I am currently a chemistry undergraduate student and have yet to attain a deep understanding of Quantum Mechanics and the underlying principles of orbitals. I am only aware that orbitals are regions of space where it is highly probable to find electrons.
from what I understand, the requirement for S-P mixing to occur is when the atomic orbital of S and P is of similar energy levels. The energy levels of S and P are similar only for the atoms Boron, Carbon and Nitrogen as the effective nuclear charge experienced by the electrons in these orbitals are similar. As a result of this similarity in energy level, when the atomic orbitals are combined to form molecular orbitals, the sigma S and sigma P molecular orbitals are able to mix in such a way that the sigma S molecular orbitals decrease in energy while the sigma P orbitals increase in energy. Hence, explaining why the π P molecular orbitals are filled first before the sigma P molecular orbitals.
Based on the initial proposition that the S and P atomic orbitals are similar in energy level, it does not specify that only the P orbital used for head on overlap (sigma P) will be similar in energy level to the S orbital. Which leads me to my question, if the entire concept is known as S-P mixing, why does the "mixing of energies" only affect the sigma P molecular orbital and not the π P molecular orbitals as well?
