The Halogens

Full Notes

Halogens

Halogens are found in group VII (17) of the periodic table and exist as diatomic molecules (F₂, Cl₂, Br₂, I₂).

Their boiling points increase down the group.

This is because:

• Halogens exist as diatomic molecules (X₂).
• Larger molecules have more electrons, leading to stronger induced dipole–dipole interactions (London Dispersion Forces).
• More energy is required to overcome these intermolecular forces.

This explains why Cl₂ is a gas, Br₂ is a liquid, and I₂ is a solid at room temperature.

Electron Configuration and Redox Behaviour

The halogens have outer shell configuration ns² np⁵ and they need one electron to complete their octet, forming X⁻ ions.

As a result, halogens act as oxidising agents. In reactions, they are reduced and the species they react with are oxidised.

Example Chlorine reduction

Cl₂ + 2e⁻ → 2Cl⁻ (Cl goes from 0 to −1)
Oxidation number decreases showing reduction.

Reactivity Trend and Displacement Reactions

A more reactive halogen displaces a less reactive halide ion from solution.

An organic solvent like hexane can be used to help identify the displaced halogen by colour.

Reactivity decreases down the group because:

• Atomic radius increases.
• Electron shielding increases.
• Nuclear attraction for incoming electron decreases, making it harder to gain an electron.

Disproportionation Reactions of Chlorine

Disproportionation reactions refer to when the same element is both oxidised and reduced simultaneously in a reaction.

You need to know the following disproportionation reactions of chlorine.

Chlorine and water:

This reaction is a disproportionation reaction because chlorine atoms (from Cl₂) are both oxidised and reduced. Their oxidation state changes from 0 (in Cl₂) to +1 (in ClO⁻) and −1 (in Cl⁻).

HClO is the active disinfecting agent in water treatment.

With cold, dilute NaOH:

Chlorine reacts with cold, dilute NaOH forming sodium chlorate(I) (NaClO) – the active ingredient in bleach.

Chlorine in Water Treatment

Chlorine is added to water to make it safe to drink. However, risks need to be balanced against benefits.

• Benefits:
  • Kills harmful bacteria.
  • Reduces risk of waterborne diseases like cholera and typhoid.
• Risks:
  • Cl₂ is a toxic gas and its use can be hazardous.
  • Can react with organic compounds in water to form chlorinated hydrocarbons, which may be carcinogenic.

Testing for Halide Ions with Silver Nitrate

Halide ions (Cl⁻, Br⁻, I⁻) can be identified using acidified silver nitrate (AgNO₃) solution followed by NH₃:

Procedure:

• Add dilute nitric acid (to remove interfering ions like CO₃²⁻).
• Add silver nitrate solution.
• Observe precipitate colour.
• Add aqueous ammonia to test solubility.

Summary

• Halogens are group 17 elements with outer electron configuration ns² np⁵.
• Boiling points increase down the group due to stronger London forces.
• Reactivity decreases down the group because of greater shielding and larger atomic radius.
• Halogens act as oxidising agents and take part in displacement and disproportionation reactions.
• Chlorine is widely used in water treatment despite associated health risks.
• Halide ions can be tested using AgNO₃ and solubility in NH₃.