9701 · 11.1
Physical properties of the Group 17 elements
Halogens show clear, predictable trends in their physical properties as you go down the group, such as increasing boiling points and deepening colour. These trends are explained by changes in electron shells, atomic size, and intermolecular forces.
Need to know
What you need to know
- Halogens are simple molecular substances. They have strong covalent bonds *within* their diatomic molecules, but only weak intermolecular forces *between* molecules.
- Boiling involves overcoming these weak intermolecular forces, not breaking the strong covalent bonds.
- The only intermolecular forces present between non-polar halogen molecules are instantaneous dipole-induced dipole (id-id) forces.
- As you go down Group 17, the number of electrons in each molecule increases (e.g., $F_2$ has 18 electrons, $Cl_2$ has 34 electrons).
- A greater number of electrons leads to a more polarisable electron cloud, resulting in larger temporary dipoles and therefore stronger id-id forces.
- More energy is required to overcome these stronger intermolecular forces, leading to a higher boiling point and lower volatility.
Explanation
Halogen Trends Unpacked
- Colour intensifies down group: pale → dark.
- Boiling point increases down group.
- Bond strength decreases: F–F anomalous weak.
- Oxidising power decreases down group.