Chemical element · reference page

Fluorine

Fluorine is element 9 (F), located in period 2, group 17 and the p block. At room temperature its reference phase is gas. Its electron configuration is [He] 2s2 2p5.

Atomic number9

Standard atomic weight18.998403162(5)

Group17

Period2

Blockp

Electron configuration[He] 2s2 2p5

Electronic structure

What controls its chemistry

Halogens have seven valence electrons. Gaining one electron completes the outer shell, so −1 halide ions are common, while covalent halogen compounds cover a wide range of oxidation states for the heavier members.

The free elements are reactive and are generally not found uncombined in nature. Reactivity and physical state change down the group as atoms become larger and more polarizable.

Chemical forms

Compounds, ions and materials

The free element is molecular or otherwise highly reactive, while halide ions are among the most common chemical forms. Hydrogen halides, metal halides, interhalogen compounds and, for the heavier halogens, oxygen-containing compounds broaden the chemistry well beyond the simple −1 state.

Physical behaviour

State and phase changes

The reference phase at room temperature is gas. The listed density is 1.696 g/L. The melting point is 53.54 K (-219.6 °C). The boiling point is 85.03 K (-188.1 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where fluorine is found and used

In practice, fluorine is encountered in fluoropolymers, dental fluorides and uranium chemistry.

In practice

Examples of real uses

Fluoropolymers

Fluorine-rich polymers use strong C–F bonds to provide chemical resistance, low friction and thermal stability.

Dental fluorides

Fluoride compounds help make tooth mineral more resistant to acid attack when used at controlled concentrations.

Uranium chemistry

Fluorine compounds are central to uranium conversion and isotope-enrichment chemistry.

Periodic position

What its neighbours tell us

Fluorine is in period 2 and group 17. In the neighbourhood shown here, it lies between oxygen and neon in the same period; chlorine is directly below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationdiatomic nonmetal

Phase at room temperatureGas

Electron shells2 · 7

Density1.696 g/L

Melting point53.54 K (-219.6 °C)

Boiling point85.03 K (-188.1 °C)

Pauling electronegativity3.98

First ionisation energy1 681 kJ/mol

Electron affinity328.2 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of fluorine is associated with André-Marie Ampère. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 18.998403162(5). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Fluorine has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The free halogen or reactive compounds of fluorine can be corrosive or toxic. Risk depends strongly on the chemical form and concentration.