Group 7: The Halogens

The halogens — fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂) — are Group 7 elements. They are non-metals that exist as diatomic molecules. Their chemistry is dominated by their high electronegativity and tendency to gain one electron to achieve a noble gas configuration, forming X⁻ halide ions.

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Trends in Physical Properties

State at Room Temperature

Halogen	State at Room Temperature	Colour
F₂	Pale yellow gas	Pale yellow
Cl₂	Greenish-yellow gas	Green-yellow
Br₂	Red-brown liquid	Red-brown
I₂	Dark grey/purple solid	Grey solid, purple vapour

The halogens change from gas to liquid to solid as you descend the group. This is because the number of electrons increases, which increases the strength of London dispersion forces (temporary dipole–induced dipole interactions) between molecules. Stronger intermolecular forces require more energy to overcome, so boiling points increase.

Boiling Points

Halogen	Boiling Point (°C)
F₂	−188
Cl₂	−34
Br₂	59
I₂	184