Molecular orbital theory (MOT) describes how molecular orbitals are formed by the combination of atomic orbitals. According to MOT, electrons in molecules occupy molecular orbitals that are distributed throughout the space between bonded atoms, rather than being localized to individual atoms. The molecular orbitals can be bonding, stabilizing the molecule by bringing the nuclei closer, or antibonding, with higher energy and less stability. The linear combination of atomic orbitals (LCAO) method describes how molecular orbitals are formed through constructive and destructive interference of atomic orbital wave functions. Bond order, determined by the number of bonding and antibonding electrons, provides information about a molecule's stability, bond length, bond strength, and magnetic character.
1. MOT ( Molecular Orbital Theory)
SALIENT FEATURES OF MOT: ( developed by F. Hund and R.S Mullikens in 1932)
• Just like atom, molecule has orbitals of definite energy levels.
• Like electrons of atoms are present in atomic orbitals, electrons of a molecule are
present in different molecular orbitals.
• Molecular orbitals are formed by the combination of atomic orbitals of proper
symmetry and comparable energies.
• An electron in an atomic orbital is under the influence of only one nucleus. However,
an electron in molecular orbital is under the influence of two or more nuclei
depending upon the number of atoms present in the molecule.
• An atomic orbital is monocentric while a molecular orbital is polycentric.
• Just as the electron probability distribution around a nucleus in an atom is given by an
atomic orbital, the electron probability distribution around a group of nuclei in a
molecule is given by molecular orbital.
2. • The number of molecular orbitals formed is equal to number of combining atomic
orbitals.
• When two atomic orbitals combine they form two molecular orbitals: bonding orbital
and antibonding orbital.
• The bonding MO has lower energy and hence greater stability whereas corresponding
antibonding MO has more energy and lesser stability.
• The molecular orbitals are filled in same way as atomic orbitals are filled in accordance
with Aufbau principle, Pauli exclusion principle and Hund’s rule.
3. LCAO ( Linear Combination of atomic orbitals) method
According to LCAO method, the linear combination of atomic orbitals can take place by
addition and by subtraction of wave functions of atomic orbitals.
4. Formation of bonding molecular orbital
• The region between the two nuclei where the two 1s atomic orbital overlap has high
electron density. Such a molecular orbital is called bonding molecular orbital.
• The electron probability densities of two atomic orbitals get cancelled in the centre (
by subtraction) so that there is no probability of finding the electron in the region of
overlap, this situation does not favour the bond formation. Such an orbital is called
anti- bonding molecular orbital.
5. Formation of bonding and antibonding molecular orbitals in terms of constructive and
destructive interference of electron waves of the combining atoms.
6. Combination of 2p atomic orbitals to form
molecular orbitals
Bonding and antibonding molecular orbitals from 2pz atomic orbitals
9. Conditions for the combination of atomic
orbitals
• The combining atomic orbitals must have same or nearly the same
energies.
• The extent of overlapping between the atomic orbitals of two atoms
should be large.
• The combining atomic orbitals must have the same symmetry about
the molecular axis.
10. Energy level diagram for
molecular orbitals
Order of increasing energy of molecular orbitals for Li2 to N2
Order of increasing energy of molecular orbitals for O2, F2 and Ne2
11. Information conveyed by bond order
1. Stability of the species
2. Bond length
3. Bond dissociation energy
4. Magnetic character ( diamagnetic or paramagnetic character)
