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

In Group 2 metals, from magnesium to barium, both atomic and ionic sizes increase. This is due to the addition of electron shells as you move down the group, which outweighs the increase in nuclear charge, resulting in larger atomic and ionic radii. This trend is important for understanding the reactivity and bonding characteristics of these metals, as covered in the Cambridge International A Levels Chemistry curriculum.

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

The reactivity of Group 2 metals increases as you move down the group from magnesium to barium. This is because the outer electrons are further from the nucleus and more easily lost due to decreased effective nuclear charge and increased shielding. This trend is crucial for predicting reactions involving these metals and is a key concept in AS-Level Inorganic Chemistry.

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

For Group 2 metal hydroxides, solubility increases from magnesium hydroxide to barium hydroxide. Conversely, the solubility of Group 2 metal sulfates decreases from magnesium sulfate to barium sulfate. These solubility trends are significant in understanding the chemical behavior of these compounds and are frequently examined in the Cambridge International A Levels Chemistry syllabus.

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 can better stabilize the carbonate ion, making it more resistant to decomposition upon heating. Understanding this trend is important for predicting the behavior of these compounds under thermal conditions, a topic covered in AS-Level Inorganic Chemistry.

What are the similarities in the chemical properties of Group 2 metals?

Group 2 metals, from magnesium to barium, share several chemical properties: they all form +2 oxidation states, react with water to form hydroxides and hydrogen gas, and react with oxygen to form oxides. These similarities arise from their similar electron configurations and are fundamental concepts in the study of Group 2 elements in the Cambridge International A Levels Chemistry curriculum.

Why is "Omitting O₂ from the nitrate decomposition." 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 one of the gaseous products. How to avoid it: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂ (both NO₂ and 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 with reactivity of the metals. How to avoid it: Compound thermal stability INCREASES down the group (even though metal reactivity also increases).

Why is "Reversing the hydroxide/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 "Giving NO₂/O₂ as products of carbonate decomposition." 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 carbonates with nitrates. How to avoid it: Carbonates → metal oxide + CO₂ only.

Group 2(AS-Level Inorganic Chemistry)Cambridge 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: Reactions and Trends — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

The reactions of the Group 2 metals (Mg to Ba) with oxygen, water and dilute acids; the reactions of their oxides, hydroxides and carbonates; the thermal decomposition and stability trend of nitrates and carbonates; and the solubility trends of the hydroxides and sulfates.

What you’ll learn

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

10.1.1 — Describe, and write equations for, the reactions of the elements with oxygen, water and dilute hydrochloric and sulfuric acids.
10.1.2 — Describe, and write equations for, the reactions of the oxides, hydroxides and carbonates with water and dilute hydrochloric and sulfuric acids.
10.1.3 — Describe, and write equations for, the thermal decomposition of the nitrates and carbonates, including the trend in thermal stabilities.
10.1.4 — Describe, and make predictions from, the trends in physical and chemical properties of the elements and their compounds.
10.1.5 — State the variation in the solubilities of the hydroxides and sulfates.

Reactions of the metals

Mg→Ba react with oxygen, water and acids, all giving the 2+ metal — and getting more vigorous down the group.

Group 2 metals lose their two outer electrons to form M²⁺ ions. Reactivity increases down the group because the outer electrons are further from the nucleus and more shielded, so they are lost more easily.

With oxygen (burn to the oxide): $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}\qquad 2\text{Ca} + \text{O}_2 \rightarrow 2\text{CaO}$

With water (→ hydroxide + hydrogen): $\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2$ Magnesium reacts only very slowly with cold water, but burns in steam: $\text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2$ . The reaction with water becomes more vigorous down the group (Ca > Mg).

With dilute acids (→ salt + hydrogen): $\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \qquad \text{Mg} + \text{H}_2\text{SO}_4 \rightarrow \text{MgSO}_4 + \text{H}_2$

Form M²⁺; reactivity increases down the group.
- O₂ → MO; + water → M(OH)₂ + H₂; + acid → salt + H₂.
Mg slow with cold water; burns in steam → MgO.

Reactions of the oxides, hydroxides and carbonates

Oxides/hydroxides are basic; carbonates react with acids giving CO₂.

Oxides with water → hydroxides (basic, alkaline): $\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$ The hydroxides become more soluble (more alkaline) down the group.

Oxides and hydroxides with dilute acids (basic → salt + water): $\text{MgO} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O}$ $\text{Mg(OH)}_2 + 2\text{HCl} \rightarrow \text{MgCl}_2 + 2\text{H}_2\text{O}$

Carbonates with dilute acids (→ salt + water + CO₂): $\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$

(With dilute H₂SO₄, an insoluble sulfate layer can again slow the reaction of the heavier carbonates.)

Oxide + water → hydroxide (basic).
Oxide/hydroxide + acid → salt + water.
Carbonate + acid → salt + water + CO₂.

Thermal decomposition and stability trend

Carbonates and nitrates decompose on heating; they get MORE thermally stable down the group.

Carbonates decompose to the oxide + carbon dioxide: $\text{MCO}_3 \rightarrow \text{MO} + \text{CO}_2$

Nitrates decompose to the oxide + nitrogen dioxide + oxygen: $2\text{M(NO}_3)_2 \rightarrow 2\text{MO} + 4\text{NO}_2 + \text{O}_2$ (NO₂ is a brown gas.)

The thermal stability trend: stability INCREASES down the group. So:

MgCO₃ decomposes at a lower temperature than BaCO₃.
You need a higher temperature to decompose the heavier compounds.

(The explanation — in terms of the larger cation being less polarising, so it distorts the anion less — is developed at A Level in 27.1. At AS you state and use the trend.)

Carbonate → oxide + CO₂; nitrate → oxide + NO₂ + O₂.
Thermal stability increases down the group.
Lighter (Mg) compounds decompose more easily.

Solubility trends: hydroxides and sulfates

Hydroxides get MORE soluble down the group; sulfates get LESS soluble.

Two opposite trends you must learn:

Hydroxides — solubility increases down the group. Mg(OH)₂ is only sparingly soluble (a weak alkali); Ca(OH)₂ more so (limewater); Ba(OH)₂ is fairly soluble (a strong alkali).
Sulfates — solubility decreases down the group. MgSO₄ is soluble; CaSO₄ is slightly soluble; BaSO₄ is insoluble.

Why BaSO₄ matters: it is the basis of the sulfate test — adding acidified Ba²⁺(aq) (barium chloride/nitrate) to a solution containing sulfate gives a white precipitate of BaSO₄. (BaSO₄ is also used as the 'barium meal' in medical X-rays.)

Hydroxide solubility ↑ down the group.
Sulfate solubility ↓ down the group (BaSO₄ insoluble).
BaSO₄ white precipitate = the sulfate test.

How it’s examined

Group 2 equations, the thermal-stability trend and the two solubility trends are reliable Paper 2 marks; BaSO₄ links to the qualitative-analysis (sulfate) test on the practical papers. Examiner reports flag wrong nitrate-decomposition products (forgetting O₂) and reversing the solubility trends.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 10.1; 9701 Examiner Reports 2022-2024; 9701/22 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.

1Group 2 metal with water (2 marks)
Getting started• reactions
Question
Write an equation for the reaction of calcium with water and predict how barium would compare.
Step-by-step solution
1. Step 1
  Calcium + water → hydroxide + hydrogen.
  $\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2$
2. Step 2
  Barium is lower in the group → more reactive → reacts more vigorously.
Answer
Ca + 2H₂O → Ca(OH)₂ + H₂; Ba reacts more vigorously.
Examiner tip
Reactivity increases down the group as the outer electrons are lost more easily (lower IE).
2Oxide formation and reaction with acid (2 marks)
Getting started• oxides
Question
Write equations for (a) magnesium burning in oxygen and (b) the oxide formed reacting with hydrochloric acid.
Step-by-step solution
1. Step 1
  (a) Basic oxide MgO.
  $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$
2. Step 2
  (b) Basic oxide + acid → salt + water.
  $\text{MgO} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O}$
Answer
2Mg + O₂ → 2MgO; MgO + 2HCl → MgCl₂ + H₂O.
3Decomposition of a Group 2 nitrate (3 marks)
Building confidence• thermal decomposition
Question
Write the equation for the thermal decomposition of calcium nitrate, and state which decomposes more easily — Ca(NO₃)₂ or Ba(NO₃)₂.
Step-by-step solution
1. Step 1
  Products: oxide + NO₂ + O₂.
  $2\text{Ca(NO}_3)_2 \rightarrow 2\text{CaO} + 4\text{NO}_2 + \text{O}_2$
2. Step 2
  Thermal stability increases down the group, so Ca(NO₃)₂ (higher up) decomposes more easily.
Answer
2Ca(NO₃)₂ → 2CaO + 4NO₂ + O₂; Ca(NO₃)₂ decomposes more easily than Ba(NO₃)₂.
Examiner tip
Don't forget the O₂ — a common omission.
4Carbonate decomposition and stability (3 marks)
Building confidence• thermal decomposition
Question
Write the equation for the thermal decomposition of magnesium carbonate, and state whether MgCO₃ or BaCO₃ decomposes at the lower temperature.
Step-by-step solution
1. Step 1
  Carbonate → oxide + carbon dioxide.
  $\text{MgCO}_3 \rightarrow \text{MgO} + \text{CO}_2$
2. Step 2
  Stability increases down the group, so MgCO₃ decomposes at the LOWER temperature.
Answer
MgCO₃ → MgO + CO₂; MgCO₃ decomposes at the lower temperature.
Examiner tip
Carbonates give only oxide + CO₂ (no NO₂/O₂) — don't mix up with nitrates.
5Solubility trends and the sulfate test (3 marks)
Stretch• solubility
Question
State the solubility trends of the Group 2 hydroxides and sulfates, and explain how this is used to test for sulfate ions.
Step-by-step solution
1. Step 1
  Hydroxides: solubility increases down the group; sulfates: solubility decreases.
2. Step 2
  BaSO₄ is insoluble, so adding acidified Ba²⁺(aq) to a sulfate gives a white precipitate.
  $\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)$
Answer
Hydroxides ↑, sulfates ↓ in solubility; white BaSO₄ precipitate confirms sulfate.
Examiner tip
Acidify with dilute HCl/HNO₃ first to remove carbonate, which would also give a white precipitate.
6Explaining the thermal-stability trend (4 marks)
Stretch• thermal stability, 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 (Mg²⁺ → Ba²⁺), so its charge density (polarising power) falls.
2. Step 2
  A smaller cation (Mg²⁺) polarises/distorts the large carbonate ion strongly, weakening a C–O bond.
3. Step 3
  This makes the carbonate decompose more easily (lower temperature) for the small cation.
4. Step 4
  Larger cations down the group polarise the anion less, so the carbonate is more stable → higher decomposition temperature.
Answer
Larger cations down the group have lower polarising power, distort the carbonate less → more stable.
Examiner tip
The key idea is cation polarising power (charge density) distorting the anion — link size to stability.

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)
$ionic radius$
size of the cation (increases down the group)
When to use
Explaining thermal stability: higher polarising power (small cation) distorts 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
Reactivity increases down the group: atoms get larger, so the outer electrons are lost more easily (lower ionisation energy).
Thermal stability (Group 2)
Examiner keyword
Resistance of a compound to decomposition on heating; for Group 2 carbonates and nitrates it 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
Adding acidified barium ions to a solution; a white precipitate of BaSO₄ confirms the presence of 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 the nitrate decomposition.
9701 Examiner Reports 2022-2024
Why it happens
Forgetting one of the gaseous products.
How to avoid it
2M(NO₃)₂ → 2MO + 4NO₂ + O₂ (both NO₂ and O₂).
✕Saying thermal stability decreases down the group.
Why it happens
Confusing with reactivity of the metals.
How to avoid it
Compound thermal stability INCREASES down the group (even though metal reactivity also increases).
✕Reversing the hydroxide/sulfate solubility trends.
Why it happens
Mixing the two up.
How to avoid it
Hydroxides more soluble down; sulfates less soluble down.
✕Giving NO₂/O₂ as products of carbonate decomposition.
Why it happens
Confusing carbonates with nitrates.
How to avoid it
Carbonates → metal oxide + CO₂ only.

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