When atoms like Carbon are sp hybridized, why are the 4 valence electrons distributed to both the sp hybridized and the 2p unhybridized orbitals before the sp orbitals are filled? Aren't sp hybridized orbitals lower energy than the 2p orbitals?
Simply, no. First of all, always keep in mind that all scientifice theories are simply attempts to make some predictive sense from our observations. At best, even “Laws” have simply been observed to make the best predictions for utility. They are still subject to change, revision or disproving.
Related to your question, the Aufbau Principle applies only to ATOMS, not molecules. The entire topic of hybridized orbitals is again an extension of previous theories into real observations. Just as we know that the “Bohr Model” does not reflect the reality of atomic structure in many regards, it is still a useful starting point for atomic discusssions. Similarly, the Aufbau Principle serves as a good guideline for understanding the observations of elemental structure.
However, it does not and was never intended to describe molecular orbitals or hybridization. We continue to observe, experiment and observe some more. We change old definitions (e.g. acdi-base theory) and develop new ones (molecular orbitals) as necessary to describe our observations more closely and to predict properties more accurately.
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