Question

What is the basic difference between Valence bond theory and Molecular orbital theory??? Very basics. Thank you.

Answer 1

DIFFERENCES

Valence bond theory assumes that electrons in a molecule are simply the electrons in the original atomic orbitals, with some used while bonding.

In other words, it does not account for the true distribution of electrons within molecules as molecules, but instead, treats electrons as if they are "localized" on the atoms themselves.

By drawing a Lewis structure and describing the electron distribution on that, you are using valence bond theory.

On the other hand, molecular orbital theory accounts for the "delocalization" of electrons (just like in `pi` systems such as benzene and pyridine, like in organic chemistry), and does not assume that the electrons are in the original atomic orbitals.

Instead, it says that the atomic orbitals combine to form the same number of new molecular orbitals as there were atomic orbitals (conservation of orbitals), and the electron distribution is based on that.

By working out a full molecular orbital diagram to see how the electrons are distributed then, you are using molecular orbital theory.

DISCOVERIES/CONCLUSIONS

As a result, a major discovery based on molecular orbital theory is that it correctly explains that `"O"_2` has unpaired electrons (is paramagnetic), whereas valence bond theory thinks `"O"_2` has all paired electrons.

From the MO diagram of `"O"_2`, you can see two electrons that are not paired in the `pi_(2p_x)^"*"` and `pi_(2p_y)^"*"` antibonding molecular orbitals.

But if you simply look at the Lewis structure, like valence bond theory has you do, it's not obvious that there are any unpaired valence electrons at all.