
| Astatine | 85 | [210] | 302 °C (est.) | — | 7 g/cm³ (pred.) | 2.2 |
| Tennessine | 117 | [294] | — | — | 7.2 g/cm³ (pred.) | — |