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

by OC2364339

This

**preview**shows half of the first page. to view the full**1 pages of the document.**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

bond

● 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

###### You're Reading a Preview

Unlock to view full version