Group 7 — Halogens

This lesson covers the properties and reactions of the Group 7 elements (halogens), as required by AQA GCSE Combined Science Trilogy (8464, Chemistry 4.1.2). The halogens are fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). You need to know their properties, reactivity trends and displacement reactions.

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Physical Properties of the Halogens

The halogens are non-metals that exist as diatomic molecules ($\text{F}_2$F2​, $\text{Cl}_2$Cl2​, $\text{Br}_2$Br2​, $\text{I}_2$I2​).

Halogen	Symbol	State at Room Temperature	Colour
Fluorine	$\text{F}_2$F2​	Gas	Pale yellow
Chlorine	$\text{Cl}_2$Cl2​	Gas	Green/yellow-green
Bromine	$\text{Br}_2$Br2​	Liquid	Red-brown/orange-brown
Iodine	$\text{I}_2$I2​	Solid	Dark grey/purple vapour

Trends in Physical Properties Going Down Group 7

• Melting and boiling points increase — because the molecules get larger with more electrons, leading to stronger intermolecular forces (London dispersion forces/van der Waals forces).
• At room temperature, the halogens change from gas → liquid → solid going down the group.

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Electronic Configuration

All halogens have seven electrons in their outer shell:

Element	Atomic Number	Configuration
Fluorine	9	$2, 7$2,7
Chlorine	17	$2, 8, 7$2,8,7
Bromine	35	$2, 8, 18, 7$2,8,18,7

When they react, they gain one electron to achieve a full outer shell, forming negative ions (halide ions) with a charge of 1−:

• $\text{F}^-$F− (fluoride), $\text{Cl}^-$Cl− (chloride), $\text{Br}^-$Br− (bromide), $\text{I}^-$I− (iodide)

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Reactivity Trends Down Group 7

graph TD
    A["Going DOWN Group 7"] --> B["Atomic radius INCREASES"]
    B --> C["Outer shell is FURTHER from nucleus"]
    C --> D["More SHIELDING by inner electrons"]
    D --> E["Nucleus has WEAKER attraction for incoming electrons"]
    E --> F["Harder to GAIN an extra electron"]
    F --> G["Reactivity DECREASES"]

    style A fill:#2c3e50,color:#fff
    style G fill:#e74c3c,color:#fff

Why Reactivity Decreases Down the Group

1. Going down the group, atoms get larger — more electron shells.
2. The outer shell is further from the nucleus.
3. There is increased shielding from inner electron shells.
4. The attraction between the nucleus and incoming electrons is weaker.
5. It is harder for the atom to gain an electron.
6. Therefore, reactivity decreases.

Exam Tip (AQA 8464): Note the opposite trend to Group 1! Group 1 gets more reactive going down (easier to lose electrons). Group 7 gets less reactive going down (harder to gain electrons). This is because Group 1 elements lose electrons while Group 7 elements gain them.

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Reactions with Metals

Halogens react with metals to form ionic compounds called metal halides:

$2\text{Na}(s) + \text{Cl}_2(g) \rightarrow 2\text{NaCl}(s)$2Na(s)+Cl2​(g)→2NaCl(s)

$2\text{Fe}(s) + 3\text{Cl}_2(g) \rightarrow 2\text{FeCl}_3(s)$2Fe(s)+3Cl2​(g)→2FeCl3​(s)

The more reactive the halogen, the more vigorously it reacts.

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Displacement Reactions

A more reactive halogen can displace a less reactive halogen from a solution of its salt. This is because the more reactive halogen is better at gaining electrons.

Displacement Reaction Summary Table

	Potassium Chloride (KCl)	Potassium Bromide (KBr)	Potassium Iodide (KI)
Chlorine ($\text{Cl}_2$Cl2​)	No reaction	Orange/brown colour appears (Br displaced)	Brown colour appears (I displaced)
Bromine ($\text{Br}_2$Br2​)	No reaction	No reaction	Brown colour appears (I displaced)
Iodine ($\text{I}_2$I2​)	No reaction	No reaction	No reaction

Example Equations

$\text{Cl}_2(aq) + 2\text{KBr}(aq) \rightarrow 2\text{KCl}(aq) + \text{Br}_2(aq)$Cl2​(aq)+2KBr(aq)→2KCl(aq)+Br2​(aq)

Chlorine is more reactive than bromine, so chlorine displaces bromine from potassium bromide.

$\text{Cl}_2(aq) + 2\text{KI}(aq) \rightarrow 2\text{KCl}(aq) + \text{I}_2(aq)$Cl2​(aq)+2KI(aq)→2KCl(aq)+I2​(aq)

$\text{Br}_2(aq) + 2\text{KI}(aq) \rightarrow 2\text{KBr}(aq) + \text{I}_2(aq)$Br2​(aq)+2KI(aq)→2KBr(aq)+I2​(aq)

Exam Tip: A displacement reaction occurs when a more reactive halogen takes the place of a less reactive halogen. If the halogen being added is less reactive than the one in the compound, no reaction occurs.

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Worked Example

Question: Bromine water is added to potassium iodide solution. State and explain what you would observe. (3 marks)

Answer:

1. The solution turns brown (or darker).
2. Bromine is more reactive than iodine because bromine's outer shell is closer to the nucleus, so bromine gains electrons more easily.
3. Bromine displaces iodine from potassium iodide: $\text{Br}_2(aq) + 2\text{KI}(aq) \rightarrow 2\text{KBr}(aq) + \text{I}_2(aq)$Br2​(aq)+2KI(aq)→2KBr(aq)+I2​(aq).

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Comparing Group 1 and Group 7 Trends

Feature	Group 1 (Alkali Metals)	Group 7 (Halogens)
Electron behaviour	Lose outer electron	Gain an electron
Ion formed	Positive (1+)	Negative (1−)
Reactivity down the group	Increases	Decreases
Reason	Easier to lose electron (further from nucleus, more shielding)	Harder to gain electron (further from nucleus, more shielding)

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Common Mistakes

Mistake	Correction
Saying reactivity increases down Group 7	Reactivity decreases down Group 7
Confusing displacement direction	A more reactive halogen displaces a less reactive one — never the reverse
Forgetting that halogens are diatomic	Always write $\text{Cl}_2$Cl2​, $\text{Br}_2$Br2​, $\text{I}_2$I2​ — not Cl, Br, I
Saying bromine is a gas	Bromine is a liquid at room temperature
Not using colour observations in displacement answers	State the colour change: orange/brown for Br, brown for I

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Summary

• Halogens have 7 electrons in their outer shell and gain 1 electron to form 1− ions.
• They exist as diatomic molecules ($\text{X}_2$X2​).
• Going down the group: melting/boiling points increase; reactivity decreases.
• Reactivity decreases because the outer shell is further from the nucleus, there is more shielding, and it is harder to attract an incoming electron.
• A more reactive halogen displaces a less reactive halogen from its salt solution.
• Displacement reactions produce observable colour changes.

Exam Tip (AQA 8464): Displacement reactions are a required practical topic. You must be able to describe the method (adding halogen water to halide solutions), state the observations (colour changes), and write balanced equations with state symbols.

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Halide Ions vs Halogens — Colour Cheatsheet