Group 7 — the Halogens

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.5 — Colours, physical states and trends in physical properties of chlorine, bromine and iodine
• 2.6 — Using the Group 7 trend to predict the properties of other halogens
• 2.7 — How halogen–halide displacement reactions show the Group 7 reactivity trend
• 2.8 — Explaining the Group 7 reactivity trend in terms of electronic configuration (bold C statement — Paper 2 only)

Specification section 2(b) Group 7 (halogens) — chlorine, bromine and iodine — also taught in Group 1 — the Alkali Metals.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The halogens are the Group 7 non-metals, and the three to know by sight are CHLORINE, a pale green gas; BROMINE, a red-brown liquid that gives off a choking brown vapour; and IODINE, a dark grey crystalline solid that forms a purple vapour when warmed. Gas, liquid, solid — going down the group in the space of three elements.

That is a physical trend and it has the explanation from Topic 1. All the halogens are simple molecular, existing as diatomic molecules — Cl₂, Br₂, I₂ — with weak forces between them. Down the group the molecules get larger, so the intermolecular forces get stronger, so more energy is needed to separate them and the MELTING AND BOILING POINTS RISE. Chlorine has already boiled at room temperature; iodine has not yet melted.

The colours deepen down the group too, from pale green through red-brown to almost black. So statement 2.6 lets you predict fluorine, which is above chlorine, and astatine, below iodine: fluorine should be a paler gas with a lower boiling point, and astatine a very dark solid with a higher melting point than iodine. Both are correct.

Now the chemical trend, and here the halogens do the opposite of Group 1. REACTIVITY DECREASES DOWN GROUP 7. Fluorine is the most reactive element in the whole periodic table; chlorine is very reactive; bromine less so; iodine less still.

Think of it like catching a ball rather than dropping one

Reaching out to CATCH a ball is easier if it comes close to your hands; a ball thrown wide, with people in the way, you will probably miss. Now compare that with DROPPING a ball you are already holding — holding it loosely at arm's length makes dropping easier, not harder. Group 1 atoms are dropping an electron and Group 7 atoms are catching one, and that single difference reverses everything. Being far from the nucleus and heavily shielded makes it easier to drop and harder to catch. So the same weakening attraction that makes Group 1 more reactive down the group makes Group 7 LESS reactive down the group.

Statement 2.8 — the explanation, and why it inverts

1. Every Group 7 atom has SEVEN electrons in its outer shell, and reacts by GAINING one to complete it.
2. Going DOWN the group, the outer shell is FURTHER from the nucleus and there are more full inner shells SHIELDING it.
3. So the attraction between the nucleus and an incoming electron is WEAKER.
4. The atom therefore attracts an electron LESS strongly and gains one LESS easily.
5. Since the reaction consists of gaining an electron, the halogen is LESS reactive. So reactivity decreases down the group.
6. Note that steps two and three are word for word the Group 1 explanation. Only the reaction differs — losing against gaining — and that is what reverses the conclusion.

Statement 2.7 is the experimental demonstration of that trend, and it is elegantly simple. A MORE reactive halogen will DISPLACE a LESS reactive one from a solution of its salt, because it takes the electron more strongly. Add chlorine water to potassium bromide solution and the colourless solution turns orange, because bromine has been displaced and released.

The equation is chlorine + potassium bromide → potassium chloride + bromine, or Cl₂ + 2KBr → 2KCl + Br₂. The solution turns orange as the displaced bromine is released, and it happens because chlorine sits above bromine in the group.

Reading a displacement table, and deducing the order

Chlorine displaces bromine from potassium bromide and iodine from potassium iodide. Bromine displaces iodine from potassium iodide. Iodine displaces neither. What is the order of reactivity, and what would happen if bromine were added to potassium chloride solution?

1. Chlorine displaces both of the others, so chlorine is the most reactive of the three.
2. Bromine displaces iodine but is itself displaced by chlorine, so bromine is in the middle.
3. Iodine displaces neither, so iodine is the least reactive.
4. The order is therefore chlorine, then bromine, then iodine — which is the order going DOWN the group, confirming reactivity decreases.
5. Bromine added to potassium chloride: bromine is BELOW chlorine and so less reactive, so NO REACTION occurs and the solution stays colourless.

Answer: Chlorine > bromine > iodine; and bromine cannot displace chloride, so there is no reaction.

That last case is the one worth expecting in an exam, because 'no reaction' is a real answer and students are reluctant to give it. A halogen can only displace one BELOW it in the group. Presented with a combination that runs the wrong way, the correct response is that nothing happens and the solution does not change colour.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 2.5 — the three halogens at room temperature

The vocabulary of this group

Halogen

A Group 7 non-metal. Each exists as diatomic molecules and reacts by gaining one electron.

Halide

The ion formed when a halogen gains an electron — chloride, bromide, iodide — or a compound containing one.

Displacement

A reaction in which a more reactive halogen takes the place of a less reactive one in a solution of its salt.

Diatomic

Existing as molecules of two atoms. All the halogens are diatomic: Cl₂, Br₂, I₂.

Statement 2.8 — explaining the reactivity trend

1. Every Group 7 atom has SEVEN outer electrons and reacts by GAINING one.
2. Down the group the outer shell is further from the nucleus, with more inner shells shielding it.
3. So the attraction for an incoming electron is WEAKER.
4. An electron is therefore gained LESS easily lower down the group.
5. Since the reaction consists of gaining an electron, reactivity DECREASES down the group.

Statement 2.7 — displacement, and what it proves

• A MORE reactive halogen DISPLACES a LESS reactive one from a solution of its salt
• So a halogen can displace any halogen BELOW it in the group, and none above it
• CHLORINE displaces bromine and iodine · BROMINE displaces iodine only · IODINE displaces neither
• The colour change is the evidence: chlorine into potassium bromide turns the solution ORANGE as bromine is released
• A combination running the wrong way gives NO REACTION and no colour change — a real and creditable answer

Why Group 1 and Group 7 trends run opposite ways

A. GROUP 1 atoms LOSE an electron. A weaker attraction lower down makes losing EASIER, so reactivity INCREASES down the group.

B. GROUP 7 atoms GAIN an electron. The same weaker attraction makes gaining HARDER, so reactivity DECREASES down the group.

Cl₂ + 2KI → 2KCl + I₂

Conditions: Chlorine displaces iodine from potassium iodide. The solution turns brown as iodine is released.

Model answer [4 marks]

Describe what is seen when chlorine water is added to colourless potassium bromide solution, and explain why. [4]

The colourless solution turns orange. Chlorine is above bromine in Group 7 and is therefore the more reactive halogen, so it displaces bromine from the potassium bromide. The bromine released is orange in solution, which produces the colour change. The equation is Cl₂ + 2KBr → 2KCl + Br₂.

Model answer [4 marks]

Explain why chlorine is more reactive than iodine. [4]

Both atoms have seven electrons in their outer shell and both react by gaining one electron. In iodine the outer shell is further from the nucleus than in chlorine, and there are more full inner shells shielding it from the nuclear charge. The attraction for an incoming electron is therefore weaker in iodine, so iodine gains an electron less easily. Since the reaction consists of gaining an electron, chlorine is the more reactive.

Not this: Reactivity increases down Group 7, as it does down Group 1.

This: It DECREASES. The explanation is identical — the outer shell is further out and better shielded — but Group 7 atoms GAIN an electron rather than losing one, so a weaker attraction makes them worse at it rather than better.

Mark-losing trap. Group 7 reactivity DECREASES down the group. Fluorine is the most reactive element there is.

Mark-losing trap. A halogen displaces only one BELOW it. Running the wrong way gives NO REACTION — say so.

Mark-losing trap. Melting points RISE down Group 7 while REACTIVITY FALLS. The two trends point opposite ways.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What are the colour and physical state of bromine at room temperature?
2. Grade 7 · Predict [2 marks] — Bromine solution is added to potassium chloride solution. What happens?
3. Grade 8 · Explain [4 marks] — Explain why iodine is less reactive than bromine.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of why reactivity decreases down Group 7, and of how displacement reactions demonstrate this.
5. 9+ · Analyse [6 marks] — A student notices that melting points rise down Group 7 while reactivity falls, and concludes that a higher melting point must cause lower reactivity. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who proved chlorine was an element and not a compound, and the one who had to make two opposite trends live in one table. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Humphry Davy — the one who proved chlorine was an element and not a compound

Chlorine was called oxymuriatic acid when Davy came to it, because Lavoisier's theory held that every acid must contain oxygen and the name simply assumed it. Davy tried repeatedly to extract oxygen from the green gas and could not, using the most powerful battery in the world to attack it. He concluded that it contained no oxygen at all and was an element in its own right, and named it chlorine from the Greek for pale green. It was a direct challenge to the most respected chemist of the age, made on the strength of a failure to find something.

• “How can failing to find something be evidence?”
• “Why was chlorine called an acid before you got to it?”
• “What was wrong with Lavoisier's theory of acids?”
• “How did you decide on the name chlorine?”
• “Was it difficult to contradict Lavoisier publicly?”

Dmitri Mendeleev — the one who had to make two opposite trends live in one table

This page and the last present a genuine puzzle: two columns of the same table, the same explanation applied to both, and opposite conclusions. Mendeleev built the arrangement in which that pattern is visible at all, ordering the elements so that properties repeat and grouping those that behave alike. He is the right person to ask why a table organised on one principle should produce trends that run in different directions in different columns, and how much he understood about WHY the pattern existed as opposed to that it did.

• “Why should two groups trend in opposite directions?”
• “Did you know why the pattern in your table existed?”
• “What made you place the halogens where you did?”
• “How did you handle elements that did not fit?”
• “What would you have made of the electron?”

Carry on through the course

• Previous lesson: Group 1 — the Alkali Metals
• Next lesson: The Gases in the Air
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• Rusting and its Prevention — Two things are needed to rust iron, and every method of stopping it removes one of them — or cheats.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification