11.1 Physical properties of the Group 17 elements
- Syllabus
- 9701–2028–2029
- Topic
- 11.1
- Level
- AS
Learning objectives
11.1.1Colours and the trend in volatility• Describe the colours and the trend in volatility of chlorine, bromine and iodine11.1.2Trend in the bond strength of the halogen• Describe/explain: the trend in the bond strength of the halogen molecules11.1.3The volatility of the elements• Interpret the volatility of the elements in terms of instantaneous dipole–induced dipole forces
Halogen colours deepen and volatility decreases from chlorine to iodine
| Element | Formula | Appearance at room conditions | Relative volatility |
|---|---|---|---|
| chlorine | Cl₂ | pale green / yellow-green gas | highest of the three |
| bromine | Br₂ | red-brown liquid with orange-brown vapour | intermediate |
| iodine | I₂ | grey-black solid; purple vapour when heated | lowest of the three |
From Cl₂ → Br₂ → I₂, colour becomes darker, melting and boiling points rise, and volatility falls. Volatility is the tendency to enter the gas phase, so a more volatile substance has a lower boiling point under comparable conditions.
State the colour for the specified physical form: iodine solid is grey-black, while iodine vapour is purple. The cause of the volatility trend is taught separately through forces between X₂ molecules, not through the X–X covalent bond.
X–X bond strength generally decreases down Group 17, with F₂ anomalous
| Molecule | Approximate X–X bond enthalpy / kJ mol⁻¹ | Comparison |
|---|---|---|
| F₂ | 158 | anomalously weaker than Cl₂ |
| Cl₂ | 243 | strongest of these four |
| Br₂ | 193 | weaker than Cl₂ |
| I₂ | 151 | weakest |
From Cl₂ → Br₂ → I₂, atomic radius and X–X bond length increase. The shared bonding pair is farther from both nuclei and attracted less strongly, so bond enthalpy and bond strength decrease.
F₂ breaks the simple trend. Fluorine atoms are so small that non-bonding electron pairs on the two atoms are very close; strong lone-pair–lone-pair repulsion weakens the F–F bond enough to make it weaker than Cl–Cl.
Bond enthalpy measures the energy needed to break the covalent bond inside X₂. It does not predict boiling point: boiling separates intact molecules and is controlled mainly by forces between them.
Larger X₂ electron clouds create stronger temporary attractions
Cl₂, Br₂ and I₂ are non-polar simple molecules. Their intermolecular attractions are instantaneous dipole–induced dipole forces: a momentary uneven electron distribution in one molecule induces a dipole in a neighbouring molecule.
| Step down the group | Consequence |
|---|---|
| each X₂ molecule contains more electrons and a larger electron cloud | the cloud is more polarisable and fluctuates more readily |
| instantaneous and induced dipoles become larger | attractions between neighbouring X₂ molecules become stronger |
| more energy is needed to separate molecules | melting and boiling points increase |
| fewer molecules escape into the gas phase at a given temperature | volatility decreases: Cl₂ > Br₂ > I₂ |
This explains the room-condition sequence gas Cl₂ → liquid Br₂ → solid I₂. The particles remain neutral diatomic molecules throughout; only the strength of attraction between molecules changes.
Do not invoke permanent dipoles: each X₂ molecule contains identical atoms and is non-polar. Do not use increasing X–X bond strength—the intramolecular bond actually weakens from Cl₂ to I₂ while boiling point rises.