In which of the following processes, the bond order increases and paramagnetic character changes to diamagnetic one ?

Solution

Chemistry ma’am · 4 steps

Check both conditions for each change: the bond order must go up, and every electron must end up paired.

1. Bond order. Bond order = 1/2(N_b - Nₐ), where N_b and Nₐ are the electrons in bonding and antibonding orbitals. Taking an electron out of an antibonding π^* orbital raises the bond order by 1/2. Adding one lowers it by 1/2.
2. Magnetic character. A species with any unpaired electron is paramagnetic. It is diamagnetic only when every electron is paired, and an odd total electron count always leaves one unpaired.

   Change	Electrons	Bond order	Magnetic character
   O₂ → O₂²⁻	16 → 18	2 → 1, falls	para → dia
   N₂ → N₂⁺	14 → 13	3 → 2.5, falls	dia → para
   NO → NO⁺	15 → 14	2.5 → 3, rises	para → dia
   O₂ → O₂⁺	16 → 15	2 → 2.5, rises	para → para

3. Why NO works. NO has 15 electrons, and its single unpaired electron sits in a π^*2p orbital. Removing that electron gives NO⁺, which has 14 electrons like N₂. Its bond order is 3 and all its electrons are paired.
4. So. Only (c), NO → NO⁺, raises the bond order and also turns a paramagnetic species into a diamagnetic one.

The idea to keep. Removing an electron from an antibonding orbital raises the bond order. If that electron was the only unpaired one, the species also becomes diamagnetic.

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