Group 7 (VII) Properties

Group 7 elements are called the halogens. They are non-metals that exist as diatomic molecules (two atoms joined), such as Cl₂, Br₂, and I₂. They need just one more electron to have a full outer shell, so they tend to gain one electron to form halide ions (Cl⁻, Br⁻, I⁻).

State and appearance at room temperature

• Chlorine (Cl₂): pale yellow-green gas
• Bromine (Br₂): red-brown liquid (gives orange-brown vapour)
• Iodine (I₂): grey-black solid (forms a purple vapour when warmed)

Trends down the group

• Density increases
• Reactivity decreases
• Melting and boiling points increase (gas → liquid → solid as you go down)
• Colour becomes darker down the group

Why reactivity decreases

Halogens react by gaining one electron. Down the group, atoms have more electron shells. The extra shells shield the nucleus, so it attracts an incoming electron less strongly. This makes gaining that electron harder, so reactivity falls from fluorine to iodine.

Displacement reactions

A more reactive halogen displaces (pushes out) a less reactive halogen from its halide salt solution.

Order of reactivity: F₂ > Cl₂ > Br₂ > I₂

Example ionic equation:

[ \mathrm{Cl_2(aq) + 2\,Br^- (aq) \rightarrow 2\,Cl^- (aq) + Br_2(aq)} ]

Observation: solution turns orange-brown as Br₂ forms.

Predicting other halogens

• Fluorine: very pale yellow gas; most reactive.
• Astatine: predicted dense, dark solid; least reactive.

Real-world links

• Chlorine is used to disinfect drinking water and swimming pools.
• Iodine is used as an antiseptic and in iodised salt.

Common misconceptions

• Not all halogens are gases at room temperature.
• A less reactive halogen cannot displace a more reactive halide.
• Diatomic means two atoms per molecule, not two different elements.