Stability Trend

Stability of alkali metal superoxides (MO₂) increases down Group 1 due to larger cation size. Small Li⁺ poorly stabilizes the large O₂⁻ ion, causing low lattice energy and instability for LiO₂. Larger cations like Rb⁺ provide better charge balance and higher stability.

Option Analysis

• (A) LiO₂ > NaO₂ > KO₂ > RbO₂: Incorrect, as stability decreases down the group, opposite to the observed trend.
• (B) LiO₂ < NaO₂ < KO₂ < RbO₂: Correct, matches increasing stability with cation size (Li⁺ < Na⁺ < K⁺ < Rb⁺).
• (C) LiO₂ < NaO₂ > KO₂ > RbO₂: Incorrect, implies peak stability at NaO₂, but trend continues upward.
• (D) LiO₂ > NaO₂ < KO₂ < RbO₂: Incorrect, wrongly shows LiO₂ most stable despite its known instability.

Introduction to Superoxide Stability

The stability order of LiO₂, NaO₂, KO₂, RbO₂ metal oxides follows a clear periodic trend crucial for CSIR NET Life Sciences and chemistry exams. Alkali metal superoxides (MO₂) gain stability down Group 1 as cation size increases from Li⁺ to Rb⁺, enabling better stabilization of the superoxide ion (O₂⁻). This guide breaks down the correct order, explains each option, and provides exam insights.

Why Stability Increases Down the Group

Superoxides contain the paramagnetic O₂⁻ ion, which is large and requires a sizable cation for ionic lattice stability. LiO₂ remains unstable due to disproportionation (2LiO₂ → Li₂O₂ + O₂), observable only at cryogenic temperatures. NaO₂ shows moderate stability, while KO₂ and RbO₂ are stable solids used in applications like oxygen generation.

Key Factors

• Cation size effect: Larger Rb⁺ (1.47 Å) vs. small Li⁺ (0.76 Å) reduces polarizing power, stabilizing O₂⁻.
• Lattice energy: Decreases appropriately down the group, favoring MO₂ formation.
• Electropositivity: Increases from Li to Rb, weakening M-O bonds but enhancing overall compound stability.

Detailed Option Breakdown for Exams

CSIR NET questions test this trend precisely. Here’s why only one option fits:

Exam Tips for Alkali Metal Oxides

• Remember: Oxides (M₂O) stable for Li; peroxides (MO₂) for Na; superoxides (MO₂) for K, Rb, Cs.
• Mnemonic: “Superoxides Stabilize Sizewise” (Li small → unstable; Cs largest → most stable).