During the change of O 2 to O 2 - , the incoming electron goes to the orbital:

Step-by-step solution

Molecular orbital configuration of O 2 ( 16 electrons) σ 1 s 2 ,   σ *   1 s 2 ,   σ 2 s 2 ,   σ * 2 s 2 ,   σ 2 p z 2 ,   π 2 p x 2 = π 2 p y 2 ,   π * 2 p x 2 = π * 2 p y 1 When an electron is added, it enters the π * 2 p x orbital and the configuration becomes σ 1 s 2 ,   σ * 1 s 2 ,   σ 2 s 2 ,   σ * 2 s 2 ,   σ 2 p z 2 ,   π 2 p x 2 = π 2 p y 2 ,   π * 2 p x