Group 2 properties and reactions

CIE A-Level Chemistry· Unit 10: Group 2 and Group 17· 45 min read

1. Physical Trends down Group 2★★☆☆☆⏱ 15 min

📘 Definition

Alkaline Earth Metals

Group 2

All Group 2 elements are s-block metals with 2 valence electrons in their outermost s orbital, forming +2 ions in almost all stable compounds.

Example:

Calcium forms $C a^{2 +}$ , barium forms $B a^{2 +}$

As you descend Group 2, the number of electron shells increases, leading to predictable changes in key physical properties:

Property	Trend down Group 2	Explanation
Atomic radius	Increases	More electron shells, increased shielding outweighs higher nuclear charge
First ionization energy	Decreases	Valence electrons are further from the nucleus, weaker electrostatic attraction
Melting point	Generally decreases	Larger atomic radius reduces the strength of metallic bonding
Density	Generally increases	Increase in atomic mass outweighs increased atomic volume

📐 Worked Example

Predict and explain the difference in first ionization energy between magnesium and strontium.

1
Strontium is below magnesium in Group 2, so it has more electron shells than magnesium.
2
This gives strontium a larger atomic radius and more inner electron shielding of the valence electrons.
3
The attraction between the nucleus and valence electrons is weaker in strontium, so less energy is required to remove an electron.
4
Therefore, the first ionization energy of strontium is lower than that of magnesium.

Exam tip:

Always link trend explanations to atomic structure (shielding, nuclear charge, atomic radius) for full marks.

2. Reactions with Oxygen and Water★★☆☆☆⏱ 20 min

Reactivity of Group 2 increases down the group, as lower ionization energy means valence electrons are lost more easily in redox reactions.

All Group 2 metals react with oxygen to form solid oxides, with the general equation:

2M(s) + O_2(g) \rightarrow 2MO(s)

📐 Worked Example

Write the balanced equation for the reaction of calcium metal with excess oxygen gas.

1
Calcium is a Group 2 metal, forms $C a^{2 +}$ ions, so its neutral oxide is $C a O$ .
2
Write the unbalanced equation:
3
$Ca(s) + O_2(g) \rightarrow CaO(s)$
4
Balance oxygen atoms by adding a coefficient of 2 to $C a O$ , then balance calcium by adding a coefficient of 2 to $C a$ :
5
$2Ca(s) + O_2(g) \rightarrow 2CaO(s)$
6
Check that all atoms are balanced, which they are, so this is the final equation.

For reaction with water, rate increases down the group: magnesium reacts very slowly with cold water, but rapidly with steam, while barium reacts vigorously with cold water. The general equation for reaction with cold water is:

M(s) + 2H_2O(l) \rightarrow M(OH)_2(s/aq) + H_2(g)

3. Reactions with Acids and Solubility Trends★★★☆☆⏱ 20 min

All Group 2 metals react with dilute acids to form a salt and hydrogen gas, with reaction rate increasing down the group. The general equation for reaction with hydrochloric acid is:

M(s) + 2HCl(aq) \rightarrow MCl_2(aq) + H_2(g)

📐 Worked Example

Write the balanced equation for the reaction of magnesium with dilute sulfuric acid, and name the products.

1
Magnesium forms $M g^{2 +}$ , the sulfate ion is $S O_{4}^{2 -}$ , so the neutral salt is magnesium sulfate ( $M g S O_{4}$ ).
2
Unbalanced equation:
3
$Mg(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + H_2(g)$
4
Check that all atoms are balanced: 1 Mg, 2 H, 1 S, 4 O on both sides.
5
Products are aqueous magnesium sulfate and hydrogen gas.

Two key solubility trends for Group 2 compounds are commonly tested:

Solubility of hydroxides increases down Group 2
Solubility of sulfates decreases down Group 2

4. Thermal Stability of Group 2 Compounds★★★☆☆⏱ 15 min

📘 Definition

Thermal stability

The resistance of a compound to decomposition when heated.

Thermal stability of Group 2 carbonates and nitrates increases down the group. A smaller Group 2 cation has higher charge density, which polarizes the anion (carbonate or nitrate) more strongly, weakening bonds within the anion and making decomposition easier.

The general equation for thermal decomposition of Group 2 carbonates is:

MCO_3(s) \rightarrow MO(s) + CO_2(g)

📐 Worked Example

Explain why magnesium carbonate decomposes at a lower temperature than barium carbonate.

1
$M g^{2 +}$ is a smaller cation than $B a^{2 +}$ , so $M g^{2 +}$ has a higher charge density.
2
The higher charge density of $M g^{2 +}$ polarizes the $C O_{3}^{2 -}$ anion more strongly, weakening the bonds within the carbonate ion.
3
Less energy is required to break the bonds and decompose $M g C O_{3}$ , so it decomposes at a lower temperature than $B a C O_{3}$ .

5. Common Pitfalls

Wrong move:

Stating that melting point always increases down Group 2

Why:

While density increases, melting point generally decreases as larger atomic size weakens metallic bonding

Correct move:

State that melting point generally decreases down Group 2

Wrong move:

Writing $M g (O H)_{2}$ as a product of magnesium reacting with steam

Why:

Hot steam dehydrates any formed hydroxide to the oxide

Correct move:

Write $M g O (s) + H_{2} (g)$ as products for Mg + steam

Wrong move:

Claiming all Group 2 hydroxides are insoluble in water

Why:

Solubility increases down the group, heavier hydroxides are highly soluble

Correct move:

Remember that hydroxide solubility increases down Group 2

Wrong move:

Stating thermal stability of carbonates decreases down Group 2

Why:

Smaller cations have higher polarizing power that destabilizes the carbonate ion

Correct move:

State that carbonate thermal stability increases down Group 2

Wrong move:

Writing Group 2 ions as +1 when balancing equations

Why:

All common Group 2 compounds have +2 cations, this leads to incorrect balancing

Correct move:

Always use +2 charge for Group 2 ions when balancing reaction equations

6. Quick Reference Cheatsheet

Trend	Direction down Group 2
Atomic radius	Increases
First ionization energy	Decreases
Melting point	Decreases
Reactivity	Increases
Hydroxide solubility	Increases
Sulfate solubility	Decreases
Carbonate thermal stability	Increases

7. Frequently Asked

Why does reactivity increase down Group 2?

Ionization energy decreases down the group due to increased atomic radius and electron shielding. Valence electrons are lost more easily, leading to higher reactivity in redox reactions.

What is the trend in solubility of Group 2 hydroxides?

Solubility of Group 2 hydroxides increases down the group: $M g (O H)_{2}$ is only slightly soluble, while $B a (O H)_{2}$ is highly soluble in water.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · 1
Trend in melting point MCQ
2022 · 2
Explain reactivity increase
2021 · 1
Solubility trend MCQ

Going deeper

syllabusCIE 9701 2025-2026 Syllabus

What's Next

The periodic trends and reaction patterns you learned for Group 2 form a critical foundation for studying other groups in the periodic table, including Group 17 halogens which frequently react with Group 2 compounds. Trends in solubility of Group 2 compounds are core to qualitative analysis tests for common anions like sulfate, and thermal decomposition trends are often tested in practical exam questions. Understanding how atomic structure dictates bulk properties is also a key transferable skill for all inorganic chemistry topics.