So far, we’ve stuck to the idea of “lone pairs”, electrons being “between” nuclei, and hybridized atomic orbitals to explain a lot of different types of bonding. But this, somewhat, deviates from the ideas that we established when talking about the quantum mechanical properties of atoms.

Electrons are notoriously tricky particles; we know they exist because it wouldn’t make sense for them to not exist. Not only that, but we have been able to detect them through experiments since we learned that atoms have electrical properties in the late 1800s. However, even though we know they’re there, we have yet to see or even isolate them from an atom, relying on estimations like the electron cloud model or electron microscopy.

Valence Bond Theory

Knowing just how difficult it is to find where an electron is, how can we be so certain that electrons are only stuck in one position when bonded? Hybridized orbitals, partially, scratch that itch somewhat by defining sigma and pi bonds as “overlapping” orbitals that create a new kind of orbital – a new space for electrons to navigate.

As we’ve learned, these overlapping orbitals – and, by extension, atomic bonding – come about because of valence electrons – a representation of electrons that makes orbital theory and bonding easy to learn. What I didn’t reveal is that this was all a part of an overarching theory – the valence bond theory (VB Theory).

The valence bond theory encompasses everything that we have discussed to this point, covering that:

• Electrons occupy orbitals that determine where they might be at any given point.

• Electrons bond by attraction to another atom’s nucleus (or, more specifically, its effective charge).

• Bonds are formed as orbitals overlap.

But there is a problem here – VB theory is limited in its scope. Firstly, and most importantly, it only accounts for the bonding activity of electrons.

For example, what if, at any point during a bond, the electrons move away from each other? Certainly, this is a possibility since electrons aren’t static particles.

Well, not only is this possible, it’s probable.

If we held that, in a bond, valence electrons from two atoms are near each other in between two attractive nuclei, even if given they had opposing spins, they’re still two particles with the same electrical charge – they will repel each other to reduce energy. What about the two nuclei themselves? If the electrons move far enough away from each other, the nuclei – both positively-charged – will repel each other too!

How does the VB theory account for this?

Molecular Orbital Theory

…Well, it doesn’t, at least not very well.

Though, fortunately, scientists recognized that there was more to bonding less than a century ago, in the middle of the quantum mechanics revolution, and developed molecular orbital theory.

Molecular orbital theory (MO theory) fills in the limitations by introducing a new way to envision electron activity and two new concepts.

To explain this covalent bonding theory, we must recall that electrons not only express particle-like behavior, but wave-like behavior. Waves, including wave-like particles, that come in contact with each other, as electrons do when they near each other, can interfere with each other.

This interference can take the form of constructive interference, where the energies of the two waves add to each other, creating a larger waveform, or destructive interference, where the energies of the two waves subtract from each other and create a smaller waveform.