Physical properties of the Group 17 elements

Element	Colour	State (room temp)	Vapour
Chlorine, Cl₂	pale green-yellow	gas	green-yellow
Bromine, Br₂	red-brown / orange	liquid	brown
Iodine, I₂	grey-black (shiny)	solid	purple/violet

Down the group the elements become darker and less volatile (gas → liquid → solid). Volatility = how easily a substance vaporises; it decreases down Group 17, so boiling points increase down the group.

The halogens are simple molecular (diatomic X₂). The forces between molecules are instantaneous dipole–induced dipole (id–id) forces (London dispersion forces).

Down the group:

• molecules have more electrons (and are larger),
• so the id–id forces between molecules are stronger,
• meaning more energy is needed to separate them → higher boiling point → lower volatility.

This is why iodine is a solid (strong id–id) while chlorine is a gas (weak id–id).

The X–X bond strength (bond energy) generally decreases down the group from Cl₂ to I₂: $\text{Cl–Cl} > \text{Br–Br} > \text{I–I}$ As the atoms get larger, the bonding electrons are further from the nuclei and the shared pair is held less strongly, so the bond is weaker.

Anomaly — F₂. The F–F bond is unexpectedly weak (weaker than Cl–Cl). This is because fluorine atoms are very small, so the lone pairs on the two adjacent F atoms repel each other strongly, weakening the bond.

This trend matters for the thermal stability of the hydrogen halides (11.2) and the reactivity of the elements.