Total number of electrons present in (π^*) molecular orbitals of O₂, O₂⁺ and O₂⁻ is ________.

Solution

Chemistry ma’am · 3 steps

Start from O₂ with 2 electrons in π^*, then add or remove one for each charge.

1. Fill O2. O₂ has 16 electrons. For oxygen the σ 2p_z orbital lies below the π 2p pair, so the valence filling ends with two unpaired electrons in the degenerate π^* 2pₓ and π^* 2p_y orbitals.

   O₂: σ 1s² σ^* 1s² σ 2s² σ^* 2s² σ 2p_z² π 2pₓ² π 2p_y² π^* 2pₓ¹ π^* 2p_y¹

2. Adjust for charge. The π^* level is the highest occupied level, so one electron is removed from it to form O₂⁺ and one is added to it to form O₂⁻. That gives 1 electron in π^* for O₂⁺ (15 electrons) and 3 for O₂⁻ (17 electrons).

   Species	Total electrons	Electrons in π^*
   O₂⁺	15	1
   O₂	16	2
   O₂⁻	17	3

3. So. The total is 1 + 2 + 3 = 6.

The common slip: 12. Counting every valence antibonding electron, including the two in σ^* 2s, gives 4 + 3 + 5 = 12. The question asks only about π^* orbitals.

The idea to keep. For O₂ and its ions, only the π^* count changes: removing an electron takes it from π^* and adding one puts it into π^*. Learn O₂ well and the ions follow.

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