Group 2: The Alkaline Earth Metals

Group 2 of the Periodic Table contains beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These are the alkaline earth metals. At A-Level (AQA 3.2.2), you need to understand trends in reactivity, reactions with water and oxygen, thermal decomposition of compounds, and solubility trends.

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Physical Properties and Trends

Property	Trend down Group 2	Reason
Atomic radius	Increases	Additional electron shells
First ionisation energy	Decreases	Greater distance from nucleus, more shielding
Electronegativity	Decreases	Bonding electrons further from nucleus
Melting point	Generally decreases (with some irregularity)	Metallic bonding weakens as ion size increases

All Group 2 elements have the electron configuration [noble gas] ns² and characteristically form M²⁺ ions by losing both s electrons.

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Reactivity Trend

Reactivity increases down Group 2.

• Down the group, ionisation energies decrease because the outer electrons are further from the nucleus and more shielded.
• It becomes progressively easier to remove the two outer electrons.
• Therefore, the metals are more readily oxidised (lose electrons more easily) going down the group.

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Reactions with Water

Group 2 metals react with water to form a metal hydroxide and hydrogen gas:

General equation: M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g)

Specific Reactions

Magnesium reacts very slowly with cold water:

Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)

• Very slow reaction; a few bubbles of hydrogen over several days.
• Magnesium reacts much more vigorously with steam: Mg(s) + H₂O(g) → MgO(s) + H₂(g)
• With steam, a bright white flame is observed and white magnesium oxide is produced.

Calcium reacts steadily with cold water:

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

• Steady stream of hydrogen bubbles.
• Solution turns milky due to sparingly soluble calcium hydroxide.
• The solution is alkaline (pH ≈ 12).

Strontium reacts more vigorously with cold water:

Sr(s) + 2H₂O(l) → Sr(OH)₂(aq) + H₂(g)

• More rapid production of hydrogen than calcium.
• Solution is strongly alkaline.

Barium reacts vigorously with cold water:

Ba(s) + 2H₂O(l) → Ba(OH)₂(aq) + H₂(g)

• Rapid effervescence.
• Barium hydroxide is more soluble than the hydroxides above it, giving a strongly alkaline solution (pH ≈ 13–14).

Exam Tip: You must know the trend in vigour of reaction: Mg (very slow with cold water) → Ca (steady) → Sr (vigorous) → Ba (very vigorous). This is because ionisation energy decreases down the group.

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Reactions with Oxygen

All Group 2 metals burn in oxygen to form white solid oxides:

General equation: 2M(s) + O₂(g) → 2MO(s)

Reaction	Equation	Observation
Magnesium	2Mg(s) + O₂(g) → 2MgO(s)	Brilliant white flame; white powder formed
Calcium	2Ca(s) + O₂(g) → 2CaO(s)	Orange-red flame; white powder formed
Strontium	2Sr(s) + O₂(g) → 2SrO(s)	Crimson flame; white powder formed
Barium	2Ba(s) + O₂(g) → 2BaO(s)	Pale green flame; white powder formed

The metal oxides are all basic. They react with water to form alkaline hydroxide solutions:

MO(s) + H₂O(l) → M(OH)₂(aq)

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Thermal Decomposition of Group 2 Carbonates

Group 2 carbonates decompose on heating to produce the metal oxide and carbon dioxide:

General equation: MCO₃(s) → MO(s) + CO₂(g)