On combustion of Li, Na and K in excess of air, the major oxides formed, respectively, are :

Solution

Chemistry ma’am · 4 steps

Bigger cation, bigger anion: as the metal ion grows, it can hold a bigger oxygen anion stably.

1. Three oxygen anions. Burning an alkali metal can give the oxide O²⁻, the peroxide O₂²⁻ or the superoxide O₂⁻. These anions get larger in that order.
2. Size match. A small cation with a high charge density pulls hard on a large anion's electron cloud and breaks it up. So a large anion is stable in the lattice only next to a large cation.

   Metal	Cation size	Major product in excess air	Anion
   Li	smallest	Li₂O	oxide, O²⁻
   Na	medium	Na₂O₂	peroxide, O₂²⁻
   K	large	KO₂	superoxide, O₂⁻

3. The reactions. 4Li + O₂ → 2Li₂O, 2Na + O₂ → Na₂O₂ and K + O₂ → KO₂. Rb and Cs also form superoxides.
4. So. The major oxides are Li₂O, Na₂O₂ and KO₂, which is option (c).

The idea to keep. Remember the ladder: Li gives the oxide, Na gives the peroxide, and K, Rb and Cs give superoxides. A larger cation can stabilise a larger anion.

Answer from the NTA final answer key. Solution by PrepWiser.