According to molecular orbital theory, the number of unpaired electron(s) in O₂²⁻ is :

Solution

Chemistry ma’am · 4 steps

Count all 18 electrons, fill the MO ladder, and look at the two π^* orbitals at the top.

1. Count electrons. Each O atom has 8 electrons, and the 2- charge adds 2 more. So O₂²⁻ has 8 + 8 + 2 = 18 electrons.
2. Fill the orbitals. For O₂ and its ions, σ 2p_z lies below the π 2p pair. Fill the orbitals in that order, two electrons per orbital.

   σ 1s² σ^* 1s² σ 2s² σ^* 2s² σ 2p_z² π 2pₓ² = π 2p_y² π^* 2pₓ² = π^* 2p_y²

3. Check the top. In neutral O₂, the two π^* orbitals hold one electron each, so it has 2 unpaired electrons. The 2 extra electrons in O₂²⁻ pair up those single electrons, so both π^* orbitals are now full.
4. So. Every orbital in O₂²⁻ is doubly filled. The number of unpaired electrons is 0, and the ion is diamagnetic.

The common slip: 2. Ignoring the 2- charge and filling only 16 electrons gives the configuration of neutral O₂, which has 2 unpaired electrons, not 0.

The idea to keep. For O₂ species, the unpaired count depends only on the π^* pair: O₂ has 2, O₂⁻ has 1, and O₂²⁻ has 0, so it is diamagnetic.

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