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Elementary gen chem books will make the careful distinction between the two theories and then proceed to claim that chemists can use both to complement each other to explain the behavior of certain electrons. The classic example is Benzene, held together by a $\sigma$ bond framework with delocalized $\pi$ orbitals. In general, resonance uses MOs and VBT describes the framework to hold the molecule together.

What is the justification for utilizing both theories at once? The construction of the wavefunctions are dramatically different. My only guess is that you need the concept of hybridization from VBT and afterwards, you are constructing molecular orbitals that are superpositions of various hybridized orbitals and other atomic orbitals, is this the right idea?

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    $\begingroup$ This is quite the can of worms you're opening. At the level of your textbook, the justification is pretty much just following the time-proven tradition in chemistry of "I don't care if it's wrong as long as it looks simple." The "orbitals" you see in your textbook are so far away from the actual maths and physics behind them that it's hard to bridge that gap without going way beyond the elementary level. For an actual discussion of how this mixing of wavefunction approaches is or isn't wrong, you'll need to go a lot deeper into a discussion that is still ongoing in theoretical chemistry. $\endgroup$
    – Antimon
    Nov 17, 2021 at 19:49
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    $\begingroup$ This might be beyond the scope of an elementary general chemistry textbook, but it is far away from a can of worms. It is quite simple actually. The two theories are essentially equivalent and once you take them to their respective mathematical limit within the same framework of approximations they produce the same wave function. They simply approach the problem from different perspectives. However, the way these theories are used and abused by some textbooks is a different story. Suffice to say, neither theory can be a justification of the other, but neither can one refute the other. $\endgroup$
    – Martin - マーチン ♦
    Nov 18, 2021 at 0:51
  • $\begingroup$ pubs.acs.org/doi/pdf/10.1021/acs.jchemed.1c00919 might be of interest $\endgroup$
    – Ian Bush
    Nov 19, 2021 at 17:18
  • $\begingroup$ @Martin-マーチン The overall wavefunction stays the same, but only for MO are the constituent orbitals actually static and have energies. In VB, you can't just pick one orbital and investigate its shape and energy, because the former isn't fixed in time and the latter doesn't exist. Thus, displaying a bonding electron pair using a localized orbital is extremely misleading. But then you have to put the caveat that this discussion is confined to a one-electron picture anyway. And then you have to discuss whether static one-electron orbitals are actually a useful physical thing or not. Can of worms. $\endgroup$
    – Antimon
    Nov 19, 2021 at 18:09

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They are both approximations that look at the problem from different angles. MO looks at the molecule as a whole, VB looks at it as a collection of bonds. In their theoretical limits, both are the same. As with all things in chemistry (and life) it is good to look at problems from different perspectives. AND combining them is more powerful than either one alone (ie. the sigma and pi systems of benzene). Both theories get much more complicated as they try to refine their respective approximations and get closer to "truth". At the introductory level, it is enough to know that they compliment each other and neither is better than the other. Chemistry is a layering process, once you get the basics you can dig deeper and deeper.

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