Describe the trend in boiling points of the halogens as you go down Group 7, and explain why this trend occurs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The halogens in Group 7 include fluorine, chlorine, bromine, iodine, and astatine. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid.
Model answer (5 marks)
Boiling points increase down Group 7.
The halogen atoms become larger and contain more electrons as you move down the group.
This increase in size and electron number strengthens London dispersion forces (van der Waals forces) between the molecules.
Stronger dispersion forces mean more energy is required to separate the molecules.
Therefore the boiling point is higher for the lower halogens (e.g. iodine > bromine > chlorine).
The halogen atoms become larger and contain more electrons as you move down the group.
This increase in size and electron number strengthens London dispersion forces (van der Waals forces) between the molecules.
Stronger dispersion forces mean more energy is required to separate the molecules.
Therefore the boiling point is higher for the lower halogens (e.g. iodine > bromine > chlorine).
Examiner tips
- Use the exact phrase "boiling points increase down the group" for the first point. Include the reason "larger atoms, more electrons" and link to stronger London dispersion forces. Finish with a clear conclusion that higher boiling points result from stronger forces.
Common mistakes
- Mixing up the order of the halogens or saying boiling points decrease. Failing to mention London dispersion forces as the key intermolecular force. Using vague terms like "more energy" without linking to dispersion forces.
Mark scheme (5 marks)
- Boiling points increase going down Group 7
- The molecules / atoms get larger / have more electrons going down the group
- Stronger intermolecular forces (London dispersion forces / van der Waals forces) between the molecules
- More energy is needed to overcome these stronger intermolecular forces
- Therefore the boiling point is higher for halogens lower in the group (e.g. iodine has a higher boiling point than chlorine)
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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