What is charge density?

The cation charge density (polarising power) decreases down the group because the charge (+2) stays the same while the radius increases. A high-charge-density cation (small Mg^2+) polarises (distorts) the electron cloud of a large anion strongly; a low-charge-density cation (Ba^2+) polarises it weakly. This single idea explains the thermal-stability trends.

SingaporeChemistrySyllabus dot point

How and why do the properties of the Group 2 elements and their compounds change down the group?

Describe the trends down Group 2 in reactivity with water and oxygen, the thermal stability and solubility of the carbonates, nitrates, hydroxides and sulfates, and explain these trends in terms of ionic radius and charge density

A focused answer to the H2 Chemistry learning outcome on Group 2. Trends in reactivity with water, thermal stability of carbonates and nitrates, solubility of hydroxides and sulfates, all explained through ionic radius and cation charge density (polarising power).

Generated by Claude Opus 4.89 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to describe the trends down Group 2 (Mg to Ba) in reactivity with water and oxygen, the thermal stability of the carbonates and nitrates, and the solubility of the hydroxides and sulfates, and to explain them using ionic radius and cation charge density (polarising power). Thermal stability and solubility trends, with the polarisation explanation, are standard Paper 2 fare.

The answer

Reactivity with water

Group 2 metals react with water to give the hydroxide and hydrogen:

\text{M} + 2\text{H}_2\text{O} \rightarrow \text{M(OH)}_2 + \text{H}_2

Reactivity increases down the group (Mg reacts slowly with cold water, Ba vigorously) because ionisation energies decrease as the atom gets larger and the outer electrons are more easily lost.

Reactivity with oxygen

The metals burn in oxygen to form basic oxides ( $2\text{M} + \text{O}_2 \rightarrow 2\text{MO}$ ), again more vigorously down the group. Magnesium burns with a bright white flame, a classic demonstration.

Charge density: the master idea

The cation charge density (polarising power) decreases down the group because the charge ( $+2$ ) stays the same while the radius increases. A high-charge-density cation (small $\text{Mg}^{2+}$ ) polarises (distorts) the electron cloud of a large anion strongly; a low-charge-density cation ( $\text{Ba}^{2+}$ ) polarises it weakly. This single idea explains the thermal-stability trends.

Thermal stability of carbonates and nitrates

Carbonates decompose to the oxide and carbon dioxide; nitrates decompose to the oxide, nitrogen dioxide and oxygen:

\text{MCO}_3 \rightarrow \text{MO} + \text{CO}_2, \qquad 2\text{M(NO}_3)_2 \rightarrow 2\text{MO} + 4\text{NO}_2 + \text{O}_2

Thermal stability increases down the group. The small, polarising $\text{Mg}^{2+}$ distorts the carbonate (or nitrate) anion, weakening an internal bond and lowering the decomposition temperature. The large $\text{Ba}^{2+}$ polarises the anion little, so its salt is more stable and needs a higher temperature.

Solubility of hydroxides and sulfates

Two opposite solubility trends:

Hydroxide solubility increases down the group. $\text{Mg(OH)}_2$ is nearly insoluble; $\text{Ba(OH)}_2$ is moderately soluble.
Sulfate solubility decreases down the group. $\text{MgSO}_4$ is soluble; $\text{BaSO}_4$ is essentially insoluble (the basis of the barium-meal and the sulfate test).

Worked example

Place $\text{MgCO}_3$ , $\text{CaCO}_3$ and $\text{BaCO}_3$ in order of increasing decomposition temperature and explain.

Order of increasing decomposition temperature: $\text{MgCO}_3 < \text{CaCO}_3 < \text{BaCO}_3$ .

Down the group the cation radius increases ( $\text{Mg}^{2+} < \text{Ca}^{2+} < \text{Ba}^{2+}$ ), so charge density and polarising power decrease.

$\text{Mg}^{2+}$ polarises the carbonate ion most strongly, weakening a C-O bond, so $\text{MgCO}_3$ decomposes at the lowest temperature.

$\text{Ba}^{2+}$ polarises the carbonate least, so $\text{BaCO}_3$ is the most thermally stable and needs the highest temperature.

Try it: Group 2 trends explorer - compare reactivity, thermal stability and solubility from Mg to Ba.

Common traps

Getting the solubility trends backwards: Hydroxide solubility increases down the group; sulfate solubility decreases. Learn both directions.
Explaining thermal stability by "bigger atoms": State it precisely: larger cation means lower charge density means weaker polarisation of the anion means greater stability.
Forgetting the products of nitrate decomposition: Group 2 nitrates give the oxide plus brown $\text{NO}_2$ plus $\text{O}_2$ , not the metal nitrite.
Saying Mg reacts vigorously with cold water: Mg reacts only very slowly with cold water (faster with steam); vigour increases down the group.
Confusing reactivity trend with stability trend: Metal reactivity increases down the group (ionisation energy); compound thermal stability also increases down the group (charge density), for different reasons.

Examples in context

Example 1. The sulfate test in qualitative analysis. Adding barium chloride (with dilute acid) to a solution containing sulfate ions gives a white precipitate of insoluble $\text{BaSO}_4$ . This is a direct consequence of the decreasing sulfate solubility down the group, and it is the standard confirmatory test for sulfate met in the Analytical Techniques content area.

Example 2. Indigestion remedies. Magnesium hydroxide and magnesium carbonate are used as antacids because they neutralise stomach acid while being only sparingly soluble, so they do not raise blood pH dangerously. The low solubility of $\text{Mg(OH)}_2$ at the top of the group is what makes it a mild, safe base, an applied use of the solubility trend.

Try this

Q1. Write the equation for the thermal decomposition of magnesium nitrate. [1 mark]

Cue. $2\text{Mg(NO}_3)_2 \rightarrow 2\text{MgO} + 4\text{NO}_2 + \text{O}_2$ .

Q2. Explain why barium reacts more vigorously with water than magnesium. [2 marks]

Cue. Ionisation energy decreases down the group; Ba loses its outer electrons more readily, so it reacts faster.

Q3. State and explain the trend in solubility of the Group 2 sulfates. [2 marks]

Cue. Solubility decreases down the group; $\text{MgSO}_4$ soluble, $\text{BaSO}_4$ insoluble (used in the sulfate test).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Specimen (9729)4 marksDescribe and explain the trend in the thermal stability of the Group 2 carbonates down the group from magnesium carbonate to barium carbonate.

Show worked answer →

Thermal stability increases down the group: MgCO3 decomposes most readily, BaCO3 least readily.

All Group 2 carbonates decompose on heating: MCO3 -> MO + CO2.

Down the group the cation radius increases, so its charge density (polarising power) decreases.

A smaller, more polarising cation (Mg2+) distorts the electron cloud of the carbonate ion more, weakening a C-O bond and making decomposition easier (lower temperature).

A larger cation (Ba2+) polarises the carbonate less, so the carbonate is more stable and needs a higher temperature to decompose.

Markers reward the direction of the trend, the decomposition equation, the decrease in charge density down the group, and the polarisation argument.

2023 (style)3 marksDescribe what is observed and write an equation for the reaction of calcium with cold water, and state how the vigour of reaction with water changes down Group 2.

Show worked answer →

Calcium reacts steadily with cold water, giving off bubbles of hydrogen gas and forming a slightly soluble white suspension of calcium hydroxide.

$\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2$ .

Reactivity with water increases down the group: magnesium reacts only very slowly with cold water (faster with steam), while barium reacts vigorously.

This is because ionisation energies decrease down the group, so the metals lose their outer electrons more readily.

Markers reward the observations, the balanced equation, the increasing reactivity down the group, and the ionisation-energy reason.