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 calledmolecular 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 σ, π etc, whereas the corresponding antibonding molecular orbitals are represented by σ*, π* 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 andHund'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) σ1s, σ*1s, σ2s, σ*2s, σ2px, π*2py, σ*2px, π2pz, π*2pz
(12) bond order and is given by
Bond order = (NB – NA/2)
where NB = number of electrons in bonding MO.
NA = number of electrons in antibonding MO.
For a stable molecule/ion, NB > NA
(13) Bond order ∝ Stability of molecule ∝ Dissociation energy ∝ 1/Bond length .
(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.
Solved example 1: Order is a concept in the molecular orbital theory. It depends on the number of electrons in the bonding and antibonding orbitals. Which of the following statements is true about it ? The bond order
(a) Can have a negative quantity
(b) Has always an integral value
(c) Can assume any positive or integral or fractional value including zero
(d) Is a non zero quantity
Solved example 2: The bond order of NO molecule is
(a) 1 (b) 2
(c) 2.5 (d) 3
Solved example 3: When two atomic orbitals combine they form
(a) One molecular orbital (b) Two molecular orbital
(c) Three molecular orbital (d) Four molecular orbital
Solved example 4: Which of the following species is the least stable
(a) O2 (b) O2–2
(c) O2+1 (d) O2–1
Solved example 5: The bond order is maximum in
(a) O2 (b) O2–1
(c) O2+1 (d) O2–2
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