Molecular orbital theory (MOT).
This theory assumes that the electrons ,shared between the atoms, are spread throughout the molecule i.e they are NOT localized on just one atom. Atomic orbitals overlap to form molecular orbitals(MOs) and the electrons occupy these MOs.
This theory is the quantum mechanical approach to bonding.The application of this theory is difficult as complicated mathematics is involved in quantum mechanical treatment of complex molecules.So, computers are generally used while using MOT for complex species.
We shall learn the basics of this theory by applying it to very simple diatomic molecules.Learning this theory is important because this theory does a better job in predicting structure, electronic spectra and paramagnetism of molecules than the VBT or hybridization theories.
Molecular orbital theory –
- # of atomic orbitals combined = # of molecular orbitals formed.
- For every combination of AOs a bonding molecular orbital (BMO) of lower energy and an anti bonding molecular orbital (ABMO) of higher energy is formed.
- Electrons will always to enter the orbital with lowest energy(Afbau principle).
- Electrons are delocalised i.e they are spread on all the atoms.
Atomic orbitals as we know are s,p,d,f orbitals on the atom.
HYBRIDIZATION ⇒ # of AOs = # of hybrid orbitals.
MOT ⇒ # of AOs = # of MOs.
Just as in hybridization , in MOT too, the no.of MOs formed exactly equal to no. of atomic orbitals(AOs) but in MOT two kinds of MOs are formed namely –
1.BONDING MOLECUAR ORBITALS- BMO- (σ) and
2.ANTI BONDING MOLECULAR ORBITALS – ABMO – (σ*).
We already know that electrons behave not only as particles but also as waves in quantum mechanics and that the orbitals are described by wave function (ψ) – (refer to post 30-32) .
Considering electrons as waves –
•Bonding molecular orbitals (BMO) are formed when the electrons in AOs constructively interfere with each other.Both waves are in phase.
•Anti bonding molecular orbitals (ABMO) are formed when the electrons in AOs destructively interfere with each other.Two waves are out of phase.These have node/s – a point where wavefunction is zero between two atoms i.e the probability of finding electrons is zero there.
•Non bonding (NBMO) are formed when –
i) the electrons in AO do not have similar energy to electrons in orbital in another atom or
ii) the constructive interference with one lobe is cancelled by destructive interference with another lobe of that orbital.
These orbitals have same energy as the AOs.
Electrons in bonding molecular orbitals make the bond stronger as these orbitals are lower in energy than the AOs , whereas electrons in anti bonding molecular orbitals make the bond less stable as these orbitals are higher in energy than the corresponding AOs.
For the overlap between the AOs to take place , the following conditions must be met –
- The AOs must have same symmetry.
- The AOs must have similar energy.
- The AOs must have spatial overlap i.e the atoms should be aligned correctly in space and must be close to each other for the overlap to take place.
In MOT we don’t talk about single , double or triple bonds but we refer to bond order. In simple terms, bond order is the number of bonds between two atoms.Bond order is the measure of stability of the bond.We will extensively talk about bond order in a later chapter. For now we shall just learn to calculate bond order w.r.t MOT.
Stability of Molecules
The stability of a molecule can be determined by the following factors –
A] Stability based on bond order – In molecules , the bond order is measure of the stability of the molecule. Higher the bond order, higher is the stability.
↑ Bond Order ↑ Stability
e.g. – Bond order for nitrogen is 3 and that for oxygen is 2.So, nitrogen is a more stable molecule than oxygen.
Bond dissociation energy for N3 molecule = 941 kJ/mol.
Bond dissociation energy for 02 molecule = 425.9 kJ/mol.
Thus, it will take more energy to break a nitrogen molecule than a oxygen molecule as nitrogen is far more stable than oxygen.
B]Stability based on # of electrons in BMO and ABMO –
If # of electrons in BMO = NB
# of electrons in ABMO = NA
i) If NB >NA → The molecule will be more stable as the net force of attraction exceeds force of repulsion.
ii)If NA > NB→ The molecule will be unstable as the net force of repusion will be more than force of attraction.
iii) If NA = NB → The molecule will be unstable as the influence of electrons in the ABMO is more than those in BMO.
In the next few posts we will continue talking about this theory.We shall see how orbitals interfere, how to draw MOT diagrams and how to write configurations w.r.t this theory.Till then ,
Be a perpetual student of life and keep learning.
Good Day !
References and Further Reading –