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Notes/Chemistry/Paper 4/Group 2
CAIEA2 Level9701§27.1

Group 2

Why Group 2 chemistry behaves as it does: anion polarisation and charge density explaining thermal stability, the lattice-energy-versus-hydration battle explaining solubility, and the Ksp calculations that put numbers on both.

180 min read 6 sub-topics
91
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2021–2025 · 34 papers
5 marks
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The AS Group 2 note handed you a list of facts: oxides and hydroxides get more alkaline down the group, carbonates fizz with acid, sulfates get less soluble. Paper 4 does not want the list — it wants the machinery that generates it. That machinery is two arguments, and this note teaches each in full. The first is polarisation: a small, doubly charged cation such as Mg2+\text{Mg}^{2+} distorts the electron cloud of a large anion, weakening bonds inside it, so the anion decomposes at a lower temperature — and everything about thermal stability follows from how this changes down the group. The second is an energy battle: dissolving an ionic solid pulls against lattice energy and for hydration enthalpy, and whether a Group 2 hydroxide or sulfate dissolves readily is decided by which term falls faster as the cation grows. Both arguments end in the same place for the exam: a three-mark chain of reasoning, written in the right vocabulary. The note closes by making the solubility story quantitative with solubility products — expressions, units, conversions and titration calculations.

The route through is: §01 charge density and anion polarisation, §02 thermal stability down the group — trends and equations, §03 evidence, decomposition temperatures and the lattice-energy argument, §04 dissolving — lattice energy versus hydration enthalpy, §05 what the solubility trends predict, and §06 applying KspK_{\text{sp}} to sparingly soluble Group 2 salts.

Before you start you should be able to

  • Group 2 chemistry as described at AS: reactions of the metals with water and oxygen, oxide/hydroxide basicity, carbonate–acid reactions, and the empirical solubility pattern of hydroxides and sulfates (this subject's own AS Group 2 note)

  • Ionic bonding, ionic radii and how radius changes down a group (this subject's own Atoms, Molecules and Stoichiometry note)

  • Lattice energy: its definition as the enthalpy change when one mole of an ionic solid forms from gaseous ions, and the charge/radius factors that make it more exothermic (this subject's own A2 Chemical Energetics note)

  • Hydration enthalpy: definition and the small-highly-charged-ion factors that make it more exothermic (this subject's own A2 Chemical Energetics note)

  • Solubility product: writing KspK_{\text{sp}} expressions with correct units, and the idea of a saturated solution in dynamic equilibrium (this subject's own A2 Ionic Equilibria note)

By the end of this page you can

  • Explain the variation in the thermal stability of the Group 2 nitrates and carbonates in terms of the charge density of the cation and the polarisation of the large anion

  • Write balanced equations, with state symbols where asked, for the thermal decomposition of Group 2 carbonates, nitrates and hydroxides, and extend the same reasoning to related anions such as ethanedioate

  • Describe what is observed when a hydrated nitrate is heated and link each observation to a decomposition product

  • Use the different rates at which the lattice energies of MO and MCO₃ change down the group to explain the trend in the enthalpy change of decomposition

  • Explain the variation in solubility and enthalpy change of solution of the Group 2 hydroxides and sulfates in terms of the relative magnitudes of the hydration enthalpy and the lattice energy

  • Predict observable consequences of the solubility trends: pH of saturated hydroxide solutions, and what forms when Group 2 metals and oxides meet dilute sulfuric acid

  • Write KspK_{\text{sp}} expressions with units for sparingly soluble salts, interconvert solubility and KspK_{\text{sp}} for 1:1 and 1:2 salts, apply the common-ion effect, and determine a solubility from titration data

01

Charge density and anion polarisation

Syllabus requirement · §27.1

“

describe and interpret the variation in the thermal stability of the Group 2 nitrates and carbonates in terms of the charge density of the cation and the polarisation of the large anion

”

Why Paper 4 asks why

The AS Group 2 note handed you a list of observations: carbonates fizz with acid, nitrates give off brown fumes on heating, sulfates get less soluble as you go down the group. Paper 4 rarely wants the list. It wants the machinery that generates it — and for thermal stability that machinery is one idea with two halves. The first half belongs to the metal ion: how small and how highly charged it is, bundled into a single idea called charge density. The second half belongs to the anion: how easily a big diffuse ion like CO32−\text{CO}_3^{2-} or NO3−\text{NO}_3^- can have its electron cloud bent out of shape by that cation. Put the two together and every decomposition trend in §02–§03 becomes predictable rather than memorised.

Charge density

Each Group 2 atom loses its two outer electrons to form M2+\text{M}^{2+} — so down the group, the charge stays the same while the radius grows as each new element opens another shell:

ionelectronic configurationionic radius / pm
Mg2+\text{Mg}^{2+}2,872
Ca2+\text{Ca}^{2+}2,8,8100
Sr2+\text{Sr}^{2+}2,8,18,8118
Ba2+\text{Ba}^{2+}2,8,18,18,8136

The same +2+2 charge spread over a growing sphere means the charge is packed less tightly. Charge density — charge per unit volume — therefore falls steadily from Mg2+\text{Mg}^{2+} to Ba2+\text{Ba}^{2+}. Small and doubly charged (Mg2+\text{Mg}^{2+}): fierce. Large and doubly charged (Ba2+\text{Ba}^{2+}): gentle. Nothing else about the group matters more than this slide.

ρcharge∝ionic charger3\rho_{\text{charge}} \propto \dfrac{\text{ionic charge}}{r^3}

Charge density rises with charge and falls steeply as radius grows. Down Group 2 the charge is fixed at +2 while r climbs from 72 pm to 136 pm, so charge density falls sharply.

·

You are never asked to compute a value — treat this as a trend statement: smaller radius at the same charge ⇒ higher charge density.

Polarisation: the cation bends the anion

Anions such as CO32−\text{CO}_3^{2-} and NO3−\text{NO}_3^- are large — their extra electrons sit in big, diffuse clouds that are not held tightly. Bring a small, fiercely charged cation nearby and those electrons are dragged towards it: the anion's electron cloud is distorted, or polarised. The distortion is not cosmetic. Bending electron density away from the anion's internal bonds — the C–O bonds of the carbonate, the N–O bonds of the nitrate — weakens those bonds from inside. A weakened anion needs less heat energy to fall apart.

So the decomposition temperature of a Group 2 compound is decided by a tug between two players:

  • the anion itself — large anions (CO32−\text{CO}_3^{2-}, NO3−\text{NO}_3^-) are easily polarised; they are the victim in this story;
  • the cation's polarising power — set by its charge density. High charge density ⇒ strong distortion ⇒ the compound decomposes more easily, at a lower temperature.
polarisation: how cation charge density controls anion stabilityMg²⁺ · small cation, high charge densityBa²⁺ · large cation, low charge densityMg²⁺+2 charge packed intoa tiny volumeelectron cloud pulledtowards the cationCOOOC–O weakenedanion polarisedbonds inside weaken → decomposes soonerBa²⁺same +2 charge spreadover a large volumeCOOOanion undistortedbonds stay strong → stable to heatdown the group: radius ↑ → charge density ↓ → less polarisation of the anion → greater thermal stability

The same nitrate ion beside two cations. The small Mg²⁺ (high charge density) pulls hard on the anion's electron cloud, stretching and weakening an N–O bond; the large Ba²⁺ (low charge density) barely disturbs it. Magnesium nitrate therefore decomposes far more easily than barium nitrate.

A clean demonstration (invented ions)

Three invented dipositive cations, with radii in the familiar Group 2 range:

cationradius / pmcharge density
X2+\text{X}^{2+}45highest
Y2+\text{Y}^{2+}80middle
Z2+\text{Z}^{2+}135lowest

All three carry +2+2, so radius alone ranks them: X distorts NO3−\text{NO}_3^- most, Z least. Predict the decomposition temperatures of their nitrates without any data: X(NO3)2\text{X(NO}_3)_2 decomposes at the lowest temperature (anion weakened most), Z(NO3)2\text{Z(NO}_3)_2 at the highest. That is the whole skill — read the cation's size, decide how much the anion suffers, and the thermal-stability order falls out.

Two nouns, two owners — use them correctly

Polarising power belongs to the CATION: it is what Mg2+\text{Mg}^{2+} has and Ba2+\text{Ba}^{2+} lacks. Polarisability belongs to the ANION: it is what makes CO32−\text{CO}_3^{2-} and NO3−\text{NO}_3^- vulnerable where tiny O2−\text{O}^{2-} is comparatively resistant. Mark schemes award the explanation marks for naming the victim explicitly — "the nitrate ion / the carbonate ion is polarised (less)" — so always write the anion's name or formula in your answer. An answer that says only "there is less polarisation" leaves the examiner guessing which way round you think the story runs.

Common mistakes

  • Saying "the cation is polarised by the anion"

    The CATION polarises; the ANION is polarised.

    The small, highly charged ion does the distorting; the large diffuse ion suffers it. Reversing the roles wrecks every downstream explanation — and examiners set questions specifically to catch it.

  • Writing only "less polarisation" without naming the anion

    "Less polarisation / distortion of the nitrate ion (or carbonate ion)", named explicitly.

    The mark scheme's second explanation mark is attached to the anion's name or formula — omitting it forfeits a mark you already understood.

  • Claiming charge density increases down the group because charge stays +2

    Charge density FALLS down the group: same charge spread over a larger radius.

    Charge density depends on both factors. The constant +2 is precisely why the growing radius wins and density drops.

Your turn

Rank cations, then convert the ranking into decomposition predictions — the two moves this section owns.

  1. 1

    Three invented Group 2-like cations have radii: P2+\text{P}^{2+} = 65 pm, Q2+\text{Q}^{2+} = 102 pm, R2+\text{R}^{2+} = 140 pm.

    (i) State which cation has the highest charge density, and explain why.

    (ii) Predict, with reasons, the order of decomposition temperatures of their carbonates PCO3\text{PCO}_3, QCO3\text{QCO}_3 and RCO3\text{RCO}_3.

    Stuck? Show hint

    Same charge on all three — so what is left to compare? And high charge density means the anion suffers MORE.

    Show solution
    1. 1

      (i) P2+\text{P}^{2+} has the highest charge density: all three ions carry +2+2, so the smallest radius packs that charge most tightly.

      Charge density compares charge per unit volume; equal charges make radius the whole decision.

    2. 2

      (ii) P2+\text{P}^{2+} distorts CO32−\text{CO}_3^{2-} most ⇒ PCO3\text{PCO}_3 decomposes at the LOWEST temperature. Order of increasing decomposition temperature: PCO3<QCO3<RCO3\text{PCO}_3 < \text{QCO}_3 < \text{RCO}_3

      More distortion weakens more C–O bonds, so less thermal energy finishes the job. R's gentle large cation leaves the anion nearly intact.

    Answer

    (i) P²⁺ — smallest radius at equal +2 charge · (ii) PCO₃ lowest, then QCO₃, RCO₃ highest

  2. 2

    Explain why Mg2+\text{Mg}^{2+} distorts a nitrate ion more strongly than Ba2+\text{Ba}^{2+} does, naming BOTH factors of charge density in your answer.

    Stuck? Show hint

    Two factors: how much charge, and over how much space.

    Show solution
    1. 1

      Both ions carry the SAME charge (+2+2), but Mg2+\text{Mg}^{2+} (72 pm) is much smaller than Ba2+\text{Ba}^{2+} (136 pm).

      Naming both factors explicitly is what the question demands — one alone cannot be 'both factors'.

    2. 2

      So Mg2+\text{Mg}^{2+} has the higher charge density, its surface attracts the anion's electron cloud more strongly, and the NO3−\text{NO}_3^- electron cloud is distorted (polarised) to a greater extent.

      Linking charge density → attraction → distortion is the full causal chain the examiner wants to see written out.

    Answer

    equal charge but Mg²⁺ much smaller ⇒ higher charge density ⇒ stronger pull on the anion's electrons ⇒ more distortion of NO₃⁻

Practise charge density and anion polarisationReal past-paper questions · Thermal stability and anion polarisation

The rest of this note

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Can you do all of these?

  • I can explain thermal-stability trends using cation radius, charge density and polarisation of the anion

  • I can write balanced equations for decomposing Group 2 carbonates, nitrates and hydroxides, and extend the argument to anions like ethanedioate

  • I can describe the observations when a hydrated nitrate is heated and link each one to a product

  • I can use the different rates of change of the lattice energies of MO and MCO₃ to explain the trend in ΔH⦵ of decomposition

  • I can define ΔH⦵sol, ΔH⦵hyd and ΔH⦵latt and combine them correctly

  • I can explain why hydroxide solubility rises but sulfate solubility falls down the group

  • I can predict observations — fizzing, white precipitates, pH changes — from the solubility trends

  • I can write Ksp expressions with correct units and convert between solubility and Ksp for 1:1 and 1:2 salts

  • I can apply the common-ion effect to a saturated Group 2 salt

  • I can determine a solubility from titration data using a redox mole ratio

Now do the questions
91 real Paper 4 parts from 2021–2025, sorted by difficulty, with mark schemes