Both my teacher and I were confused in class today about ionization energies. Which is more difficult to ionize? Her notes say that

a single electron in an anti-bonding orbital is the easiest to remove and

a paired electron in a bonding orbital is the hardest.

 

Is this correct? It seems logically that a paired electron would be easier to remove because it is being repelled already by the other electron in the pair. So it seems that a paired anti-bond should be easiest and a single lone bonded should be hardest. So which is it? She was unsure of the accuracy of her notes and we could not find this in the book so I wanted to get a second opinion. Thanks.

The thing is, a paired electron is energetically stable while an unpaired electron is energetically unstable. So by removing one electron from a set of paired electrons you are taking a stable configuration and making it unstable. This requires energy. This is why the Alkaline-Earth metals are much less reactive than their comparitive Alkali Metals of Group 1. Those Group 2 metals have a somewhat stable configuration of paired electrons in the s-shells so in order to remove an electron from it you need to input a greater deal of energy. In the Group 1 metals the electron is not paired up so it is easier to be pulled away as it doesn't have the electron pair stability.