Chapter-5 Part-A.pptx - Chapter – 5 Spring – 2018 Part...

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Chapter – 5 Part - A Spring – 2018 Dr. Jay Gilbert
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LCAO Method Consider atoms a and b which have atomic orbitals described by the wave functions  (a) and (b) . If the electron clouds of these atoms overlap when these atoms approach each other, then the wave function for the molecule [molecular orbital (ab) ] can be obtained by the linear combination of the atomic orbitals ( LCAO ) (a) and (b) . (ab) = N[c a (a) + c b (b) ] Where N is a normalizing constant which ensures that probability of finding an electron in whole of the space is unity The c a and c b are constant chosen to give minimum energy for (AB) . If the atoms 'a' and 'b' are similar then c a and c b have similar values. If atoms A and B are the same then c a and c b have equal values.
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We will consider the atomic orbitals between two s orbitals present in the two hydrogen atoms in H 2 . The two hydrogen atoms are labeled H a and H b to differentiate from one another, even though they are identical in all aspects. Molecular Orbitals from 's' orbitals When the two atoms approach toward each other, at the appropriate distance their electron clouds overlap and form larger molecular orbitals. The molecular orbitals thus formed based on LCAO are due to the sum of the two atomic orbitals and the difference between the two atomic orbitals.
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Bonding MO results when the signs of the two wave functions are the same.
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