Trends in Group 2 properties
All Group 2 atoms end in ns² and lose two electrons to form M²⁺ ions. Going down the group (Mg → Ba):
- Atomic radius increases: one extra electron shell each period.
- First ionisation energy decreases: the outer electron is further from the nucleus and more shielded by inner shells, so the attraction is weaker. This outweighs the larger nuclear charge.
- Melting point generally decreases: metallic bonding is the attraction between M²⁺ ions and delocalised electrons. Bigger ions have the same charge spread over a larger size, so the attraction is weaker. Magnesium is lower than expected because its crystal structure is different.
- Reactivity increases, because the outer electrons are lost more easily.
Reactions with water
With cold water: M(s) + 2H₂O(l) → M(OH)₂(aq or s) + H₂(g). The reaction gets more vigorous down the group. The solution becomes alkaline (pH about 10–12) because hydroxide ions form.
- Magnesium reacts very slowly with cold water, forming a weakly alkaline solution of Mg(OH)₂ that barely dissolves.
- With steam, heated magnesium burns with a bright white flame: Mg(s) + H₂O(g) → MgO(s) + H₂(g).
- Calcium fizzes steadily; strontium and barium react faster still.
Oxidation states: the metal goes from 0 to +2 (oxidised); hydrogen goes from +1 to 0 (reduced).
Solubility of hydroxides and sulfates
- Hydroxides: solubility increases down the group. Mg(OH)₂ is almost insoluble; Ba(OH)₂ dissolves well and gives a strongly alkaline solution.
- Sulfates: solubility decreases down the group. MgSO₄ dissolves well; BaSO₄ is insoluble.
Test for sulfate ions
Add acidified barium chloride solution. A white precipitate of BaSO₄ shows sulfate: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The acid (hydrochloric or nitric) is added first to remove carbonate ions, which would also give a white precipitate (BaCO₃). Sulfuric acid is never used because it contains sulfate.
Uses of Group 2 compounds
- Mg(OH)₂: antacid, neutralises excess stomach acid. Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.
- Ca(OH)₂ (slaked lime): raises the pH of acidic soils.
- CaO and CaCO₃: remove SO₂ from flue gases (flue-gas desulfurisation): CaO + SO₂ → CaSO₃; CaCO₃ + SO₂ → CaSO₃ + CO₂.
- BaSO₄: 'barium meal' for X-ray images of the gut. Ba²⁺ ions are toxic, but BaSO₄ is insoluble so it is not absorbed.
- BaCl₂: test for sulfate ions.
- Mg metal: extracts titanium: TiCl₄ + 2Mg → Ti + 2MgCl₂.
Key formulas and definitions
- M(s) + 2H₂O(l) → M(OH)₂ + H₂(g)
- Mg(s) + H₂O(g) → MgO(s) + H₂(g)
- Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) (white precipitate)
- Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
- CaO + SO₂ → CaSO₃; CaCO₃ + SO₂ → CaSO₃ + CO₂
- TiCl₄ + 2Mg → Ti + 2MgCl₂
Worked examples
1. Explain why the first ionisation energy of barium is lower than that of magnesium.
Barium's outer electron is in the 6s shell, much further from the nucleus than magnesium's 3s electron, and it is shielded by more inner shells. So the nucleus attracts it less, and less energy is needed to remove it, even though barium has more protons.
2. Write the equation for calcium reacting with water, with state symbols, and give the oxidation state changes.
Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g). Ca: 0 → +2 (oxidised). H: +1 → 0 (reduced).
3. How many moles of H₂ form when 0.48 g of Mg reacts fully with steam? (Mr Mg = 24.3)
n(Mg) = 0.48 ÷ 24.3 ≈ 0.0198 mol. Mg : H₂ = 1 : 1, so n(H₂) ≈ 0.0198 mol (about 0.020 mol).
4. Why must the barium chloride be acidified when testing for sulfate?
Carbonate ions also form a white precipitate (BaCO₃). Adding HCl first reacts with carbonate (giving CO₂), so only sulfate gives the white precipitate.
5. Why is BaSO₄ safe for X-ray meals when Ba²⁺ ions are toxic?
BaSO₄ is insoluble, so almost no Ba²⁺ ions dissolve to be absorbed into the blood. It passes through the gut and blocks X-rays, outlining the gut.
6. What mass of CaO is needed to remove 6.4 kg of SO₂? (Mr CaO = 56.1, SO₂ = 64.1)
n(SO₂) = 6400 ÷ 64.1 ≈ 99.8 mol. CaO : SO₂ = 1 : 1, so mass CaO = 99.8 × 56.1 ≈ 5600 g = 5.6 kg.
Common mistakes
- Saying ionisation energy falls because nuclear charge falls. Nuclear charge rises; distance and shielding win.
- Writing MgO for magnesium with cold water. With cold water it forms Mg(OH)₂; MgO forms with steam.
- Mixing up solubility trends. Hydroxides MORE soluble down; sulfates LESS soluble down.
- Acidifying the barium chloride test with sulfuric acid. That adds sulfate ions; use HCl or HNO₃.