Group 7(17), the halogens
The halogens in Group 7 are very reactive non-metals.
Full topic guide: the detailed syllabus page with worked examples and common mistakes lives at studyvector.co.uk/a-level/chemistry/inorganic-chemistry/group-7-halogens.
Topic preview: Group 7(17), the halogens
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Topic explanation
The Group 7 elements, the halogens, are reactive non-metals with seven outer electrons. This topic covers the trends in their physical properties, such as boiling points (increase down the group due to stronger van der Waals forces) and electronegativity (decreases down the group). A key chemical property is their ability to act as oxidising agents, with their oxidising power decreasing down the group. This leads to displacement reactions where a more reactive halogen will displace a less reactive halide ion from solution. The reactions of halide ions with concentrated sulfuric acid and with silver nitrate solution are also important identifying tests.
Group 7(17), the halogens is easiest to revise when it is treated as a precise exam behaviour, not a loose note-taking category. In A-Level Chemistry, the goal is to recognise how the topic appears in a question, identify the command word, and decide what evidence, method, or vocabulary earns marks. StudyVector keeps this page tied to AQA · Edexcel · OCR language where coverage is available, then routes practice towards the same topic so revision moves from explanation into retrieval.
A strong revision session starts with a short recall check. Write down the rule, definition, process, or method linked to Group 7(17), the halogens before looking at any notes. Then answer one exam-style prompt and compare your answer with the mark-scheme logic: did you make a clear point, support it with the right step, and avoid drifting into a nearby topic? This matters because many lost marks come from almost-correct answers that do not match the expected structure.
Use this guide as the first layer: understand the topic, look at the worked examples, complete the mini quiz, then move into full practice. The full StudyVector practice loop is designed to capture whether mistakes are caused by knowledge, method, language, or timing. That distinction is important. If the error is factual, you need reteaching. If the error is method-based, you need a worked retry. If the error is wording, you need command-word calibration. That is how Group 7(17), the halogens becomes a controlled revision target rather than another page in a folder.
Lost marks → repair task
Why marks are usually lost here
These are the error patterns StudyVector looks for after an attempt. The goal is not a generic explanation; it is one repair move and one follow-up question.
Unit, formula, or method slip
Examiner move: Select the correct method and keep units, substitutions, signs, and rounding visible.
Repair drill: Redo the calculation or method line slowly, naming the formula before substituting values.
Missing chain of reasoning
Examiner move: Show the link between point, method, evidence, and conclusion instead of jumping to the final line.
Repair drill: Write the missing because/therefore step, then retry one isomorphic question.
Command-word miss
Examiner move: Answer the action in the command word before adding extra detail.
Repair drill: 60-second rewrite: start the answer with explain, compare, evaluate, state, or calculate in mind.
Mini quiz
Use these checks before full practice. They test topic recognition, exam technique, and whether you can connect the explanation to a marked response.
1. What should you check first when a Group 7(17), the halogens question appears in A-Level Chemistry?
- A.The command word and the exact topic focus
- B.The longest paragraph in your notes
- C.A memorised answer from a different topic
2. Which revision action gives the strongest evidence that Group 7(17), the halogens is improving?
- A.Rereading the explanation twice
- B.Answering a timed exam-style question and reviewing lost marks
- C.Highlighting every key phrase in the topic notes
Sample questions
Topic-specific public question previews are still being reviewed. We keep them off public pages until the topic match is safe.
Exam tips
- Read the command word carefully — "explain" needs reasons; "state" expects a short fact.
- For Group 7(17), the halogens, show structured working even when you are practising multiple choice — it builds accuracy under time pressure.
- Mark yourself against the mark scheme style: one clear point per mark, in logical order.
- Come back to this topic after a day or two; short spaced reviews beat one long cram.
Worked examples
Example 1
Modelled exam response
Write an ionic equation for the displacement reaction between chlorine water and potassium bromide solution. Step 1: Identify the more reactive halogen. Chlorine is above bromine in Group 7, so it is more reactive and a stronger oxidising agent. Step 2: Chlorine will oxidise bromide ions to bromine. Chlorine itself is reduced to chloride ions. Step 3: The ionic equation is Cl2(aq) + 2Br-(aq) -> 2Cl-(aq) + Br2(aq).
Example 2
Identify the task before answering
Question type: a Group 7(17), the halogens prompt asks for a clear response in A-Level Chemistry. Step 1: underline the command word. Step 2: name the exact part of Group 7(17), the halogens being tested. Step 3: decide whether the mark scheme wants a definition, method, explanation, comparison, or calculation. Why it works: most weak answers fail before the content starts because they answer the topic generally rather than the exact exam task.
Example 3
Turn feedback into a repair task
Suppose your answer shows partial understanding but loses marks for precision. First, rewrite the missing mark as a short target: "I need to state the mechanism, unit, reason, or evidence explicitly." Then answer one similar question without notes. Finally, compare the second attempt with the first and check whether the same mark was recovered. Why it works: Group 7(17), the halogens improves faster when feedback creates a specific retry, not another passive reading session.
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Common mistakes
- Confusing the trend in reactivity of the halogens with that of the halides. Halogen reactivity decreases down the group, whereas the reducing power of the halide ions increases down the group.
- Incorrectly identifying the products of the reaction between halide ions and concentrated sulfuric acid. For example, with bromide ions, sulfuric acid is reduced to sulfur dioxide, not hydrogen sulfide.
- Mixing up the colours of the silver halide precipitates. Silver chloride (AgCl) is white, silver bromide (AgBr) is cream, and silver iodide (AgI) is yellow.
Exam board notes
All boards cover the characteristic trends and displacement reactions of the halogens. AQA often includes questions on the reactions of halides with concentrated sulfuric acid, testing the different products formed. Edexcel places emphasis on the use of silver nitrate and ammonia solution to distinguish between halide ions. OCR may ask more about the industrial importance of halogens, such as the use of chlorine in water treatment.
FAQs
Why do boiling points increase down Group 7?
As you go down the group, the number of electrons in the halogen molecules increases. This leads to stronger temporary dipole-induced dipole forces (van der Waals forces) between the molecules, which require more energy to overcome, resulting in higher boiling points.
What is disproportionation?
Disproportionation is a redox reaction where the same element is both oxidised and reduced. An example is the reaction of chlorine with cold, dilute sodium hydroxide: Cl2(aq) + 2NaOH(aq) -> NaCl(aq) + NaClO(aq) + H2O(l), where chlorine is reduced to -1 in NaCl and oxidised to +1 in NaClO.
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