Pearson Edexcel International GCSE Chemistry 4CH1: Group 1 (alkali metals) and Group 7 (halogens) – Revision Notes

These notes condense sub-topics (a) Group 1 (alkali metals) and (b) Group 7 (halogens) of Topic 2, Inorganic chemistry, in the Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 (September 2024). They cover points 2.1–2.8. The qualification is untiered; points 2.4C and 2.8C are bold with a C reference, so they are Paper 2 only. For full explanations and worked examples, use the Group 1 and Group 7 study guide.

Links: 4CH1 Chemistry course hub, printable 4CH1 checklist, free 4CH1 all-topics diagnostic. The wider topic is in the Reactivity and group trends revision notes. For exam-style questions, use the Groups 1 and 7, acids, salts and tests practice set and the Reactivity and group trends practice set.

The two trends in one line each

• Group 1: reactivity increases down the group (Li < Na < K).
• Group 7: reactivity decreases down the group (Cl₂ > Br₂ > I₂).

Learn them together. Same causes, opposite directions.

2.1 Group 1 as a family

Similar reactions with water show Li, Na and K belong together.

Shared observation	Why
Floats	Less dense than water
Fizzes	Hydrogen gas given off
Moves around	Pushed by the hydrogen
Gets smaller, disappears	Metal is used up
Indicator turns blue/purple	Alkaline hydroxide forms

General:  alkali metal + water → metal hydroxide + hydrogen
Pattern:  2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)

Every Group 1 metal forms a 1+ ion, so the hydroxide is always MOH.

2.2 Evidence for the Group 1 trend

	Lithium	Sodium	Potassium
Water	Fizzes steadily, stays solid	Fizzes rapidly, melts into a ball	Violent; hydrogen ignites, lilac flame
Air (cut surface)	Tarnishes slowly	Tarnishes quickly	Tarnishes almost at once

• Tarnishing = an oxide layer forming on the shiny surface.
• All three are stored under oil to keep out air and water vapour.
• Burning in air gives the oxide, e.g. 4Na(s) + O₂(g) → 2Na₂O(s).

Method: answering “give evidence for the trend”

1. Name the reaction (water or air).
2. Give a comparative observation for at least two metals (“potassium fizzes faster than lithium”).
3. Link it: faster reaction = more reactive.

2.3 Predicting other alkali metals

Continue the trend, and say its direction first.

Property	Trend down the group	Rubidium/caesium prediction
Reactivity	Increases	Explosive with water
Melting point	Decreases (Li 181 °C → K 63 °C)	Lower than potassium (Rb 39 °C, Cs 28 °C)
Hardness	Softer	Very soft
Density	Generally increases (K is an exception)	Rb and Cs are denser than water and sink
Formulae	Same 1+ ion	RbOH, CsCl, Rb₂O

Products and formulae follow the family pattern exactly. Physical trends are general, not perfect.

2.4C Explaining the Group 1 trend (Paper 2 only)

Configurations: Li 2.1, Na 2.8.1, K 2.8.8.1.

Four-link chain (learn it word for word)

1. More occupied shells, so the outer electron is further from the nucleus.
2. More inner shells, so more shielding.
3. So weaker attraction between nucleus and outer electron.
4. So the outer electron is lost more easily: more reactive.

Key equation: K → K⁺ + e⁻

2.5 Halogen physical properties

All are diatomic: Cl₂, Br₂, I₂.

Halogen	Room-temperature appearance	Boiling point (°C)
Chlorine	Pale green (yellow-green) gas	−34
Bromine	Red-brown liquid; orange-brown vapour	59
Iodine	Grey-black solid; purple vapour	184

Down the group:

• colour darkens
• gas → liquid → solid
• melting and boiling points increase (bigger molecules, stronger intermolecular forces; point 1.48)

2.6 Predicting other halogens

	Fluorine (above Cl)	Astatine (below I)
State	Gas	Solid
Colour	Paler than chlorine (pale yellow)	Darker than iodine (black)
Boiling point	Well below −34 °C	Above 184 °C
Reactivity	Most reactive halogen	Less reactive than iodine

2.7 Displacement: the evidence for the Group 7 trend

Rule: a more reactive halogen displaces a less reactive halogen from a solution of its halide.

Colours in solution: chlorine very pale green; bromine orange (yellow if dilute); iodine brown. Potassium and sodium halide solutions are colourless, so a colour that appears after mixing comes from the halogen produced. Where there is no reaction, the mixture simply keeps the colour of the halogen water you added.

Added ↓ / Solution →	KCl	KBr	KI
Cl₂	—	Orange	Brown
Br₂	✗	—	Brown
I₂	✗	✗	—

Two reactions for chlorine, one for bromine, none for iodine, so Cl₂ > Br₂ > I₂.

Method: writing a displacement equation

1. Check the halogen added is above the halide’s halogen. If not, write “no reaction”.
2. Swap: the added halogen becomes the halide ion; the halide becomes the halogen (diatomic).
3. Balance: one X₂ needs two halide ions.
4. Ionic form: delete the metal ion (spectator).

Full:   Br₂(aq) + 2NaI(aq) → 2NaBr(aq) + I₂(aq)
Ionic:  Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq)

Worked reminder: identifying an unknown halide

Solution Z + chlorine water → turns brown
Solution Z + bromine water  → turns brown
Brown = iodine formed, and both Cl₂ and Br₂ displaced it.
So Z contains iodide ions (e.g. potassium iodide).

If chlorine had given orange and bromine no change, Z would contain bromide ions.

In electron terms, the halogen gains electrons (reduced, the oxidising agent) and the halide ion loses electrons (oxidised). These terms come from point 2.20.

2.8C Explaining the Group 7 trend (Paper 2 only)

Halogens have 7 outer electrons (Cl 2.8.7) and react by gaining one electron: Br₂ + 2e⁻ → 2Br⁻.

Four-link chain

1. Down the group the outer shell is further from the nucleus.
2. More shielding by inner shells.
3. Weaker attraction for an incoming electron.
4. Electron gained less easily: less reactive.

Must-know distinctions

Group 1	Group 7
Metals	Non-metals
Lose 1 electron → 1+ ion	Gain 1 electron → 1− ion
Reactivity increases down	Reactivity decreases down
Melting point decreases down	Melting/boiling point increases down
Evidence: water and air	Evidence: displacement

Also keep apart: halogen (the element, Cl₂) and halide (the ion, Cl⁻, or its compound, KCl).

Quick self-test

1. State what you would see if universal indicator is added to the water after lithium has reacted.
2. Write the formula of caesium hydroxide and of rubidium oxide.
3. Give one observation that shows sodium is more reactive than lithium in water.
4. Why are alkali metals stored under oil?
5. Rubidium has a density of 1.53 g/cm³. Predict whether it floats on water.
6. Bromine is added to sodium chloride solution. What do you observe, and why?
7. Write the ionic equation for chlorine reacting with bromide ions.
8. Name the substance that is oxidised in question 7.
9. Predict the colour and state of fluorine at room temperature.
10. Iodine and astatide ions: predict whether a reaction happens.
11. (Paper 2 only) Lithium is 2.1 and potassium is 2.8.8.1. In one sentence, why is potassium’s outer electron lost more easily?
12. (Paper 2 only) Explain why chlorine attracts an extra electron more strongly than iodine.

Answers

1. It turns blue or purple: lithium hydroxide is an alkali.
2. CsOH; Rb₂O.
3. Any one: sodium fizzes faster; sodium melts into a ball while lithium stays solid; sodium moves around faster.
4. To stop them reacting with oxygen and water vapour in the air.
5. It sinks (1.53 g/cm³ is greater than water’s 1.00 g/cm³).
6. No change (stays orange). Bromine is less reactive than chlorine, so it cannot displace chlorine from chloride ions.
7. Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
8. The bromide ions (they lose electrons).
9. A pale yellow gas (paler than chlorine; boiling point below chlorine’s).
10. Yes: iodine is more reactive than astatine, so it displaces astatine.
11. It is further from the nucleus with more shielding, so it is attracted less strongly.
12. Chlorine’s outer shell is closer to the nucleus with less shielding, so the attraction for an incoming electron is stronger.

Where marks are usually lost

• Writing “halogens lose electrons” in a 2.8C explanation. They gain electrons.
• Stopping the explanation chain at “more shells”, without “weaker attraction” and “lost/gained more easily”.
• Saying “more protons, so stronger attraction” for the lower element. Distance and shielding outweigh the extra protons.
• Giving iodine solid as purple. The solid is grey-black.
• Describing bromine as a gas, or as “brown gas”. It is a red-brown liquid.
• Writing Cl instead of Cl₂ in equations, or forgetting the 2 in front of the halide.
• Saying bromine water “goes clear” or “decolourises” in displacement. That is the alkene test; here a new colour appears.
• Quoting an observation for only one metal when the question asks for a comparison.
• Using “fizzes” and “bubbles” as two separate observations.
• Stating a trend without its direction (“melting point changes down the group”).

Official syllabus

Pearson Edexcel International GCSE in Chemistry (4CH1) specification, Issue 3, September 2024, published by Pearson Education Limited – Topic 2 Inorganic chemistry, sub-topics (a) Group 1 (alkali metals) and (b) Group 7 (halogens), points 2.1–2.8C.