Group 7 – The Halogens

OCR

GCSE

Group 7 elements, the halogens, are non-metals that exist as diatomic molecules with distinct periodic trends in physical and chemical properties. Candidates must analyze the trend of increasing melting and boiling points down the group, attributed to increasing intermolecular forces, alongside the trend of decreasing reactivity, explained by atomic radius and shielding effects. Mastery of this topic requires the construction of balanced symbol equations for displacement reactions, which serve as key evidence for the reactivity series and introduce redox concepts through electron transfer mechanisms.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating that reactivity decreases down the group due to increased atomic radius and shielding effects
Credit responses that identify the physical states at room temperature: Chlorine (gas), Bromine (liquid), Iodine (solid)
Award marks for constructing balanced symbol equations where a more reactive halogen displaces a less reactive halide from solution
Candidates must explain that halogens exist as diatomic molecules held together by weak intermolecular forces
Award 1 mark for predicting the properties of Astatine (e.g., dark solid, low reactivity) by extrapolating group trends

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You correctly identified the trend, but you must explain *why* using terms like 'shielding' and 'nuclear attraction'"
"Check your equation balancing—remember that halogens are diatomic (e.g., Cl2, not Cl)"
"You have confused the element with the ion; ensure you write 'chloride' when it is part of a compound"
"Excellent prediction of Astatine's properties; ensure you explicitly link this back to the trend seen in the upper group"

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating that reactivity decreases down the group due to increased atomic radius and shielding effects
Credit responses that identify the physical states at room temperature: Chlorine (gas), Bromine (liquid), Iodine (solid)
Award marks for constructing balanced symbol equations where a more reactive halogen displaces a less reactive halide from solution
Candidates must explain that halogens exist as diatomic molecules held together by weak intermolecular forces
Award 1 mark for predicting the properties of Astatine (e.g., dark solid, low reactivity) by extrapolating group trends

Examiner Tips

Expert advice for maximising your marks

💡When explaining reactivity trends, explicitly reference the 'weaker attraction between the nucleus and the incoming electron' for lower elements
💡For prediction questions involving Astatine, extrapolate strictly from the trends of Chlorine, Bromine, and Iodine without introducing external knowledge
💡In displacement questions, always check if the free halogen is higher in the group than the halogen in the compound; if not, write 'no reaction'
💡Memorise the specific colours: Chlorine (pale green), Bromine (orange/brown), Iodine (grey solid/purple vapour)

Common Mistakes

Pitfalls to avoid in your exam answers

Stating that reactivity increases down the group by confusing Group 7 trends with Group 1 alkali metals
Attributing low melting/boiling points to the breaking of covalent bonds rather than weak intermolecular forces
Confusing the terms 'halogen' (element) and 'halide' (ion) when describing displacement products
Omitting state symbols or using incorrect symbols (e.g., writing Br2(aq) as Br(l)) in ionic equations

Key Terminology

Essential terms to know

Periodic trends in physical properties (melting/boiling points)

Reactivity trends explained by atomic structure

Displacement reactions and redox processes

Formation of ions and ionic salts with metals

Likely Command Words

How questions on this topic are typically asked

State

Explain

Predict

Describe

Write

Compare

Practical Links

Related required practicals

•{"code":"PAG C4","title":"Distinguishing between Halide Ions","relevance":"Use of silver nitrate solution to identify chloride, bromide, and iodide ions"}
•{"code":"Displacement Investigation","title":"Reactivity of Halogens","relevance":"Adding halogen solutions to halide salts to observe colour changes"}

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