Chemical Bonding and Molecular Structure
13.0 Molecular Orbital Theory
13.0 Molecular Orbital Theory
According to this theory, as the electrons of an atom are present in various atomic orbitals, electrons of a molecule are present in various molecular orbitals. Molecular orbitals are formed by the combination of atomic orbitals of comparable energy and proportional symmetry. While an electron in atomic orbital is influenced by one nucleus, in a molecular orbital, it is influenced by two or more nuclei depending upon the number of atoms in the molecule.
Thus, an atomic orbital is monocentric while a molecular orbital is polycentric. The number of molecular orbitals formed is equal to the number of combining atomic orbitals. When two atomic orbitals combine, two molecular orbitals are formed. One is known as bonding molecular orbital (BMO) whereas other is anti-bonding molecular orbital (ABMO).
BMO has lower energy and hence greater stability than the corresponding ABMO. First BMO are filled, then ABMO starts filling because BMO has lower energy than that of ABMO.
Molecular orbitals like the atomic orbitals are filled in accordance with the Aufbau Principle obeying the Pauli’s Principle and the Hund’s rule.
Order of energy of various molecular orbitals is as follows:
- For $O_2$ and higher molecules$$\sigma 1s,\sigma *1s,\sigma 2s,\sigma *2s,\sigma 2{p_x},\left[ {\pi 2{p_y} = \pi 2{p_z}} \right],\left[ {\pi *2{p_y} = \pi *2{p_z}} \right],\sigma *2{p_x}$$
- For $N_2$ and lower molecules$$\sigma 1s,\sigma *1s,\sigma 2s,\sigma *2s,\left[ {\pi 2{p_y} = \pi 2{p_z}} \right],\sigma 2{p_x},\left[ {\pi *2{p_y} = \pi *2{p_z}} \right],\sigma *2{p_x}$$
