Group 17: the halogens

This page covers a single Form 4 content standard: Group 17, the halogens. It is the perfect partner to Group 1, because the halogens show the opposite reactivity trend and are explained by the same idea, atomic size. Learn the colours and states, the reactions, the displacement rule, and the reason reactivity decreases down the group, and you will have everything the exam asks for on this standard.

What the halogens are

Group 17 contains fluorine (F), chlorine (Cl), bromine (Br) and iodine (I), with astatine below them. They are reactive non-metals and exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). Every halogen atom has seven valence electrons, so it needs to gain just one more electron to reach a stable, full outer shell, this is the key to their chemistry.

Physical properties: colour and state

You must know how the colour and physical state change down the group:

• Chlorine is a greenish-yellow gas.
• Bromine is a reddish-brown liquid that gives off a brown vapour.
• Iodine is a purplish-black solid that sublimes to a purple vapour when warmed.

So going down the group, the colour gets darker, and the physical state changes from gas → liquid → solid. The melting point and boiling point increase down the group, and the density increases.

Chemical properties

Because each atom has seven valence electrons, a halogen reacts by gaining one electron to form an ion with a −1 charge (for example Cl becomes Cl⁻, called a chloride ion). Two reactions you should know:

• With water. Chlorine dissolves in water to form chlorine water, which is acidic and acts as a bleach, so it can decolourise dyes.
• With hot iron. A halogen reacts with heated iron to form an iron(III) halide. For chlorine: 2Fe + 3Cl₂ → 2FeCl₃.

The reactivity trend and its reason

The most examined idea is that reactivity decreases down Group 17: chlorine is more reactive than bromine, which is more reactive than iodine. You must be able to explain this:

Explanation. Going down the group, each atom has one more electron shell, so the atomic size increases. The outermost shell is therefore further from the nucleus, and the nucleus attracts an incoming electron less strongly. This means the atom gains an electron less easily, so the halogen is less reactive. That is why reactivity decreases from chlorine to iodine.

Displacement reactions

A more reactive halogen displaces a less reactive halogen from a solution of its halide. Because reactivity decreases down the group, a halogen higher up will displace one lower down. For example, chlorine (higher) displaces bromine (lower) from potassium bromide:

Cl₂ + 2KBr → 2KCl + Br₂

The colourless solution turns brown as free bromine is produced, which confirms the displacement has happened.

Worked example

Question. Chlorine gas is bubbled through a colourless solution of potassium iodide. (a) State what you would observe. (b) Write a balanced chemical equation. (c) Explain, in terms of reactivity, why this reaction happens.

Step 1, Predict from reactivity. Chlorine is above iodine in Group 17, so chlorine is more reactive than iodine.

Step 2, State the observation. The more reactive chlorine displaces iodine from the solution. Iodine is produced, so the solution turns brown.

Step 3, Write the equation. Cl₂ + 2KI → 2KCl + I₂.

Step 4, Explain. Chlorine is more reactive because it is smaller, so it attracts an electron more strongly and gains an electron more easily than iodine. It therefore takes the place of iodine in the compound, releasing free iodine.

Answer. (a) The solution turns brown. (b) Cl₂ + 2KI → 2KCl + I₂. (c) Chlorine is more reactive than iodine (smaller atom, gains an electron more easily), so it displaces iodine from potassium iodide.

Practice question

Question. (a) Arrange chlorine, bromine and iodine in order of decreasing reactivity. (b) State whether bromine can displace chlorine from potassium chloride, and explain your answer.

Answer. (a) In order of decreasing reactivity: chlorine > bromine > iodine. (b) Bromine cannot displace chlorine, because bromine is less reactive than chlorine (bromine is lower in the group, so it is a larger atom that gains an electron less easily). A less reactive halogen cannot displace a more reactive one.

Exam tip

Learn the colours and states exactly, chlorine (greenish-yellow gas), bromine (reddish-brown liquid), iodine (purplish-black solid), because a description question expects the precise words. For any reactivity or displacement question, always explain the trend using increasing atomic size, the outer shell being further from the nucleus, and the atom therefore gaining an electron less easily. Note that this is the mirror image of Group 1: for the metals a larger atom loses an electron more easily (more reactive), while for the halogens a larger atom gains an electron less easily (less reactive). Getting these the right way round is the single most common source of lost marks.

Where this fits

This standard sits alongside Group 1 in the The Periodic Table of Elements chapter and reinforces the same size-based reasoning. Revise the colours, states and equations with the revision notes and drill the displacement reactions with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure you never confuse the Group 1 and Group 17 trends and that your displacement equations and observations are complete, which is exactly where the marks are on this standard across SPM Chemistry.

Quick recap

• Group 17 halogens have 7 valence electrons and form −1 ions.
• Down the group: colour darkens, state goes gas → liquid → solid, boiling point rises.
• A more reactive halogen displaces a less reactive one from its halide.
• Reactivity decreases down the group because the larger atom gains an electron less easily.