Question

Why does hybridization of orbitals occur?

Answer 1

because by creating hybridizated orbitals you get lower energy levels than in plain bonding

Answer 2

Hybridization is not a phsyical process. So it doesn't "occur" in the usual sense you mean that, like low tide or an eclipse occurs. Hybridizaiton is a mathematical trick we use to interpret the orbitals electrons occupy in a MOLECULE (molecular orbitals) in terms of the orbitals they occupy in ATOMS (atomic orbitals).

In truth, there does not need to be any connection. It could be that electrons in molecules occupy orbitals that have nothing in common with the orbitals they occupy in atoms. But that rarely happens. Usually, we can imagine the molecular orbitals as being "constructed" from the overlap of atomic orbitals. But to do that, we usually need to imagine constructing "new" atomic orbitals which are blends of the atomic orbitals we use to describe the isolated atom. These are the hybrid orbitals. But they don't have any real existence: they're just building blocks for describing the molecular orbitals, which do exist.

It's important to realize that describing bonding in molecules is a trade-off. The most accurate and physically realistic thing we can do is construct molecular orbitals for any possible arrangement of the atoms, and figure out which arrangement and which set of MOs has the lowest energy. This will correspond to reality. But that means doing at least hundreds of very complex calculations for each and every molecule you want to understand. If you want a very accurate picture of a molecule, this is indeed what you do.

But that's wholly unrealistic for a broad description of many molecules -- thousands, if not millions -- which is what you are trying to achieve in general chemistry, or organic chemistry. So you compromise. You describe the true molecular orbital picture with an approximation, sometimes crude, sometimes pretty good, which is made up of putting together atomic orbitals on each atom, following certain empirical rules, like the octet rule, VSEPR, resonance, the rules about hybridization, and so forth. All of this paraphrenalia -- all the rules in Valence Bond theory -- are just a crude approximation to the full very complicated calculation you don't have time to do. (As a bonus, they sometimes give you physical insight into why electrons choose the bonding patterns they do, although sometimes they also lead you into error.)

Answer 3

While I do appreciate your long and thorough answer, it does not really address my question. In fact, you confused me even more. Besides lower energy MO's, is there another reason that atoms hybridize?

Answer 4

Certainly it addresses your question. Perhaps you need to study it more carefully, or perhaps you need additional background knowledge to thoroughly understand it. Without more information about where your understanding breaks down, I won't be able to help you further.

Your follow-on question reveals a deep misconception about the physical driving forces for chemical bonding. The only such conceivable driving force is minimizing the energy (at a constant entropy). This is basic thermodynamics, and governs absolutely every drive to equilibrium. I am curious what other factor you can imagine might be at work. Any idea?

Answer 5

I thought hybridization existed to stabilize the molecule. I learned that lower energy orbitals mean more stable and that's why atoms hybridize, to make lower energy bonds.