When atoms engage to form molecules, they do so by the overlap of atomic orbitals, creating a bigger molecular orbital that now encompasses more than one atom of a molecule.
These molecular orbitals become the region where electron density concentrates and further reveal whether the bonding interactions are favourable or not. The molecular orbitals favouring bonding interactions are called bonding molecular orbitals, and those weakening the bonding interactions are called antibonding molecular orbitals. The orbitals that abstained from bonding forming interactions but still part of molecular orbitals are called nonbonding orbitals.
The various ways by which a molecular orbital can form depend on the directionality or the approach of the atomic orbitals. Therefore, molecular orbitals can be further categorised as
- sigma (σ) orbitals (head-on overlap of atomic orbitals),
- pi (π) orbitals (lateral overlap of atomic orbitals).
In reality, molecular orbitals are mathematical functions that take into account the wave nature of electrons and follow certain rules that preserve the orbital number, attain maximum overlap, and only allow the orbitals having similar energies and compatible symmetries to mix.
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