Electron energy query.

[Gareth56]

I read that an electron in a molecular anti-bonding orbital is at a higher energy compared to the atomic orbital than an electron in a bonding orbital however it does not explain why an electron in an anti-bonding orbital is at a higher energy. See attached image.

 

Without going into advanced chemical principles could someone explain why this is the case?

 

Thanks.

[Enthalpy]

If two orbitals interact weakly (so this is an approximation, because bonds are rather strong interactions) their combination, or molecular orbital, can be a sum or a difference of the atomic orbitals.

 

The sum has a bigger volume, the difference a smaller one. The bigger volume means less kinetic energy, which makes the orbital bonding. The smaller volume makes the anti-bonding orbital.

 

In addition (or even the main reason), the uniform phase over both atomic orbitals means also a longer wavelength (as the phase spreads over two atoms) and a smaller kinetic energy, while the opposite phase means a smaller wavelength.