Fluorine, chlorine, bromine and iodine are all members of Group 7. Describe the trend in physical appearance of the halogens from fluorine to iodine at room temperature and pressure, and explain what this tells us about the reactivity of the halogens down the group.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
At room temperature and pressure, fluorine is a pale yellow gas, chlorine is a yellow-green gas, bromine is a red-brown liquid, and iodine is a grey-black solid.
Model answer (5 marks)
Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid at room temperature and pressure.
The colour becomes darker down the group – fluorine is pale yellow, chlorine yellow‑green, bromine red‑brown, iodine grey‑black.
Reactivity decreases down the group.
This is because the atomic radius increases, so the outermost electrons are further from the nucleus.
The further distance makes it harder to attract an extra electron, so the halogen is a weaker oxidising agent.
The colour becomes darker down the group – fluorine is pale yellow, chlorine yellow‑green, bromine red‑brown, iodine grey‑black.
Reactivity decreases down the group.
This is because the atomic radius increases, so the outermost electrons are further from the nucleus.
The further distance makes it harder to attract an extra electron, so the halogen is a weaker oxidising agent.
Examiner tips
- State the physical state change and colour change first. Link the trend to atomic radius and electron attraction. Use the phrase "reactivity decreases" and explain why.
- common_mistakes
- :
- Confusing the order of states (e.g. saying bromine is a gas). Forgetting to mention the role of atomic radius. Using vague terms like "less reactive" without explaining the electron‑attraction reason.
Mark scheme (5 marks)
- The halogens change from gas to liquid to solid going down the group (fluorine and chlorine are gases, bromine is a liquid, iodine is a solid)
- The colour darkens / becomes deeper going down the group (from pale yellow to grey-black)
- Reactivity decreases going down Group 7
- Because atomic radius increases down the group, so the outer shell is further from the nucleus
- So it is harder to attract an extra electron to complete the outer shell / gain an electron, making the halogen a weaker oxidising agent
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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