[reyrey389]

I know that the sigma 2p bonding orbital could be less/higher in energy than the pi 2p bonding (based on if it is C2,N2,B2 etc), but

Why is the sigma 2p antibonding orbital always higher in energy than the pi 2p antibonding one?

[fledarmus]

Bonding orbitals are called bonding orbitals because they are lower in energy than the orbitals they are made from - this means that the electrons in the atomic orbitals can work together to lower the total energy of the system, and the two atoms making up the bond are more stable bonded together than separated.

The antibonding orbitals are the exact opposite of the bonding orbitals - the electrons in the interacting orbitals are directly opposed to each other, increasing the total energy of the system. The better the overlap between the atomic orbitals, the stronger both the antibonding and bonding interactions can be.

Think of two large men pushing a heavy sled. If they are both on the same side of the sled, pushing together directly against the sled, there is a lot of energy combined to push the sled. If they are on opposite sides of the sled pushing directly against the sled, there is a lot of energy keeping the sled from going anywhere. That is your sigma bonding and antibonding orbitals.

Now think of the same two men pushing the same sled in the same direction, but instead of standing behind the sled, they are standing beside it, with their arms stretched out to the side to try to push the sled. When they are pushing the same direction, they can still apply force to move the sled and work together, but not as much as when they were directly behind it. When they are pushing opposite directions, they still apply force against each other, but again, not as strong as when one was directly behind and the other directly in front. That is your pi bonding and antibonding orbitals.

In general, however far the bonding orbitals drop in energy over the non-bonded orbitals is how far the anti-bonding orbitals increase in energy over the non-bonded orbitals. If both orbitals are filled, the energy of the system is the same as the non-bonded system, and entropy prefers the non-bonded system. Since the sigma bonding orbitals show a larger drop in energy than the pi bonding orbitals, the sigma anti-bonding orbitals will show a correspondingly larger increase in energy than the pi bonding orbitals.