9701 · 11.1

Physical properties of the Group 17 elements — practice questions

Practice and worked examples for 9701 Physical properties of the Group 17 elements. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

Explain why the boiling point of iodine ( $184^\circ C$ ) is much higher than that of chlorine ( $-34^\circ C$ ). [4 marks]

Show solution outline

Both chlorine ( $Cl_2$ ) and iodine ( $I_2$ ) are simple molecular substances with weak intermolecular forces between their molecules. [1]
The intermolecular forces are instantaneous dipole-induced dipole (id-id) forces. [1]
An iodine molecule has more electrons (106) than a chlorine molecule (34). [1]
This results in stronger id-id forces between iodine molecules, which require more energy to overcome, leading to a higher boiling point. [1]

Worked example 2

The bond enthalpy of the Cl-Cl bond is +243 kJ mol⁻¹, while that of the F-F bond is only +158 kJ mol⁻¹. Explain this difference.

Show solution outline

A fluorine atom has a smaller atomic radius than a chlorine atom.
In a diatomic $F_2$ molecule, this small size brings the non-bonding lone pairs of electrons on the two atoms into close proximity.
This causes significant electrostatic repulsion between the lone pairs.
This repulsion weakens the covalent F-F bond, lowering its bond enthalpy compared to the Cl-Cl bond where the atoms are larger and repulsion is less significant.

Mark a 9701 question on this topic Full lesson Open in course studio