Group 7: The Halogens

This lesson covers the properties, trends and reactions of the Group 7 elements (the halogens), as required by the Edexcel GCSE Combined Science specification (1SC0). You need to know their physical and chemical properties, how reactivity changes down the group, and be able to explain halogen displacement reactions.

---

The Group 7 Elements

The halogens are the non-metal elements in Group 7 of the periodic table:

Element	Symbol	Atomic Number	State at Room Temperature	Colour	Electron Configuration
Fluorine	F₂	9	Gas	Pale yellow	2, 7
Chlorine	Cl₂	17	Gas	Yellow-green	2, 8, 7
Bromine	Br₂	35	Liquid	Red-brown	2, 8, 18, 7
Iodine	I₂	53	Solid	Dark grey/purple vapour	2, 8, 18, 18, 7
Astatine	At₂	85	Solid (radioactive, very rare)	—	—

Key Points

• All halogens have seven electrons in their outer shell.
• Halogens exist as diatomic molecules (two atoms bonded together): F₂, Cl₂, Br₂, I₂.
• They are all non-metals.
• They are toxic and must be handled with care (especially chlorine and fluorine).

---

Physical Properties and Trends

Trends Going Down Group 7

Property	Trend Down Group 7
Melting point	Increases
Boiling point	Increases
State at room temperature	Gas → Liquid → Solid
Colour	Gets darker
Reactivity	Decreases

The increase in melting and boiling points is due to stronger intermolecular forces between the larger molecules as you go down the group.

Exam Tip: The reactivity trend in Group 7 is the opposite of Group 1. In Group 1, reactivity increases going down. In Group 7, reactivity decreases going down. Make sure you can explain both trends.

---

Why Reactivity Decreases Down Group 7

Halogens react by gaining one electron to form a negative ion with a −1 charge (called a halide ion):

Cl + e⁻ → Cl⁻

As you go down Group 7:

1. The atoms get larger — there are more electron shells.
2. The outer shell is further from the nucleus.
3. There is more shielding from inner electron shells.
4. The attraction between the nucleus and an incoming electron is weaker.
5. Therefore it is harder to gain an extra electron → the halogen is less reactive.

graph TD
    A["Going down Group 7"] --> B["More electron shells"]
    B --> C["Outer shell further<br/>from nucleus"]
    C --> D["Weaker attraction for<br/>incoming electron"]
    D --> E["Harder to gain<br/>an extra electron"]
    E --> F["Reactivity<br/>DECREASES"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#e67e22,color:#fff
    style F fill:#27ae60,color:#fff

---

Reactions of Halogens

Reaction with Metals

Halogens react with metals to form metal halides (ionic compounds). For example:

Sodium + Chlorine → Sodium chloride 2Na(s) + Cl₂(g) → 2NaCl(s)

Iron + Bromine → Iron(III) bromide 2Fe(s) + 3Br₂(g) → 2FeBr₃(s)

Reaction with Hydrogen

Halogens react with hydrogen to form hydrogen halides, which dissolve in water to form acidic solutions:

H₂(g) + Cl₂(g) → 2HCl(g)

The vigour of the reaction with hydrogen decreases down the group (fluorine reacts explosively, iodine reacts only slowly and with a catalyst).

---

Halogen Displacement Reactions

A more reactive halogen can displace (push out) a less reactive halogen from an aqueous solution of its salt. This is called a displacement reaction.

The Rule

A halogen will displace any halogen below it in Group 7 from a solution of its salt.

Displacement Reactions Table

	Potassium chloride (KCl)	Potassium bromide (KBr)	Potassium iodide (KI)
Chlorine (Cl₂)	No reaction	✓ Displacement occurs	✓ Displacement occurs
Bromine (Br₂)	No reaction	No reaction	✓ Displacement occurs
Iodine (I₂)	No reaction	No reaction	No reaction

Example Reactions

Chlorine + potassium bromide → potassium chloride + bromine Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

The solution turns orange/brown as bromine is produced.

Chlorine + potassium iodide → potassium chloride + iodine Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)

The solution turns brown/dark as iodine is produced.

Bromine + potassium iodide → potassium bromide + iodine Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)

Exam Tip: Displacement reactions are one of the most commonly tested topics for Group 7. Remember: a more reactive halogen displaces a less reactive one. Chlorine can displace bromine and iodine; bromine can only displace iodine; iodine cannot displace either of the others.

---

Observing Displacement Reactions

You can observe displacement reactions by the colour changes that occur:

Halogen/Halide Ion	Colour in Solution
Chlorine (Cl₂)	Pale green/colourless
Chloride (Cl⁻)	Colourless
Bromine (Br₂)	Orange/brown
Bromide (Br⁻)	Colourless
Iodine (I₂)	Brown/dark brown
Iodide (I⁻)	Colourless

When a displacement reaction occurs, you see the colour of the displaced halogen appear in the solution.

---

Uses of Halogens

Halogen	Uses
Fluorine	Added to water and toothpaste to prevent tooth decay; making Teflon
Chlorine	Sterilising water supplies; making bleach and PVC
Bromine	Making flame retardants and some pharmaceuticals
Iodine	Antiseptic for wounds; used in medical imaging

---

Comparing Group 7 to Group 1

Feature	Group 1 (Alkali Metals)	Group 7 (Halogens)
Type	Metals	Non-metals
Outer electrons	1	7
Ion formed	+1 (lose 1 electron)	−1 (gain 1 electron)
Reactivity trend	Increases down group	Decreases down group
Melting point trend	Decreases down group	Increases down group

Exam Tip: Comparing Group 1 and Group 7 trends is a favourite exam question. The key difference is that Group 1 metals lose electrons (easier further from nucleus → more reactive down group) while Group 7 non-metals gain electrons (harder further from nucleus → less reactive down group).

---

Summary