Question

On combustion in excess of air, lithium forms mainly the oxide, sodium forms peroxide while potassium or rubidium or caesium forms superoxides. It suggests that as cationic size increases the

• A. anionic polarisability increases, thus stability decreases.
• B. anionic size increases, polarisability increases, covalency increases, thus stability increases
• C. anionic size increases, coordination number increases, lattice energy increases, thus stability increases
• D. anionic size decreases, stability increases.

Answer 1

Answer: (C)

The stability of the peroxides and superoxides increases as the metal ions become larger. Basically, large cations are stabilised by large anions, because if both ions are similar in size the coordination number will be high, and this gives a high lattice energy.