How does molecular orbital theory differ from valence bond theory?

Molecular orbital theory considers electrons in molecules as delocalised over the entire molecule, unlike valence bond theory.

Molecular orbital (MO) theory and valence bond (VB) theory are two fundamental ways of understanding the structure and bonding of molecules in chemistry. They both aim to explain how atoms combine to form molecules, but they do so in different ways.

Valence bond theory, which was developed first, treats bonds as localised between two atoms. It suggests that a chemical bond forms when the atomic orbitals of two atoms overlap and the electrons in these overlapping orbitals can be shared between the atoms. This theory is particularly good at explaining the formation of single bonds and the geometries of molecules. However, it struggles to accurately describe some phenomena, such as the colour of transition metal complexes or the paramagnetism of oxygen.

Molecular orbital theory, on the other hand, considers electrons to be delocalised over the entire molecule. Instead of being confined to the region between two atoms, electrons are thought to occupy molecular orbitals that can extend over two or more atoms. This theory provides a more accurate description of the electronic structure of molecules, especially those with delocalised electrons like benzene or the oxygen molecule. It can also explain phenomena that valence bond theory cannot, such as why oxygen is paramagnetic and why transition metal complexes are coloured.

In essence, while valence bond theory views a molecule as a collection of individual atoms held together by localised bonds, molecular orbital theory sees it as a collection of positive nuclei surrounded by a 'cloud' of delocalised electrons. Both theories have their strengths and weaknesses, and the choice of which to use often depends on the specific properties of the molecule being studied. For example, valence bond theory is often more useful for understanding the structure and reactivity of simple molecules, while molecular orbital theory is more powerful for describing the properties of complex molecules and ions.