Group 7 of the periodic table contains the halogens – a group of non-metals with seven electrons in their outer shell. They are known for being reactive and for forming salts when they react with metals.

Elements in Group 7

Electron structure and reactivity

All halogens have seven outer electrons, so they:

• Are one electron short of a full outer shell
• Tend to gain one electron in reactions to form 1⁻ ions (halide ions)

As you go down the group:

• Reactivity decreases
• Atoms become larger
• Outer electrons are further from the nucleus
• There is less attraction, so it’s harder to gain an extra electron

Trends down Group 7

These changes are linked to increasing atomic mass and intermolecular forces.

Reactions with metals

Halogens react with metals to form ionic compounds called metal halides.

Example:

Sodium + chlorine → sodium chloride
2Na + Cl₂ → 2NaCl

Each halogen atom gains one electron to become a halide ion (Cl⁻, Br⁻, etc.).

Displacement reactions

A more reactive halogen can displace a less reactive halogen from a compound.

This shows their reactivity trend.

Example:

Chlorine + potassium iodide → potassium chloride + iodine
Cl₂ + 2KI → 2KCl + I₂

Chlorine is more reactive than iodine, so it displaces it.

🧪 You can often see a colour change during displacement:

• Iodine appears as a brown solution when it’s displaced