Linear Combination of Atomic Orbitals (LCAO)
Linear Combination of Atomic Orbitals (LCAO)
According to wave mechanics, the
atomic orbitals can be expressed by wave functions (ψ's) which represent the
amplitude of the electron waves. These are obtained from the solution of
Schrodinger wave equation.
However, since it cannot be solved for any system containing more than one
electron, molecular orbitals which are one electron wave functions for
molecules are difficult to obtained directly from the solution of Schrodinger
wave equation. To overcome this problem, an approximate method known as linear
combination of atomic orbitals (LCAO) has been adopted.
Let us apply this method to the
homonuclear diatomic hydrogen molecule. Consider the hydrogen molecule
consisting of two atoms A and B. each hydrogen atom in the ground state has one
electron in 1s orbital. The atomic orbitals of these atoms may be represented
by the wave functions ψA and ψB. Mathematically, the formation of
molecular orbitals may be described by the linear combination of atomic
orbitals that can take place by addition and by subtraction of wave functions
of individual atomic orbitals as shown below:
Therefore, the two molecular
orbitals o and o*are formed as :
The molecular orbital o formed by
the addition of atomic orbitals is called the bonding molecular orbital while
the molecular orbital o* formed by the subtraction of atomic orbital is called
antibonding molecular orbital as depicted in below figure:
Qualitatively, the formation of
molecular orbitals can be understood in terms of the constructive or
destructive interference of the electron waves of the combining atoms. In the
formation of bonding molecular orbital, the two electron waves of the bonding
atoms reinforce each other due to constructive interference while in the
formation of antibonding molecular orbital, the electron waves cancel each
other due to destructive interference. As a result, the electron density in a
bonding molecular orbital is located between the nuclei of the bonded atoms
because of which the repulsion between the nuclei is very less while in case of
an antibonding molecular orbital, most of the electron density is located away
from the space between the nuclei, there is a nodal plane (on which the
electron density is zero) between the nuclei and hence the repulsion between
the nuclei is high. Electrons placed in a bonding molecular orbital tend to
hold the nuclei together and stabilize a molecule. Therefore, a bonding
molecular orbital always possesses lower energy than either of the atomic
orbitals that have combined to form it. In contrast, the electrons placed in
the antibonding molecular orbital destabilize the molecule. This is because the
mutual repulsion of the electrons in this orbital is more than the attraction
between the electrons and the nuclei, which causes a net increase in energy.
In may be
noted that the energy of the antibonding orbital is raised above the energy of
the parent atomic orbitals that have combined and the energy f the bonding
orbital has been lowered than the parent orbitals. The total energy of two molecular
orbitals, however, remains the same as that of two original atomic orbitals.
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