# Halogens

The reactive non-metals of group 7, such as fluorine, chlorine, bromine and iodine. They have 7 outer electrons and form molecules of two atoms.

Chemistry (school) · Part 02: The periodic table · Card 7 of 9 · Last updated 2026-10-19

## Analogy

A halogen is one electron short of a full set, so it grabs one from other atoms.

## Example

**Halogens at room temperature**

Chlorine: green gas
Bromine: red-brown liquid
Iodine: grey solid

## In practice

Chlorine is used to kill germs in swimming pools. Reactivity goes down as you move down the group.

## Practice

### Flashcards

- **How does reactivity change down group 7?** It decreases. Chlorine is more reactive than iodine.

### Interview questions

**Describe the trend in reactivity of the halogens and give an example reaction that shows it.**

- Reactivity decreases down group 7
- Smaller atoms attract an extra electron more strongly
- A more reactive halogen displaces a less reactive one from a salt
- Chlorine displaces bromine from potassium bromide

## Questions people ask

### Why are halogens found as pairs of atoms?

Each halogen atom has 7 outer electrons and needs one more for a full shell. Two atoms can share a pair of electrons, so each gets a full outer shell. This forms a stable molecule such as Cl2, held by a covalent bond.

### What is a halogen displacement reaction?

A more reactive halogen pushes a less reactive one out of its compound. When chlorine water is added to potassium bromide, chlorine takes the potassium and bromine is released, turning the solution orange. Bromine cannot do the same to potassium chloride because it is less reactive.

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