Group 7: the halogensEdexcel A-Level Chemistry: Revision notes
Section 1
Physical properties and their trends
The halogens (fluorine, chlorine, bromine, iodine, astatine) exist as diatomic molecules, X₂, held together by London forces (instantaneous dipole–induced dipole forces).
| Halogen | State at room temperature | Colour |
|---|---|---|
| F₂ | gas | pale yellow |
| Cl₂ | gas | pale green |
| Br₂ | liquid | red-brown (orange vapour) |
| I₂ | solid | grey-black (purple vapour) |
Melting and boiling temperatures increase down the group. Molecules have more electrons, so the London forces between them are stronger and more energy is needed to overcome them. It is the intermolecular forces that are overcome, not the covalent X–X bond.
Saying the covalent bond gets stronger down the group. The bond is not broken when a halogen boils; London forces between molecules are.
Section 2
Electronegativity and reactivity
Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. It decreases down Group 7 from fluorine (the most electronegative element) to iodine.
Reason: going down the group the atomic radius increases and there is more shielding by inner shells, so the nucleus attracts the bonding pair (or an incoming electron) less strongly, even though nuclear charge rises.
Halogens react by gaining an electron, so they are oxidising agents. Reactivity therefore decreases down the group: an atom gains an electron less readily when the incoming electron is further from the nucleus and more shielded. Fluorine is the strongest oxidising agent; iodine is the weakest of the common four.
Always give both factors for trends: larger radius AND more shielding. Quote nuclear charge only to say that it does not outweigh them.
Section 3
Displacement reactions of halogens
A more reactive halogen oxidises the halide ions of a less reactive one, displacing the halogen. Examples (in aqueous solution):
- Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq): solution turns orange
- Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq): solution turns brown
- Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq): solution turns brown
- Iodine does not displace chloride or bromide; bromine does not displace chloride.
Adding an organic solvent such as cyclohexane, shaking and letting it settle makes the colours clearer. The halogen dissolves in the upper organic layer: chlorine pale green or colourless, bromine orange, iodine violet.
In terms of oxidation numbers, the halogen goes from 0 to −1 (reduced) and the halide ion goes from −1 to 0 (oxidised).
Colours in water are dilute and hard to tell apart; the organic layer gives orange for Br₂ and violet for I₂.
Section 4
Oxidation of metals and disproportionation
Halogens oxidise metals to halides: 2Na + Cl₂ → 2NaCl (Na 0 → +1) and Mg + Cl₂ → MgCl₂ (Mg 0 → +2). The halogen is reduced from 0 to −1.
Disproportionation is a reaction in which the same element is simultaneously oxidised and reduced.
- With water: Cl₂ + H₂O ⇌ HCl + HClO (Cl 0 → −1 and 0 → +1). Chlorine is used to treat drinking water because chlorate(I) kills bacteria; this prevents disease but chlorine is toxic and can form harmful chlorinated organic compounds.
- With cold dilute NaOH: Cl₂ + 2NaOH → NaCl + NaClO + H₂O (Cl −1 and +1). The mixture is household bleach.
- With hot NaOH: 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O (Cl −1 and +5), giving sodium chlorate(V).
Bromine and iodine react in analogous ways, for example Br₂ + 2NaOH → NaBr + NaBrO + H₂O when cold.
Quoting the oxidation state of the whole compound. State the oxidation number of chlorine in each species: NaClO is +1 and NaClO₃ is +5.
Section 5
Predicting properties of fluorine and astatine
Trends let you predict properties of elements and compounds at the ends of the group.
- Fluorine: pale yellow gas, most electronegative, strongest oxidising agent; it would oxidise all other halide ions.
- Astatine: below iodine, so a black solid with a higher melting and boiling temperature than iodine; weaker oxidising agent than iodine so it would not displace iodide ions; the astatide ion, At⁻, would be a stronger reducing agent than I⁻.
Predictions rest on regular trends, but astatine is radioactive and very short-lived, so they cannot easily be tested.
Must know
- Halogens are diatomic with London forces; melting and boiling temperatures rise down the group
- States: F₂ and Cl₂ gases, Br₂ liquid, I₂ solid
- Electronegativity and oxidising power decrease down the group (larger radius, more shielding)
- Displacement: Cl₂ displaces Br⁻ and I⁻; Br₂ displaces I⁻; organic layer orange (Br₂), violet (I₂)
- Disproportionation: Cl₂ with water, cold NaOH (+1), hot NaOH (+5)
- Predict: fluorine strongest oxidising agent; astatine a black solid
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Group 7: the halogens
- A technician compares the Group 7 elements chlorine, bromine and iodine in a fume cupboard at room temperature (about 20 °C).Explain why iodine has a higher boiling temperature than chlorine.2 marks
- A student investigates displacement reactions of the halogens. She has aqueous solutions of potassium chloride, potassium bromide and potassium iodide, together with chlorine water, bromine water, aqueous iodine and the organic solvent cyclohexane, which is less dense than water and immiscible with it.The student adds aqueous iodine to potassium bromide solution and sees no reaction. Explain this, referring to the relative oxidising power of bromine and iodine.2 marks
- A water company treats drinking water with chlorine to kill bacteria. A separate factory makes household bleach by passing chlorine into sodium hydroxide solution.Chlorine reacts with water as it dissolves. Write an equation for the reaction and explain, in terms of oxidation numbers, why it is a disproportionation reaction.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).