Hello,

The Valence Bond Theory fails to answer certain questions like why He ₂ molecule does not exist and why O ₂ is paramagnetic. So, in 1932, the molecular orbital theory was introduced in order to answer the solutions of questions like above.

• According to the Molecular Orbital Theory, individual atoms combine to form molecular orbitals. Thus the electrons of an atom are present in various atomic orbitals and are associated with several nuclei.
• We can describe an electron in an atom as occupying an atomic orbital, or by a wave function Ψ. These are solutions to the Schrodinger wave equation. Electrons in a molecule occupy molecular orbitals. We can obtain the wave function of a molecular orbital by the following methods.

1. Linear Combination of Atomic Orbitals (LCAO)

2. United Atom Method

1. LCAO :

As per this method, the formation of orbitals is because of Linear Combination (addition or subtraction) of atomic orbitals which combine to form the molecule. Consider two atoms A and B which have atomic orbitals described by the wave functions Ψ _A and Ψ _B . If the electron cloud of these two atoms overlaps, then we can obtain the wave function for the molecule by a linear combination of the atomic orbitals Ψ _A and Ψ _B. The below equation forms two molecular orbitals.

Ψ _MO = Ψ _A + Ψ _B

_{( + for Bonding Molecular Orbitals and - for Antibonding Molecular Orbitals )}

We have to follow certain rules while filling up molecular orbitals with electrons in order to write correct molecular configurations. They are

• Aufbau Principle – This principle states that those molecular orbitals which have the lowest energy are filled first.
• Pauli’s Exclusion Principle – According to this principle, each molecular orbital can accommodate a maximum of two electrons having opposite spins.

Best Wishes.