JEE Main 2022 · 30 June · morning · Single correct · +4 / −1
Correct statement about alkali metal oxides is :
- (a) peroxides are colored.
- (b) superoxides are paramagnetic.
- (c) oxides are paramagnetic.
- (d) peroxides are both colored and paramagnetic.
Answer and solution
Answer (b) superoxides are paramagnetic.
Solution
Chemistry ma’am · 3 steps
Look at the anion, not the metal: only the superoxide ion carries an unpaired electron.
- Three oxygen anions. Alkali metal ions such as Na⁺ and K⁺ have no unpaired electrons. So colour and magnetism depend only on the anion: oxide O²⁻, peroxide O₂²⁻ or superoxide O₂⁻.
- Fill the orbitals. Start from O₂, which has 2 electrons in its antibonding π^* orbitals. Each extra electron goes into π^*. One extra electron gives π^(*3), which leaves one unpaired. Two extra electrons give π^(*4), which pairs them all.
Anion Electron picture Unpaired electrons Magnetism and colour Oxide O²⁻ closed shell, like neon 0 diamagnetic, colourless Peroxide O₂²⁻ π^(*4) 0 diamagnetic, colourless Superoxide O₂⁻ π^(*3) 1 paramagnetic, coloured (KO₂ is yellow-orange) - So. Only the superoxide has an unpaired electron, so only superoxides are paramagnetic. The correct statement is (b).
The idea to keep. For oxygen anions, count the π^* electrons. Oxide and peroxide are fully paired, so they are colourless and diamagnetic. Superoxide has one odd electron, so it is coloured and paramagnetic.
Answer from the NTA final answer key. Solution by PrepWiser.