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Lecture 25

CHM 114 Lecture Notes - Lecture 25: Linear Combination Of Atomic Orbitals, Alkali Metal, Bond Order

Course Code
CHM 114
George Wolf

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Molecular Orbitals Expressed as LCAO’s
One of the most efficient approaches used to calculate molecular orbitals is to mathematically
express the molecular orbitals as a linear combination of atomic orbitals (LCAO). This is a
reasonable concept since in the dissociated atom limit the electrons will reside in atomic orbitals
As a general rule for the LCAO method, if n atomic orbitals are used as a mathematical basis
set, then only n molecular orbitals can be derived
For example, to describe the electronic molecular orbitals of the H2 molecule, the simplest
LCAO approach would be to describe these MO’s as a linear combination for the 1s atomic
orbitals centered on each H atomic center
Therefore, only 2 MO’s can be derived
Antibonding vs bonding
Sigma type bonding →
Symmetrical line if spin it
Bond Order (BO) =
½(#bonding e’s - #antibonding e’s)
Bond Order of 0 → does not
For the alkali and alkaline earth diatomics, we will assume that the core electrons will
remain in core states in the molecule. The valence electrons will occupy MO’s derived
from the LCAO of the valence ns atomic orbitals
For alkali metal diatomic molecules, the two valence electrons are enough to fill the
signma ns (bonding) level, gving a bond order of 1
Li2, Na2
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