Section 1 of 4Features of the Halogens
The halogens are Group 7 non-metals that share a common pattern of structure and bonding.
Non-metals
- Halogens are poisonous non-metals that exist as diatomic molecules, such as F₂ and Cl₂, rather than single atoms.
Outer electrons
- Each halogen atom has seven outer electrons and needs one more to achieve a full outer shell.
Covalent bond
- The two atoms in a halogen molecule share one pair of electrons in a single covalent bond.
Halide ions
- Halogens react with metals to form ionic compounds containing halide ions with a charge of −1.
Section 2 of 4Trends Down the Group
Going down Group 7, the physical properties change in a regular way.
Trends Down the Group
Melting and boiling points
- Both increase going down the group because the molecules are larger and have stronger intermolecular forces.
Colour
- The halogens become darker going down the group, from pale yellow fluorine to dark grey-black iodine.
State
- The state changes from gas to liquid to solid going down the group, because the melting and boiling points rise.
Atomic size
- Atoms get larger going down the group because each step adds another electron shell.
| Halogen | State at 20 °C | Colour |
|---|---|---|
| Fluorine, F₂ | Gas | Pale yellow |
| Chlorine, Cl₂ | Gas | Pale green-yellow |
| Bromine, Br₂ | Liquid | Red-brown |
| Iodine, I₂ | Crumbly solid | Dark grey-black |
Section 3 of 4Why the Trends Happen
The explanation follows the change in molecule size.
1
Going down Group 7, each halogen atom has one more electron shell than the atom above it.
2
Larger molecules contain more electrons, which creates stronger intermolecular forces between the molecules.
3
More energy is needed to overcome these stronger forces, so the melting and boiling points increase.
4
The stronger forces explain why iodine is a solid at room temperature while fluorine and chlorine are gases.
Section 4 of 4Exam Checklist & Tips
7.1.2 – Key Points for the Exam
- Halogen A Group 7 non-metal that forms diatomic molecules and reacts with metals to form halides.
- Diatomic Describes a molecule made of two atoms, such as Cl₂.
- Intermolecular forces Attractions between separate molecules, which are weaker than the covalent bonds inside them.
- Halide ion A negative ion of a halogen, such as Cl⁻ or Br⁻.
Common Mistakes
- Writing the halogen molecules as F or Cl; they are diatomic, so the formulae are F₂ and Cl₂.
- Saying halogens lose electrons to form ions; they gain one electron to form −1 halide ions.
- Confusing intermolecular forces with covalent bonds;
- melting breaks the forces between molecules, not the bonds inside them.
- Claiming melting points rise because the bonds become stronger;
- the increase is due to stronger intermolecular forces.