JEE Main 2021 · 18 March · evening · Single correct · +4 / −1
The oxide that shows magnetic property is :
- (a) MgO
- (b) SiO₂
- (c) Mn₃O₄
- (d) Na₂O
Answer and solution
Answer (c) Mn₃O₄
Solution
Chemistry ma’am · 3 steps
Magnetism needs unpaired electrons, so look for the oxide whose metal ion has a partly filled d subshell.
- What causes magnetism. A substance is attracted by a magnet only when it has unpaired electrons. If every electron is paired, it is diamagnetic. Oxide ions O²⁻ have all electrons paired, so the answer depends on the other ion.
Oxide Cation Configuration Unpaired electrons MgO Mg²⁺ [Ne] 0 SiO₂ Si in the +4 state [Ne], all bonds paired 0 Mn₃O₄ Mn²⁺ and Mn³⁺ [Ar]3d⁵ and [Ar]3d⁴ 5 and 4 Na₂O Na⁺ [Ne] 0 - Inside Mn3O4. Mn₃O₄ is a mixed oxide, MnO.Mn₂O₃. It contains one Mn²⁺ and two Mn³⁺ ions per formula unit. Both ions have many unpaired d electrons, and their spins line up in a way that makes the solid strongly magnetic.
- So. MgO, SiO₂ and Na₂O have only noble-gas-like ions and are diamagnetic. Only (c) Mn₃O₄ shows magnetic behaviour.
The idea to keep. To judge magnetism, write the configuration of each ion and count unpaired electrons. Ions with a noble gas configuration give nothing; partly filled d subshells give magnetism.
Answer from the NTA final answer key. Solution by PrepWiser.