Trends in the properties of the Group 2 metals, magnesium to barium

Chemistry A2 · Group 2. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Two trends down Group 2 must be explained. The first is thermal stability. The carbonates and nitrates become harder to decompose from magnesium to barium. A small 2+ ion has a high charge density and distorts (polarises) the large anion, pulling electron density towards itself. This weakens a bond inside the anion, so it breaks more easily on heating. Larger cations polarise less.

The second is solubility. The hydroxides become more soluble down the group, but the sulfates become less soluble. The reason is how lattice energy and enthalpy change of hydration both change as the cation gets bigger, and which one changes more.

Rules to remember

Carbonates: MCO₃ → MO + CO₂. Nitrates: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.
Thermal stability increases down the group: MgCO₃ decomposes most easily, BaCO₃ least easily.
Smaller cation → higher charge density → more polarisation of the anion → lower decomposition temperature.
Hydroxides: solubility increases down the group; Mg(OH)₂ is the least soluble, Ba(OH)₂ the most (highest pH).
Sulfates: solubility decreases down the group; MgSO₄ is soluble, BaSO₄ is insoluble.
Sulfates: SO₄²⁻ is large, so lattice energy changes little, while ΔH_hyd of the cation becomes much less exothermic. ΔH_sol becomes more endothermic.
Hydroxides: OH⁻ is small, so lattice energy becomes less exothermic faster than ΔH_hyd does. ΔH_sol becomes more exothermic.

Common mistake

Students say the hydroxides and sulfates follow the same solubility trend. They are opposite: hydroxides become more soluble down the group and sulfates become less soluble.

Worked example

Equal amounts of magnesium nitrate and strontium nitrate are heated in the same way. Which gives off brown fumes first, and why?

The brown gas is NO₂, from 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.
Mg²⁺ is smaller than Sr²⁺ with the same charge, so it has a higher charge density.
Mg²⁺ polarises the nitrate ion more, weakening an N−O bond more.
So magnesium nitrate decomposes at a lower temperature.

Answer: Magnesium nitrate gives brown fumes first, because Mg²⁺ polarises the nitrate ion more than Sr²⁺ does.

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Watch: Crash Course on Group 2 (Inorganic : 9701 AS Chemistry)

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