What is the number of unpaired electron(s) in the highest occupied molecular orbital of the following species : N₂;N₂ + ;O₂;O₂ + ?

Solution

Chemistry ma’am · 3 steps

Fill the MOs up to the last electron, then look only at that top level and apply Hund's rule there.

1. Two filling orders. Up to N₂ (14 electrons or fewer), s-p mixing pushes σ 2p_z above π 2pₓ = π 2p_y. For O₂ and its ions, σ 2p_z lies below the π pair. The highest occupied MO (HOMO) is the last level that receives electrons.
2. Degenerate HOMO. When the HOMO is a pair of equal-energy orbitals like π^* 2pₓ and π^* 2p_y, Hund's rule puts one electron in each before any pairing.

   Species	Electrons	HOMO and its filling	Unpaired in HOMO
   N₂	14	σ 2p_z², paired	0
   N₂⁺	13	σ 2p_z¹	1
   O₂	16	π^* 2pₓ¹ π^* 2p_y¹	2
   O₂⁺	15	π^* 2pₓ¹ π^* 2p_y⁰	1

3. So. The counts are 0, 1, 2, 1, which is option (c). The two unpaired π^* electrons in O₂ are also why oxygen is paramagnetic.

The idea to keep. For unpaired electrons in the HOMO, find the last level filled. If it is a degenerate π or π^* pair, spread electrons singly first. Remember that O₂ has two unpaired π^* electrons.

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