Trends in Group 2 properties and reactionsAQA A-Level Chemistry: Revision notes

Group 2 elements (Mg, Ca, Sr, Ba) each have two outer electrons in an s sub-shell.

Atomic radius increases down the group because each element has an extra electron shell, so the outer electrons are further from the nucleus and more shielded.

First ionisation energy decreases down the group (Mg 738, Ca 590, Sr 550, Ba 503 kJ mol⁻¹):

• the atomic radius increases
• shielding by the inner shells increases
• these outweigh the increase in nuclear charge, so the attraction on the outer electron is weaker and less energy is needed to remove it.

The first ionisation of calcium is Ca(g) → Ca⁺(g) + e⁻.

Group 2 metals have a giant metallic lattice: positive M²⁺ ions held together by attraction to delocalised electrons (two per atom).

The melting point generally decreases down the group because the ions get larger. The delocalised electrons are then further from the nuclei of the ions, so the attraction is weaker and less energy is needed to overcome it.

The trend is not perfectly smooth: Mg 923 K, Ca 1115 K, Sr 1050 K, Ba 1000 K. Use the structure and bonding in any explanation, not covalent bonds or London forces.

Reactivity increases down the group because the elements lose their two outer electrons more easily as the ionisation energy falls.

• Mg reacts very slowly with cold water: Mg + 2H₂O → Mg(OH)₂ + H₂. It burns in steam to form the oxide: Mg(s) + H₂O(g) → MgO(s) + H₂(g).
• Ca, Sr and Ba react with cold water to give the hydroxide and hydrogen, more vigorously down the group: M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g).

The solutions are alkaline because they contain OH⁻ ions. Ca(OH)₂ is only sparingly soluble, so the mixture looks cloudy.

Titanium is extracted by heating titanium(IV) chloride with magnesium at about 1000 K in an atmosphere of argon:

TiCl₄ + 2Mg → Ti + 2MgCl₂

• Magnesium is oxidised (0 to +2) and titanium is reduced (+4 to 0).
• Magnesium is a stronger reducing agent than titanium, so it can displace it.
• Argon is unreactive, so it stops titanium and magnesium reacting with oxygen or nitrogen in air.

Worked example: 100 g TiCl₄ (189.9 g mol⁻¹) is 0.527 mol, which needs 1.05 mol Mg, or 25.6 g.