Molecular Orbital Theory: Why is There Mixing?

[semc]

Hi guys i came across molecular orbital theory and i don't understand why sometimes there is orbital mixing. For example B,C,N have no orbital mixing while O,F have orbital mixing. May i know how to know when there is orbital mixing?

 

[Borek]

Perhaps my English fails me - what do you mean by "mixing"? Molecular orbitals are always linear combinations of the atomic orbitals, so in each molecule - regardless of the elements involved - atomic orbitals are "mixed" to create molecular orbitals.

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[semc]

Hi borek! I meant if you are using molecular orbital theory to explain diatomic molecule like B₂ the sigma 2p bonding orbital will be promoted to a higher energy level then the pi 2p bonding orbital due to mixing of s and p orbitals. But I don't understand why this does not apply for O₂ or F₂.