What are the general trends in atomic and ionic radii of Group 2 metals from magnesium to barium?

In Group 2 metals, from magnesium to barium, both atomic and ionic radii increase. This is because each successive element has an additional electron shell, which increases the size of the atom and its corresponding ion. This trend is important in A-Level Inorganic Chemistry as it affects the reactivity and properties of these metals and their compounds.

How does the reactivity of Group 2 metals change from magnesium to barium?

The reactivity of Group 2 metals increases down the group from magnesium to barium. This is due to the decreasing ionization energy as the atomic size increases, making it easier for these metals to lose their two outer electrons and form cations. This trend is crucial for understanding reactions involving these metals in Cambridge International A Levels Chemistry.

What are the trends in solubility of Group 2 metal hydroxides and sulfates?

For Group 2 metal hydroxides, solubility increases down the group from magnesium hydroxide to barium hydroxide. Conversely, the solubility of Group 2 metal sulfates decreases from magnesium sulfate to barium sulfate. These trends are significant in A-Level Chemistry as they influence the practical applications and reactions of these compounds.

Why do Group 2 metals have higher melting points compared to Group 1 metals?

Group 2 metals have higher melting points than Group 1 metals because they have two valence electrons that contribute to metallic bonding, compared to just one in Group 1 metals. This results in stronger metallic bonds in Group 2 metals, requiring more energy to break. Understanding this concept is essential for students studying A-Level Inorganic Chemistry.

How does the thermal stability of Group 2 carbonates change from magnesium to barium?

The thermal stability of Group 2 carbonates increases from magnesium carbonate to barium carbonate. This is because larger cations like barium have a lower charge density, which stabilizes the carbonate ion more effectively, making it less prone to decomposition. This trend is important for predicting the behavior of these compounds in thermal decomposition reactions in A-Level Chemistry.

Why is "Omitting O₂ from nitrate decomposition (or giving NO₂ from carbonates)." a common mistake in Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds?

Why it happens: Mixing up carbonate and nitrate products. How to avoid it: Carbonate → oxide + CO₂; nitrate → oxide + NO₂ + O₂. Source: 9701 Examiner Reports 2022-2024

Why is "Saying thermal stability decreases down the group." a common mistake in Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds?

Why it happens: Confusing it with metal reactivity (also increases down). How to avoid it: Compound thermal stability INCREASES down the group.

Why is "Reversing the hydroxide and sulfate solubility trends." a common mistake in Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds?

Why it happens: Mixing the two up. How to avoid it: Hydroxides more soluble down; sulfates less soluble down.

Why is "Not acidifying before the sulfate test." a common mistake in Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds?

Why it happens: Forgetting carbonate interference. How to avoid it: Acidify with HCl/HNO₃ first to remove carbonate (also a white precipitate).

Group 2 - IICambridge International A Level Chemistry 9701

Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds

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Group 2: Similarities and Trends (Mg–Ba) — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

Reactions of the Group 2 metals and their compounds, the reactivity, thermal-stability and solubility trends, the polarising-power explanation, and the main uses.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

27.1 — Describe the reactions of the Group 2 elements with oxygen and water.
27.1 — Describe the reactions of the oxides, hydroxides and carbonates, and the trends in solubility and thermal stability.
27.1 — Explain the trend in thermal stability in terms of cation polarising power.

Reactions and the reactivity trend

Form basic oxides and hydroxides; reactivity rises down the group as ionisation energy falls.

The Group 2 metals all form 2+ ions and basic oxides: $2\text{Ca} + \text{O}_2 \rightarrow 2\text{CaO} \qquad \text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2$

The oxides react with water to give alkaline hydroxides: $\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$

Reactivity increases down the group. As the atoms get larger, the outer electrons are further from the nucleus and more shielded, so they are lost more easily (lower ionisation energy). Magnesium reacts only slowly with cold water (but readily with steam), whereas barium reacts vigorously.

Form 2+ ions and basic oxides/hydroxides.
Ca + 2H₂O → Ca(OH)₂ + H₂.
Reactivity increases down (lower ionisation energy).

See the full worked example for similarities and trends in the properties of the group 2 metals, magnesium to barium, and their compounds →

Solubility and thermal-stability trends

Hydroxides more soluble down; sulfates less soluble down; carbonates/nitrates more stable down.

Solubility:

Hydroxides become more soluble down the group (Mg(OH)₂ sparingly soluble → Ba(OH)₂ soluble), so solutions become more alkaline.
Sulfates become less soluble down the group (MgSO₄ soluble → BaSO₄ insoluble). This is the basis of the sulfate test: acidified Ba²⁺(aq) gives a white precipitate of BaSO₄.

Thermal stability of the carbonates and nitrates increases down the group: $\text{MgCO}_3 \rightarrow \text{MgO} + \text{CO}_2 \qquad 2\text{Mg(NO}_3)_2 \rightarrow 2\text{MgO} + 4\text{NO}_2 + \text{O}_2$ So MgCO₃ decomposes at the lowest temperature, BaCO₃ at the highest.

Hydroxides ↑ solubility down; sulfates ↓ solubility down.
Sulfate test: acidified Ba²⁺ → white BaSO₄.
Carbonate/nitrate thermal stability increases down.

See the full worked example for similarities and trends in the properties of the group 2 metals, magnesium to barium, and their compounds →

Polarising power and uses

Small cations polarise the anion strongly (less stable); larger cations down the group are more stable.

The thermal-stability trend is explained by cation polarising power (≈ charge ÷ radius):

$\text{polarising power} \propto \frac{\text{charge}}{r}$

A small cation like Mg²⁺ has a high charge density, so it polarises (distorts) the large carbonate or nitrate anion strongly, weakening a bond within it — so the compound decomposes easily (low temperature). Down the group the cations get larger, polarise the anion less, and the compounds become more stable.

Uses. Ca(OH)₂ and CaCO₃ neutralise acidic soils (liming); Mg(OH)₂ and CaCO₃ are antacids; BaSO₄ is a 'barium meal' for X-rays (insoluble, so the toxic Ba²⁺ is not absorbed).

Polarising power ∝ charge/radius.
Small Mg²⁺ distorts the anion → least stable carbonate.
Uses: liming soil, antacids, barium meal.

See the full worked example for similarities and trends in the properties of the group 2 metals, magnesium to barium, and their compounds →

How it’s examined

Paper 4 often pairs an equation/observation question with a polarising-power explanation of the thermal-stability trend, plus the sulfate test. Examiner reports flag reversed solubility trends, missing O₂ in nitrate decomposition, and stating the wrong stability direction.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 27.1; 9701 Examiner Reports 2022-2024; 9701/41 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-20.

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Step-by-step worked examples — Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds

Step-by-step solutions to past-paper-style questions on similarities and trends in the properties of the group 2 metals, magnesium to barium, and their compounds, written exactly the way a tutor would explain them at the board.

1Reactions with oxygen and water (3 marks)
Getting started• reactions
Question
Write equations for (a) calcium burning in oxygen and (b) calcium reacting with water.
Step-by-step solution
1. Step 1
  (a) Basic oxide forms.
  $2\text{Ca} + \text{O}_2 \rightarrow 2\text{CaO}$
2. Step 2
  (b) Hydroxide + hydrogen.
  $\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2$
Answer
2Ca + O₂ → 2CaO; Ca + 2H₂O → Ca(OH)₂ + H₂.
Examiner tip
Reactivity with water increases down the group (Mg slow with cold water; Ba vigorous).
2Reactivity trend down the group (2 marks)
Getting started• reactivity
Question
Explain why reactivity increases from magnesium to barium.
Step-by-step solution
1. Step 1
  Atoms get larger down the group, so the outer electrons are further from the nucleus and more shielded.
2. Step 2
  The outer electrons are lost more easily (lower ionisation energy) → more reactive.
Answer
Larger atoms with more shielding lose their outer electrons more easily (lower IE) → more reactive down the group.
3Oxides and hydroxides with water (3 marks)
Building confidence• oxides, solubility
Question
Write the equation for calcium oxide reacting with water, and state how the alkalinity of the hydroxides changes down the group.
Step-by-step solution
1. Step 1
  Oxide + water → hydroxide (alkaline).
  $\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$
2. Step 2
  Hydroxide solubility increases down the group (Mg(OH)₂ sparingly soluble → Ba(OH)₂ soluble), so the solutions become more alkaline.
Answer
CaO + H₂O → Ca(OH)₂; hydroxides become more soluble (more alkaline) down the group.
Examiner tip
Hydroxides MORE soluble down; sulfates LESS soluble down — don't reverse them.
4Thermal decomposition (3 marks)
Building confidence• thermal stability
Question
Write the equations for the thermal decomposition of (a) magnesium carbonate and (b) magnesium nitrate, and state which carbonate decomposes at the lowest temperature.
Step-by-step solution
1. Step 1
  (a) Carbonate → oxide + CO₂.
  $\text{MgCO}_3 \rightarrow \text{MgO} + \text{CO}_2$
2. Step 2
  (b) Nitrate → oxide + NO₂ + O₂.
  $2\text{Mg(NO}_3)_2 \rightarrow 2\text{MgO} + 4\text{NO}_2 + \text{O}_2$
3. Step 3
  Stability increases down the group, so MgCO₃ decomposes at the lowest temperature.
Answer
MgCO₃ → MgO + CO₂; 2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂; MgCO₃ is least stable (lowest decomposition temperature).
Examiner tip
Nitrates give NO₂ AND O₂; carbonates give only CO₂.
5Explaining the stability trend (4 marks)
Stretch• polarising power
Question
Explain why the thermal stability of the Group 2 carbonates increases down the group. (4)
Step-by-step solution
1. Step 1
  Down the group the cation gets larger, so its charge density (polarising power) falls.
2. Step 2
  A small cation (Mg²⁺) polarises/distorts the large carbonate ion strongly, weakening a C–O bond.
3. Step 3
  So the small-cation carbonate decomposes more easily (lower temperature).
4. Step 4
  Larger cations down the group polarise the anion less → more stable carbonate → higher decomposition temperature.
Answer
Larger cations down the group have lower polarising power, distort the carbonate ion less → more stable.
Examiner tip
The key idea is cation polarising power (charge density) distorting the anion.
6Sulfate solubility, test and uses (4 marks)
Stretch• solubility, uses
Question
Describe the sulfate solubility trend, the test for sulfate ions, and give two uses of Group 2 compounds. (4)
Step-by-step solution
1. Step 1
  Sulfate solubility decreases down the group (BaSO₄ insoluble).
2. Step 2
  Sulfate test: add acidified Ba²⁺(aq); a white precipitate of BaSO₄ confirms sulfate.
  $\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)$
3. Step 3
  Uses: Ca(OH)₂ / CaCO₃ to neutralise acidic soils; Mg(OH)₂ / CaCO₃ as antacids; BaSO₄ as a 'barium meal' in medical X-rays (insoluble, so non-toxic).
Answer
Sulfates less soluble down (BaSO₄ insoluble); test = white BaSO₄ with acidified Ba²⁺; uses: liming soil, antacids, barium meal.
Examiner tip
Acidify first (HCl/HNO₃) to remove carbonate, which would also give a white precipitate.

Key Formulae — Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds

The formulae you need to memorise for similarities and trends in the properties of the group 2 metals, magnesium to barium, and their compounds on the Cambridge International A Level 9701 paper, with every variable defined in plain English and a note on when to use it.

Cation polarising power
$\text{polarising power} \propto \frac{\text{charge}}{\text{ionic radius}}$
$charge$
charge on the cation (2+ for Group 2)
$ionic radius$
increases down the group
When to use
Explaining thermal stability: smaller cations polarise the anion more → less stable compound.

Key Definitions and Keywords — Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds

Definitions to memorise and the exact keywords mark schemes credit for similarities and trends in the properties of the group 2 metals, magnesium to barium, and their compounds answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Reactivity trend (Group 2)
Examiner keyword
Increases down the group: larger atoms lose their outer electrons more easily (lower ionisation energy).
Thermal stability trend
Examiner keyword
The thermal stability of the carbonates and nitrates increases down the group.
Polarising power
Examiner keyword
A cation's ability to distort a nearby anion's electron cloud; greater for small, highly charged cations.
Solubility trends
Examiner keyword
Group 2 hydroxides become MORE soluble down the group; the sulfates become LESS soluble.
Sulfate test
Examiner keyword
Acidified barium ions give a white precipitate of BaSO₄, confirming sulfate ions.

Common Mistakes and Misconceptions — Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds

The traps other students keep falling into on similarities and trends in the properties of the group 2 metals, magnesium to barium, and their compounds questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Omitting O₂ from nitrate decomposition (or giving NO₂ from carbonates).
9701 Examiner Reports 2022-2024
Why it happens
Mixing up carbonate and nitrate products.
How to avoid it
Carbonate → oxide + CO₂; nitrate → oxide + NO₂ + O₂.
✕Saying thermal stability decreases down the group.
Why it happens
Confusing it with metal reactivity (also increases down).
How to avoid it
Compound thermal stability INCREASES down the group.
✕Reversing the hydroxide and sulfate solubility trends.
Why it happens
Mixing the two up.
How to avoid it
Hydroxides more soluble down; sulfates less soluble down.
✕Not acidifying before the sulfate test.
Why it happens
Forgetting carbonate interference.
How to avoid it
Acidify with HCl/HNO₃ first to remove carbonate (also a white precipitate).

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