Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 33: 20 April

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Molecular Orbital Model of Bonding

As with atoms, we will begin with the simplest system, in this case the dihydrogen molecule, H2. (Strictly speaking, the simplest molecule is the dihydrogen molecular ion, H2+, with a single electron.)

As I noted in the beginning of our discussion of modern bonding, orbitals are conserved, so if we add two hydrogen atoms, Ha & Hb together, the two 1s orbitals should give us two molecular orbitals, MO1 and MO2:

MO1 = 1sa + 1sb

MO2 = 1sa - 1sb

Note that one orbital will have a lower energy and the second a higher energy as expected from the approximate conservation of orbital energies we noted earlier. [text Fig 9.27]And when we add and subtract the two atomic orbitals they give molecular orbitals of quite different shapes. [text Fig 9.25]

H2 molecular orbital diagram

dihydrogen MO diagram

illustration from Wikimedia Commons

The molecular orbitals resulting from this combination are symmetrical along the atomic axis between the bonded atoms, and as before are referred to as sigma (sigma) molecular orbitals. The two orbitals, however have much different properties.

Bond Order = (#bonding electrons - # antibonding electrons)/2. Divide by two to get "classical" two electron bond. Bond order gives a measure of bond strength in units of an electron-pair bond.

So far we've looked only at atoms with s-electrons and s-orbitals. What happens when we have p-electrons? The first element with p-electrons is boron, with a valence electronic configuration of 2s22p1. So what happens if we combine two boron atoms and calculate the new energy levels for the potential molecule?

Lets now go back and and look at the bond orders and bonding of the homonuclear diatomic molecules of the second period. [text Fig 9.38] As we can see in each case the bonding is as predicted from molecular orbital theory. ( used clickers to predict bond orders and paramagnetism)

Molecular Orbital Energy Levels and Bonding in Diatomic Homonuclear Molecules, Li2-Ne2
  Li2 Be2 B2 C2 N2 O2 F2 Ne2
sigma2p*               sigma orbital diagram with paired arrows
pi2p*           pi orbital diagram with 2 unpaired arrows pi orbital diagram with 2 sets of paired arrows pi orbital diagram with 2 sets of paired arrows
sigma2p         sigma orbital diagram with paired arrows pi orbital diagram with 2 sets of paired arrows pi orbital diagram with 2 sets of paired arrows pi orbital diagram with 2 sets of paired arrows
pi2p     pi orbital diagram with 2 unpaired arrows pi orbital diagram with 2 sets of paired arrows pi orbital diagram with 2 sets of paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows
sigma2s*   sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows
sigma2s sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows sigma orbital diagram with paired arrows
Bonds  1 0 1 2 3 2 1 0
Paramagnetic N N Y N N Y N N

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© R A Paselk

Last modified 20 Apr 2015