2.3.1Inorganic Chemistry
Trends in Properties
5 min read 5 parts Lesson 9 of 27
Part 1 of 5Physical Properties
Halogens exist as diatomic molecules, X₂, held together by covalent bonds.
Fluorine
- A pale yellow gas at room temperature, made of diatomic molecules F₂ with very weak London forces.
Chlorine
- A pale green gas, which liquefies at about −35 °C.
Bromine
- A red-brown volatile liquid that gives off an orange-brown vapour.
Iodine
- A grey-black shiny solid that forms a purple vapour on gentle warming.
| Halogen | State at 298 K | Boiling point / °C |
|---|---|---|
| F₂ | Gas | −188 |
| Cl₂ | Gas | −35 |
| Br₂ | Liquid | 59 |
| I₂ | Solid | 184 |
Part 2 of 5Explaining Boiling Points
Explaining Boiling Points
London forces
- Down the group molecules contain more electrons, so instantaneous dipoles are larger and London forces are stronger.
Energy needed
- More energy is required to overcome intermolecular forces, so boiling points rise from fluorine to iodine.
Covalent bonds
- The covalent bond within each molecule is not broken during boiling, only the forces between molecules.
Volatility
- Volatility therefore decreases down the group as attraction between molecules grows.
Part 3 of 5Electronegativity
1
Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond.
2
Going down the group, atomic radius increases because each element has an extra electron shell.
3
The bonding pair is further from the nucleus and is also more shielded by the extra inner shells.
4
These effects outweigh the increase in nuclear charge, so attraction for the bonding pair decreases.
5
Pauling values fall from fluorine, 4.0, to chlorine 3.0, bromine 2.8 and iodine 2.5.
Part 4 of 5Oxidising Ability
Oxidising Ability
Trend
- The oxidising power of the halogens decreases down the group, so chlorine is a stronger oxidising agent than iodine.
Cause
- An incoming electron is added to a shell further from the nucleus and more shielded, so it is attracted less strongly.
Reducing ions
- Halide ions are the reverseiodide is the strongest reducing agent, as its outer electron is easiest to lose.
Evidence
- A more reactive halogen will displace a less reactive halide from solution.
Part 5 of 5Exam Checklist & Tips
2.3.1 – Key Points for the Exam
- Halogen An element in Group 17 (Group 7), with seven outer-shell electrons.
- Electronegativity The ability of an atom to attract the bonding electrons in a covalent bond.
- Volatility How easily a substance turns into a gas.
- London forces Weak attractions between induced dipoles in neighbouring molecules.
Common Mistakes
- Saying that covalent bonds break when halogens boil.
- Claiming electronegativity increases down the group because the nuclear charge increases.
- Calling chlorine a liquid at room temperature; it is a gas.
- Confusing the trend in oxidising power of halogens with the trend in reducing power of halides.