The magnetic behaviour of Li₂O,Na₂O₂ and KO₂, respectively, are :

Solution

Chemistry ma’am · 3 steps

Ignore the metal ions and look only at the oxygen anion: oxide, peroxide or superoxide.

1. Cations do not matter. Li⁺, Na⁺ and K⁺ all have noble gas configurations, so all their electrons are paired. Any unpaired electron must come from the oxygen anion.
2. Fill antibonding orbitals. For O₂²⁻ and O₂⁻, fill the molecular orbitals as in O₂ and look at the two antibonding π^* orbitals, which hold up to 4 electrons. The single oxide ion O²⁻ has a neon configuration, so it has no unpaired electrons.

   Compound	Anion	Total electrons in anion	Unpaired electrons	Behaviour
   Li₂O	oxide, O²⁻	10, neon configuration	0	diamagnetic
   Na₂O₂	peroxide, O₂²⁻	18, π^* holds 4	0	diamagnetic
   KO₂	superoxide, O₂⁻	17, π^* holds 3	1	paramagnetic

3. So. The order is diamagnetic, diamagnetic and paramagnetic, which is option (d).

The idea to keep. Oxide and peroxide ions have all electrons paired. The superoxide ion O₂⁻ has 17 electrons, an odd count, so it always has one unpaired electron and is paramagnetic.

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