Group 1 and 2 trends and reactionsEdexcel International A Level Chemistry: Revision notes

The first ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms: X(g) → X⁺(g) + e⁻. It decreases down both Group 1 and Group 2.

First ionisation energies (kJ mol⁻¹): Li 520, Na 496, K 419, Rb 403, Cs 376 and Be 900, Mg 738, Ca 590, Sr 550, Ba 503.

Down a group the atomic radius increases and there are more inner shells, so shielding increases. These outweigh the increase in nuclear charge, so the nucleus attracts the outer electron less strongly and less energy is needed to remove it.

Reactivity increases down Group 1 (Li to K) and Group 2 (Mg to Ba). Group 1 atoms lose one electron to form M⁺; Group 2 atoms lose two to form M²⁺. These are oxidation reactions: the metal's oxidation number rises from 0.

Down the group the outer electron(s) are further away and more shielded, so less energy is needed to form the ion. For Group 2, explain with the first and second ionisation energies together.

Reactivity depends on how easily the metal loses electrons, not on how many it loses.

With oxygen: lithium burns with a red flame to form Li₂O (4Li + O₂ → 2Li₂O). Magnesium burns with a bright white flame: 2Mg + O₂ → 2MgO. Group 1 oxides and Group 2 oxides are ionic solids.

With chlorine: the metals form ionic chlorides, e.g. 2Na + Cl₂ → 2NaCl and Mg + Cl₂ → MgCl₂.

With water: Group 1 metals give the hydroxide and hydrogen: 2M + 2H₂O → 2MOH + H₂. Lithium fizzes steadily; sodium melts; potassium melts and burns with a lilac flame. Calcium, strontium and barium fizz in cold water to give the hydroxide and hydrogen: M + 2H₂O → M(OH)₂ + H₂. Magnesium reacts extremely slowly with cold water, but burns in steam: Mg + H₂O(g) → MgO + H₂.

Group 1 and 2 oxides are basic oxides. With water they form alkaline hydroxides:

• Na₂O + H₂O → 2NaOH
• CaO + H₂O → Ca(OH)₂

Magnesium oxide is only sparingly soluble, so its solution is weakly alkaline (about pH 10).

With dilute acid they form a salt and water: MgO + 2HCl → MgCl₂ + H₂O. The ionic equation is MO + 2H⁺ → M²⁺ + H₂O.

Worked example: 0.40 g of MgO (M = 40.3 g mol⁻¹) neutralises 2 × 0.40 ÷ 40.3 = 0.020 mol of HCl.

Group 1 and 2 hydroxides neutralise dilute acids to form a salt and water.

• NaOH + HCl → NaCl + H₂O
• Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O

The ionic equation is the same for all of them: OH⁻ + H⁺ → H₂O.

A Group 2 hydroxide such as Mg(OH)₂ is only sparingly soluble, but it dissolves as the acid removes OH⁻ ions: this is why it works as an antacid.

Hydroxides become more soluble down Group 2. Mg(OH)₂ is sparingly soluble (weakly alkaline), Ba(OH)₂ is fairly soluble (strongly alkaline).

Sulfates become less soluble down Group 2. MgSO₄ is soluble; CaSO₄ is slightly soluble; SrSO₄ and BaSO₄ are insoluble.

A white precipitate on adding dilute sulfuric acid or sodium sulfate to a solution shows barium ions: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The trends are opposite for hydroxides and sulfates.