Pearson Edexcel IGCSE Chemistry: Reactivity and Group Trends — Revision Notes

Condensed for the final weeks. For the full explanation, use the Reactivity and Group Trends study guide.

Group 1: alkali metals (a)

Lithium, sodium and potassium react similarly with water (evidence they’re a family), but with increasing vigour down the group (evidence of the reactivity trend). Reactivity increases down Group 1 because the single outer electron is progressively easier to lose as it sits further from the nucleus, with more shielding.

Group 7: halogens (b)

Chlorine, bromine and iodine — physical states/colours change down the group (chlorine: pale green gas; bromine: red-brown liquid; iodine: grey-black solid). Displacement reactions between halogens and halide solutions provide the evidence: a more reactive halogen displaces a less reactive one from its salt solution. Reactivity decreases down Group 7 because the outer shell needing one more electron is progressively harder to fill as it sits further from the nucleus, with more shielding.

Memorise the opposite directions: Group 1 reactivity increases down; Group 7 reactivity decreases down. A common exam-pressure error is applying the wrong direction to the wrong group.

Gases in the atmosphere (c)

Gas	Approx. % by volume in dry air
Nitrogen	~78%
Oxygen	~21%
Argon	~1%
Carbon dioxide	~0.04%

Measuring % oxygen experimentally: react a metal (e.g. iron) or non-metal (e.g. phosphorus) with the oxygen in a sealed volume of air, and measure the volume decrease. Combustion of magnesium, hydrogen and sulfur in oxygen. Thermal decomposition of metal carbonates (e.g. copper(II) carbonate) releases CO₂. CO₂ is a greenhouse gas, linked to climate change.

Reactivity series (d)

MOST reactive -> LEAST reactive
Potassium, Sodium, Lithium, Calcium, Magnesium, Aluminium, Zinc,
Iron, Copper, Silver, Gold

Learn this eleven-metal order as one memorised sequence — nearly every question in this section assumes it as background knowledge. Determined from: reactions with water, reactions with dilute HCl/H₂SO₄, and displacement reactions (metal + metal oxide; metal + aqueous metal salt).

Rusting: iron rusts in the presence of both water and oxygen together (neither alone is sufficient). Prevention: barrier methods (paint, oil — physically excludes water/oxygen), galvanising (zinc coating — also provides sacrificial protection if scratched), sacrificial protection (a more reactive metal, e.g. zinc or magnesium, corrodes preferentially, protecting the iron).

Oxidation, reduction and redox

Definition style	Oxidation	Reduction
Oxygen-based	Gain of oxygen	Loss of oxygen
Electron-based	Loss of electrons	Gain of electrons

Redox = a reaction where oxidation and reduction happen simultaneously. Practise both definitions on the same reaction — exam questions may use either convention.

Worked example: predicting a displacement reaction

Will zinc react with copper sulfate solution?

Reactivity order (relevant part): ... zinc, iron, copper ...
Zinc is MORE reactive than copper (higher in the series).
Prediction: zinc displaces copper -- forms zinc sulfate + copper metal.
Redox view: zinc is OXIDISED (loses electrons, Zn -> Zn2+);
            copper ions are REDUCED (gain electrons, Cu2+ -> Cu).

Worked example: identifying the more reactive halogen

Chlorine gas is bubbled through a solution of potassium bromide. A colour change is observed (colourless to orange-brown).

Observation:  colourless solution turns orange-brown
Interpretation: chlorine has displaced bromine from potassium
                bromide, forming potassium chloride and free
                bromine (which colours the solution orange-brown)
Conclusion:   chlorine is MORE reactive than bromine, consistent
              with chlorine sitting above bromine in Group 7 and
              reactivity decreasing down the group

If the reaction had been attempted the other way round (bromine added to potassium chloride solution), no reaction would occur, since bromine cannot displace a halogen more reactive than itself – being able to predict which direction a Group 7 displacement will and won’t work in, not just describe one direction, is what this sub-topic’s practical evidence is really testing.

Linking (a)-(d) together

These four sub-topics build a single connected argument even though they’re taught separately: Groups 1 and 7 establish that reactivity trends exist and can be explained by electron configuration; the reactivity series (d) extends the same underlying idea (how easily an element loses or gains electrons) across metals generally, beyond just Group 1; and gases in the atmosphere (c) sits alongside as the specification’s required practical context for measuring reactivity experimentally (via combustion and thermal decomposition reactions). Revising all four as one connected story about how and why elements react differently, rather than four disconnected fact-lists, makes the reactivity series easier to reconstruct even under exam pressure if a specific fact briefly slips your mind.

Key terms

Reactivity series — metals ranked by chemical reactivity, from potassium (most) to gold (least). Displacement reaction — a more reactive element takes the place of a less reactive one in a compound. Oxidation — gain of oxygen, or loss of electrons. Reduction — loss of oxygen, or gain of electrons. Sacrificial protection — a more reactive metal corrodes preferentially to protect a less reactive one.

Common mistakes

• Applying Group 1’s “reactivity increases down” rule to Group 7 (where it decreases), or vice versa.
• Reciting the reactivity series with a metal out of place, especially around aluminium/zinc/iron in the middle.
• Correctly defining oxidation/reduction in one convention but applying the other convention’s rule by mistake.
• Treating barrier methods, galvanising and sacrificial protection as interchangeable, rather than three distinct mechanisms.

Quick self-test

• State the reactivity trend direction for Group 1 and for Group 7, and explain the electron-configuration cause of each.
• Recite the eleven-metal reactivity series in order.
• Predict whether magnesium will displace zinc from zinc sulfate solution, and explain using both oxidation/reduction conventions.
• Name the two conditions both required for iron to rust.
• Explain why galvanising still protects iron even after the zinc coating is scratched.

Official syllabus

Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 — qualifications.pearson.com.