Concept:The stability of complex oxygen anions (like peroxide \( \text{O}_2^{2-} \) and superoxide \( \text{O}_2^- \)) requires a large, highly polarizable metal cation to prevent the crystal lattice from collapsing.
Solution:- As we travel down Group I-A, the atomic size of the alkali metals becomes very large (Potassium, Rubidium, and Cesium).
- Their large ionic radii possess a low charge density, which perfectly matches and stabilizes the large, unstable superoxide ion (\( \text{O}_2^- \)).
- When combusted in an excess of oxygen, these heavy alkali metals bypass normal oxide and peroxide phases to immediately form bright orange/yellow Superoxides (e.g., \( \text{KO}_2, \text{RbO}_2, \text{CsO}_2 \)).
Why other options are incorrect:- Lithium forms normal oxides (\( \text{Li}_2\text{O} \)).
- Sodium mostly forms peroxides (\( \text{Na}_2\text{O}_2 \)).
- K, Rb, and Cs are uniquely large enough to form superoxides.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.