What is the number of unpaired electron(s) in the highest occupied molecular orbital of the following species : ; ; ; ?
- A.2, 1, 0, 1
- B.0, 1, 0, 1
- C.0, 1, 2, 1
- D.2, 1, 2, 1
Show answer and solution
Correct option: C
Solution
The MO configurations and their HOMOs:
N₂ (10 electrons): Configuration ends in ...σ2p², with the HOMO being σ2p (bonding), which is fully filled. 0 unpaired electrons.
N₂⁺ (9 electrons): One electron is removed from the HOMO of N₂ (the σ2p orbital), leaving it with 1 electron. 1 unpaired electron.
O₂ (12 electrons): Configuration ends in ...π*2p¹, π*2p¹ — the two degenerate antibonding π* orbitals each have 1 electron (Hund's rule). The HOMO is doubly degenerate π*2p, so there are 2 unpaired electrons.
O₂⁺ (11 electrons): One electron is removed from one of the π*2p orbitals of O₂, leaving only 1 electron total in the π* level. 1 unpaired electron.
Thus the answer is 0, 1, 2, 1 → Option 3.
N₂ (10 electrons): Configuration ends in ...σ2p², with the HOMO being σ2p (bonding), which is fully filled. 0 unpaired electrons.
N₂⁺ (9 electrons): One electron is removed from the HOMO of N₂ (the σ2p orbital), leaving it with 1 electron. 1 unpaired electron.
O₂ (12 electrons): Configuration ends in ...π*2p¹, π*2p¹ — the two degenerate antibonding π* orbitals each have 1 electron (Hund's rule). The HOMO is doubly degenerate π*2p, so there are 2 unpaired electrons.
O₂⁺ (11 electrons): One electron is removed from one of the π*2p orbitals of O₂, leaving only 1 electron total in the π* level. 1 unpaired electron.
Thus the answer is 0, 1, 2, 1 → Option 3.
[JEE Main 2023 · 24 Jan · Shift 2]