What are the general chemical properties of halogen elements in Group 17?

Halogen elements, found in Group 17 of the periodic table, are highly reactive non-metals. They have seven valence electrons, making them eager to gain one more electron to achieve a stable octet configuration. This results in their high electronegativity and strong oxidizing properties. Halogens can form salts when they react with metals and are known for their ability to form diatomic molecules, such as Cl2 and Br2. Their reactivity decreases down the group, with fluorine being the most reactive.

How do the chemical properties of hydrogen halides vary across the halogen group?

Hydrogen halides are binary compounds formed between hydrogen and halogens. Their chemical properties vary across the group due to differences in bond strength and acidity. As you move down the group from HF to HI, the bond strength decreases, making HI the weakest and most easily dissociated in water. Consequently, the acidity of hydrogen halides increases down the group, with HI being the strongest acid. HF is an exception, as it forms strong hydrogen bonds, making it less acidic than expected.

Why is fluorine the most reactive halogen, and how does this affect its chemical behavior?

Fluorine is the most reactive halogen due to its high electronegativity and small atomic size, which allows it to attract electrons more effectively than other halogens. This high reactivity means fluorine can form compounds with almost all elements, including noble gases under certain conditions. Its strong oxidizing ability makes it capable of displacing other halogens from their compounds, a property utilized in various industrial applications.

What role do halogens play in redox reactions, and how is this relevant to AS-Level Chemistry?

In redox reactions, halogens act as strong oxidizing agents due to their high electronegativity and ability to gain electrons. This property is crucial in many chemical processes, such as the extraction of metals and organic synthesis. For AS-Level Chemistry students, understanding the role of halogens in redox reactions is essential for predicting reaction outcomes and balancing redox equations, which are key skills in the Cambridge International A Levels curriculum.

How does the reactivity of halogens influence their industrial applications?

The reactivity of halogens significantly influences their industrial applications. For instance, chlorine is widely used in water purification and the production of PVC due to its ability to kill bacteria and form stable compounds. Bromine is used in flame retardants, while iodine is essential in pharmaceuticals and antiseptics. Understanding these applications helps AS-Level Chemistry students appreciate the practical implications of halogen reactivity and their importance in various industries.

Why is "Saying oxidising power increases down the group." a common mistake in The chemical properties of the halogen elements and the hydrogen halides?

Why it happens: Confusing with halide-ion reducing power. How to avoid it: Halogen oxidising power decreases down the group (Cl₂ > Br₂ > I₂). Source: 9701 Examiner Reports 2022-2024

Why is "Thinking displacement works in both directions." a common mistake in The chemical properties of the halogen elements and the hydrogen halides?

Why it happens: Not ranking oxidising power. How to avoid it: A halogen only displaces the halides BELOW it (Cl₂ displaces Br⁻/I⁻, but I₂ displaces neither).

Why is "Explaining HX stability without bond strength." a common mistake in The chemical properties of the halogen elements and the hydrogen halides?

Why it happens: Giving the trend without the cause. How to avoid it: Longer, weaker H–X bond down the group → less stable.

Group 17(AS-Level Inorganic Chemistry)Cambridge International A Level Chemistry 9701

The chemical properties of the halogen elements and the hydrogen halides

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Chemical Properties of the Halogens and Hydrogen Halides — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

The halogens as oxidising agents (and displacement reactions), their reactions with hydrogen, and the trend in thermal stability of the hydrogen halides explained by bond strength.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

11.2.1 — Describe the relative reactivity of the elements as oxidising agents.
11.2.2 — Describe the reactions of the elements with hydrogen and explain their relative reactivity in these reactions.
11.2.3 — Describe the relative thermal stabilities of the hydrogen halides and explain these in terms of bond strengths.

The halogens as oxidising agents

Oxidising power falls down the group; a more reactive halogen displaces a less reactive halide.

The halogens are oxidising agents — they gain electrons to form halide ions ( $\text{X}_2 + 2e^- \rightarrow 2\text{X}^-$ ).

Oxidising power decreases down the group: $\text{Cl}_2 > \text{Br}_2 > \text{I}_2$ Going down, the atom is larger with more shielding, so it attracts an extra electron less strongly → weaker oxidising agent.

Displacement reactions demonstrate this: a more reactive halogen displaces a less reactive halide ion from solution.

$\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$ (orange solution forms).
$\text{Cl}_2 + 2\text{KI} \rightarrow 2\text{KCl} + \text{I}_2$ (brown solution / grey-black solid).
$\text{Br}_2 + 2\text{KI} \rightarrow 2\text{KBr} + \text{I}_2$ .
Iodine cannot displace Cl⁻ or Br⁻ (it is the weakest oxidising agent).

Oxidising power: Cl₂ > Br₂ > I₂.
Larger atom + more shielding → gains e⁻ less easily.
More reactive halogen displaces less reactive halide.

Reactions with hydrogen

All give HX; the vigour decreases down the group.

Each halogen reacts with hydrogen to form a hydrogen halide: $\text{H}_2 + \text{X}_2 \rightarrow 2\text{HX}$

The reactivity (vigour) decreases down the group:

F₂: reacts explosively, even in the cold and dark.
Cl₂: reacts explosively in sunlight/UV (via free-radical substitution; see 14.1).
Br₂: reacts slowly, needs heat and a catalyst.
I₂: reacts very slowly and reversibly (partial reaction), even when heated.

This mirrors the decreasing reactivity (oxidising power) of the elements down the group.

H₂ + X₂ → 2HX.
Vigour decreases F₂ → I₂.
F₂ explosive in dark; I₂ slow and reversible.

Thermal stability of the hydrogen halides

HX gets less thermally stable down the group as the H–X bond weakens.

The thermal stability of the hydrogen halides decreases down the group: $\text{HCl} > \text{HBr} > \text{HI}$

Why? Down the group the halogen atom is larger, so the H–X bond is longer and weaker (lower bond energy). A weaker bond is more easily broken by heat.

So HI is the least stable and decomposes most readily back into its elements when heated: $2\text{HI} \rightarrow \text{H}_2 + \text{I}_2$ (You'd see purple iodine vapour.) HCl is very stable and barely decomposes.

Thermal stability: HCl > HBr > HI.
H–X bond gets longer/weaker down the group.
HI decomposes most easily (→ H₂ + I₂).

How it’s examined

Displacement reactions, the oxidising-power trend and HX thermal stability are common Paper 1/2 marks (and link to halide tests). Examiner reports flag reversing the oxidising/stability trends and not explaining them via atomic size / bond strength.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 11.2; 9701 Examiner Reports 2022-2024; 9701/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-20.

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Step-by-step worked examples — The chemical properties of the halogen elements and the hydrogen halides

Step-by-step solutions to past-paper-style questions on the chemical properties of the halogen elements and the hydrogen halides, written exactly the way a tutor would explain them at the board.

1Reactivity with hydrogen (2 marks)
Getting started• reactions
Question
Compare the vigour of the reactions of chlorine and iodine with hydrogen.
Step-by-step solution
1. Step 1
  Chlorine reacts explosively in sunlight; iodine reacts slowly and reversibly.
2. Step 2
  Reactivity decreases down the group.
Answer
Cl₂ reacts vigorously (explosive in light); I₂ reacts slowly and reversibly.
Examiner tip
Reactivity (oxidising power) falls down the group, so reactions with H₂ get less vigorous.
2Predicting displacement (2 marks)
Getting started• displacement
Question
Will (a) chlorine displace bromide ions, and (b) iodine displace chloride ions? Justify.
Step-by-step solution
1. Step 1
  (a) Yes — Cl₂ is a stronger oxidising agent than Br₂, so it displaces Br⁻.
2. Step 2
  (b) No — I₂ is a weaker oxidising agent than Cl₂, so it cannot displace Cl⁻.
Answer
Cl₂ displaces Br⁻ (yes); I₂ does not displace Cl⁻ (no) — a halogen only displaces those below it.
3Halogen displacement (3 marks)
Building confidence• displacement
Question
Chlorine is bubbled into potassium bromide solution. State the observation, write the equation and explain.
Step-by-step solution
1. Step 1
  Observation: solution turns orange (bromine formed).
2. Step 2
  Equation:
  $\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$
3. Step 3
  Explanation: Cl₂ is a stronger oxidising agent than Br₂, so it displaces Br⁻.
Answer
Orange Br₂ forms; Cl₂ + 2KBr → 2KCl + Br₂; Cl₂ is the stronger oxidising agent.
4Thermal stability of HX (3 marks)
Building confidence• thermal stability
Question
Explain why hydrogen iodide is less thermally stable than hydrogen chloride.
Step-by-step solution
1. Step 1
  Iodine atom is larger than chlorine, so the H–I bond is longer.
2. Step 2
  Longer bond → weaker bond (lower bond energy).
3. Step 3
  Weaker bond breaks more easily on heating → HI less stable (decomposes to H₂ + I₂).
Answer
H–I is longer and weaker than H–Cl, so HI decomposes more readily.
Examiner tip
Link stability to bond length/strength, not to the molecule's size in general.
5Explaining the oxidising-power trend (4 marks)
Stretch• oxidising power
Question
Explain why oxidising power decreases down Group 17 (Cl₂ > Br₂ > I₂). (4)
Step-by-step solution
1. Step 1
  Oxidising means gaining an electron to form X⁻.
2. Step 2
  Down the group the atom is larger and the incoming electron enters a shell further from the nucleus.
3. Step 3
  More shielding by inner electrons reduces the nuclear attraction.
4. Step 4
  So the atom attracts an electron less strongly → weaker oxidising agent down the group.
Answer
Larger, more shielded atoms down the group gain an electron less easily → oxidising power falls.
Examiner tip
Frame it as ease of GAINING an electron — the opposite reasoning to halide reducing power.
6Displacement ionic equations and colours (4 marks)
Stretch• displacement, ionic equation
Question
Chlorine water is added to separate solutions of bromide and iodide ions, then shaken with cyclohexane. Write the ionic equations and state the cyclohexane colours.
Step-by-step solution
1. Step 1
  With bromide:
  $\text{Cl}_2 + 2\text{Br}^- \rightarrow 2\text{Cl}^- + \text{Br}_2$
2. Step 2
  Br₂ in cyclohexane = orange.
3. Step 3
  With iodide:
  $\text{Cl}_2 + 2\text{I}^- \rightarrow 2\text{Cl}^- + \text{I}_2$
4. Step 4
  I₂ in cyclohexane = violet (purple).
Answer
Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ (orange in cyclohexane); Cl₂ + 2I⁻ → 2Cl⁻ + I₂ (violet in cyclohexane).
Examiner tip
Ionic equations omit the spectator K⁺; the cyclohexane colour confirms which halogen was displaced.

Key Formulae — The chemical properties of the halogen elements and the hydrogen halides

The formulae you need to memorise for the chemical properties of the halogen elements and the hydrogen halides on the Cambridge International A Level 9701 paper, with every variable defined in plain English and a note on when to use it.

Halogen reduction half-equation
$\text{X}_2 + 2e^- \rightarrow 2\text{X}^-$
$X_2$
the halogen (Cl₂, Br₂, I₂) acting as an oxidising agent
When to use
Showing the halogen gaining electrons; the ease of this decreases down the group (Cl₂ > Br₂ > I₂).

Key Definitions and Keywords — The chemical properties of the halogen elements and the hydrogen halides

Definitions to memorise and the exact keywords mark schemes credit for the chemical properties of the halogen elements and the hydrogen halides answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Oxidising power (halogens)
Examiner keyword
The ability of a halogen to gain electrons; decreases down the group (Cl₂ > Br₂ > I₂).
Displacement reaction
Examiner keyword
A more reactive halogen displaces a less reactive halide ion from solution (a halogen displaces those below it).
Reactivity trend (Group 17)
Examiner keyword
Reactivity (oxidising power) decreases down the group, so reactions with hydrogen and metals get less vigorous.
Thermal stability of HX
Examiner keyword
Resistance of a hydrogen halide to decomposition on heating; decreases down the group as the H–X bond weakens.

Common Mistakes and Misconceptions — The chemical properties of the halogen elements and the hydrogen halides

The traps other students keep falling into on the chemical properties of the halogen elements and the hydrogen halides questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Saying oxidising power increases down the group.
9701 Examiner Reports 2022-2024
Why it happens
Confusing with halide-ion reducing power.
How to avoid it
Halogen oxidising power decreases down the group (Cl₂ > Br₂ > I₂).
✕Thinking displacement works in both directions.
Why it happens
Not ranking oxidising power.
How to avoid it
A halogen only displaces the halides BELOW it (Cl₂ displaces Br⁻/I⁻, but I₂ displaces neither).
✕Explaining HX stability without bond strength.
Why it happens
Giving the trend without the cause.
How to avoid it
Longer, weaker H–X bond down the group → less stable.

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