Group 7 Halogens: Trends, Reactivity and Displacement

Boiling points increase down Group 7 because the halogen molecules get bigger, so the forces between molecules (intermolecular forces) get stronger and more energy is needed to overcome them. Reactivity decreases down the group because the outer shell gets further from the nucleus and is more shielded, so the atom attracts an extra electron less strongly.

Those two sentences are the core of almost every Group 7 question. The marks are lost in the wording, not the idea, so this guide covers the trends, the colours and states, displacement reactions with ionic equations, and the answers that examiners do not credit.

Group 7 elements: what you need to know

The halogens are fluorine, chlorine, bromine, iodine and (in some specifications) astatine. Each has seven electrons in its outer shell, and each exists as a diatomic molecule, for example Cl₂ and Br₂. The atoms in each molecule are held together by a single covalent bond. Hold on to that last point, because it matters in the next section.

The physical properties you are expected to recall:

• Fluorine — pale yellow gas
• Chlorine — pale green gas (often described as yellow-green)
• Bromine — red-brown liquid that gives off an orange-brown vapour
• Iodine — grey-black solid that gives off a purple vapour when warmed

The colour gets darker and the state moves from gas to liquid to solid as you go down the group. That change in state is itself evidence for the trend in melting and boiling points.

Why does boiling point increase down Group 7?

This is also the answer to “what is the trend in melting points in Group 7”: both melting and boiling points increase down the group. The reason is the size of the molecules. Going from Cl₂ to Br₂ to I₂, each molecule has more electrons and a larger surface, so the intermolecular forces between molecules become stronger. Stronger forces take more energy to overcome, so more heating is needed to melt or boil the substance.

The trap here is the covalent bond. Melting or boiling a halogen does not break the Cl–Cl or I–I bond. The molecules stay intact; only the weak forces between them are overcome. Writing “the covalent bonds get stronger” or “it takes more energy to break the bonds” is the most common way students lose these marks, because it describes the wrong thing entirely.

A full-mark answer looks like this:

• The molecules get larger down the group.
• So the intermolecular forces between the molecules are stronger.
• So more energy is needed to overcome these forces, giving higher melting and boiling points.

Why does reactivity decrease down Group 7?

Yes, reactivity decreases down Group 7. Fluorine is the most reactive halogen and iodine (or astatine) the least. A halogen reacts by gaining one electron to complete its outer shell, so the question is how easily the atom can pull that electron in.

Going down the group:

• The atoms have more electron shells, so the outer shell is further from the nucleus.
• There is more shielding from the inner shells.
• So the attraction between the nucleus and an incoming electron is weaker.
• So it is harder to gain an electron, and reactivity decreases.

Examiners want all three links: distance, shielding, and the consequence for gaining an electron. “The atoms are bigger so they are less reactive” is too thin to score on its own, and so is any answer that talks about losing electrons. Halogens gain electrons.

The contrast with Group 1

In Group 1 the trend runs the other way: reactivity increases down the group. The explanation uses the same ideas, but the reaction is the opposite. A Group 1 metal reacts by losing its single outer electron. Further down, that electron is further from the nucleus and more shielded, so it is lost more easily. For Group 1, a weaker hold on the outer electron means more reactive. For Group 7, a weaker pull on an incoming electron means less reactive. If you are asked to compare them, say what each group does with its outer electron before you give the trend.

Displacement reactions

A more reactive halogen displaces a less reactive halogen from a solution of its salt. This is the practical evidence for the reactivity trend, and it is a standard exam question.

• Chlorine water added to potassium bromide solution: the solution turns orange as bromine forms.
• Chlorine water added to potassium iodide solution: the solution turns brown as iodine forms.
• Bromine water added to potassium iodide solution: the solution turns brown as iodine forms.
• Iodine added to potassium chloride or potassium bromide solution: no reaction, because iodine is less reactive.

The word equation for the first reaction is chlorine + potassium bromide → potassium chloride + bromine. The balanced symbol equation is:

Cl₂ + 2KBr → 2KCl + Br₂

Higher-tier and IGCSE questions often ask for the ionic equation. The potassium ions are spectator ions and do not change, so they are left out:

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

The same pattern gives the other two:

• Cl₂ + 2I⁻ → 2Cl⁻ + I₂
• Br₂ + 2I⁻ → 2Br⁻ + I₂

In each case the halogen molecule gains electrons (it is reduced) and the halide ion loses electrons (it is oxidised). If your course covers redox, you can add that: the more reactive halogen is the better electron acceptor. Check the charges balance on both sides. The most common slip is writing Br₂ with a charge, or leaving out the 2 in front of the halide ion.

Answers that lose marks

• “Stronger covalent bonds.” The bond inside the molecule is not what changes when a halogen melts or boils.
• “Stronger bonds between the atoms.” Be explicit: say “intermolecular forces between molecules”.
• “Iodine is bigger so it is less reactive” with no mention of shielding, distance from the nucleus or gaining an electron.
• “Chlorine is stronger than bromine.” Use “more reactive”. Reactivity is the term the mark scheme uses.
• Mixing up the colours. Bromine solution is orange and iodine solution is brown. Marks are given for the observation, so write the colour change, not just “it reacts”.
• Forgetting the ionic equation is about ions. Do not include K⁺ in the final line; it is a spectator ion.

Wording differs slightly by board, but these ideas are the same in GCSE and IGCSE chemistry. If you are revising the topic for a specific board, the GCSE AQA Group 7 notes, the GCSE Edexcel Group 7 notes, the IGCSE Edexcel halogens notes and the IGCSE Oxford AQA Group 7 notes follow the structure of each course. To test recall of the colours and states, try the Group 7 flashcards.

Understanding why is one thing, and writing it in the exact form the mark scheme wants is another. It also helps to know how command words change what an “explain” answer needs. Practise explanation questions on Exaim and every written answer is marked against the board’s mark scheme, so you can see which link in the chain you missed, whether that was intermolecular forces, shielding or the ionic equation.