Chlorine, bromine and iodine are all halogens found in Group 7 of the periodic table. Explain why the halogens become less reactive as you go down Group 7.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Halogens react with metals and non-metals by gaining one electron to form a negative ion. The reactivity of halogens decreases going down the group.
Model answer (5 marks)
1. Halogens gain one electron to complete their outer shell.
2. Going down the group, atoms have more electron shells.
3. The outer shell is further from the nucleus.
4. Inner shells provide greater shielding of the nucleus.
5. The attraction between the nucleus and the incoming electron is weaker, making it harder to gain an electron and so the halogen is less reactive.
2. Going down the group, atoms have more electron shells.
3. The outer shell is further from the nucleus.
4. Inner shells provide greater shielding of the nucleus.
5. The attraction between the nucleus and the incoming electron is weaker, making it harder to gain an electron and so the halogen is less reactive.
Examiner tips
- Use the exact wording ‘gain one electron’ and ‘complete the outer shell’. Mention the increase in electron shells and shielding. Show the link between weaker attraction and reduced reactivity. Keep the answer concise and point‑wise to match the 5‑mark structure.
Common mistakes
- Confusing the halogen’s reactivity with that of the metals. Omitting the role of shielding or the distance of the outer shell. Using vague terms like ‘less reactive’ without explaining why.
Mark scheme (5 marks)
- Atoms gain one electron (to complete their outer shell / form a full outer shell)
- Going down the group, atoms have more electron shells / the outer shell is further from the nucleus
- There is greater shielding of the nucleus by inner electron shells going down the group
- The attraction between the nucleus and the incoming electron is weaker (going down the group)
- So it is harder / less easy to gain an electron, making the halogen less reactive going down the group
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Inorganic chemistry and the Periodic Table questions
- Sodium and potassium are both alkali metals in Group 1 of the periodic table. Wh…
- Explain why elements in Group 1 of the periodic table become more reactive as yo…
- Explain why the noble gases in Group 0 of the periodic table are described as ch…
- Transition metals and Group 1 metals are both found in the periodic table, but t…