In O−2, O2 and O2−2 molecular species, the total number of anti-bonding electrons respectively are:
7, 6, 8
To know the number of anti-bonding electrons, lets draw the MO energy level diagrams of O−2, O2 and O2−2
The following is the MOT diagram for Dioxygen O2 molecule:
As you can see, we have omitted the 1s orbitals for convenience. The 1s portion will be similar to the MOT diagram for O−2 ion show above.
Now let us take a look at the MOT diagram for the peroxide ion O2−2
Now, we know that
Bonding electrons are represented by σ and π.
Anti-bonding electrons are represented by σ* and π*.
Therefore, from the above diagrams, we can see that,
Number of anti-bonding electrons in:
O−2 ⇒ 7
O2 ⇒ 6
O2−2 ⇒ 8