MOT: LINEAR COMBINATION OF ATOMIC
ORBITALS (LCAO)
Molecular orbital theory is quite successful in solving the mystery of molecules. In
VBT we have learnt that only valence electrons and their corresponding orbitals of
different atoms take part in bonding and form a molecule. The MOT suggests that all
atomic orbitals of different atoms come together and produce molecular orbitals, thus
whole atoms participate in bonding to form a molecule. And these molecular orbitals
are associated with all the nuclei in the molecule (nucleus of all bonded atoms).
Do you ever imagine how do any two atoms combine to form a molecule? Two
different atoms come closer to each other, but their nuclei repel each other and their
electrons are attracted by other's nuclei. Both attractive force and repulsive force work
between them, but at certain distance these forces balance each other. This optimum
distance, where both forces are in equilibrium with each other, is called the bond
length. It means that at this distance these atoms can make a stable bond together.
As I have told you that MOT is based on the dual nature of electron. So here we will
describe an electron occupying an atomic orbital by its wave function ψ and we will
see how MOT finds the wave function of a molecular orbital.
But we will try ourselves first. If we have 2 atoms A and B, each has 2 orbitals and all
of them will combine to produce molecular orbitals. So we have to produce 2+2=4
molecular orbitals. Let’s suppose these orbitals are made up of clay. a1, a2 orbitals and
atom B has b1, b2 orbitals. We can take half of the clay from each orbital from each
atom and mix them to form one molecular orbital like:
