JEE Main & Advanced Chemistry Chemical Bonding and Molecular Structure / रासायनिक आबंधन एवं आणविक संरचना Molecular Orbital Theory Or MOT

Molecular orbital theory was given by Hund and Mulliken in 1932.         

The main ideas of this theory are,             

(1) When two atomic orbitals combine or overlap, they lose their identity and form new orbitals. The new orbitals thus formed are called molecular orbitals.             

(2) Molecular orbitals are the energy states of a molecule in which the electrons of the molecule are filled just as atomic orbitals are the energy states of an atom in which the electrons of the atom are filled.           

(3) In terms of probability distribution, a molecular orbital gives the electron probability distribution around a group of nuclei just as an atomic orbital gives the electron probability distribution around the single nucleus.           

(4) Only those atomic orbitals can combine to form molecular orbitals which have comparable energies and proper orientation.           

(5) The number of molecular orbitals formed is equal to the number of combining atomic orbitals.             

(6) When two atomic orbitals combine, they form two new orbitals called bonding molecular orbital and antibonding molecular orbital.             

(7) The bonding molecular orbital has lower energy and hence greater stability than the corresponding antibonding molecular orbital.           

(8) The bonding molecular orbitals are represented by  etc, whereas the corresponding antibonding molecular orbitals are represented by  etc.           

(9) The shapes of the molecular orbitals formed depend upon the type of combining atomic orbitals.           

(10) The filling  of molecular orbitals in a molecule takes place in accordance with Aufbau principle, Pauli's exclusion principle and Hund's rule. The general order of increasing energy among the molecular orbitals formed by the elements of second period and hydrogen and their general electronic configurations are given below.             

(11) Electrons are filled in the increasing energy of the MO which is in order           

(a) \[\sigma 1s<{{\sigma }^{*}}1s<\sigma 2s<{{\sigma }^{*}}2s<\sigma 2{{p}_{z}}<\pi 2{{p}_{y}}\]

\[=\pi 2{{p}_{x}}<{{\pi }^{*}}2{{p}_{x}}={{\pi }^{*}}2{{p}_{y}}={{\pi }^{*}}2{{p}_{z}}\]

(b) \[=\frac{Increasing\text{ }energy\text{ }\left( for\text{ }electrons\text{ }>\text{ }14 \right)}{\sigma 1s<{{\sigma }^{*}}1s<\sigma 2s<{{\sigma }^{*}}2s<\pi 2{{p}_{x}}=\pi 2{{p}_{y}}<\sigma 2{{p}_{z}}<{{\pi }^{*}}2{{p}_{x}}}\]

\[={{\pi }^{*}}2{{p}_{y}}<{{\sigma }^{*}}2{{p}_{z}}\]

(12) \[\frac{Increasing\text{ }energy\text{ }(for\text{ }electrons\le 14)}{Number\text{ }of\text{ }bonds\text{ }between\text{ }two\text{ }atoms\text{ }is\text{ }called}\]           

bond order and is given by           

where number of electrons in bonding MO.                     

number of electrons in antibonding MO.           

For a stable molecule/ion,           

(13) Bond order µ Stability of molecule µ Dissociation energy µ .           

(14) If all the electrons in a molecule are paired then the substance is a diamagnetic on the other hand if there are unpaired electrons in the molecule, then the substance is paramagnetic. More the number of unpaired electron in the molecule greater is the paramagnetism of the substance.