Molecular Nitrogen and Related Diatomic Molecules

Lewis Structure

Let's look at the Lewis structure of and N2.

Atomic nitrogen has 5 valence electrons and 4 valence orbitals (2s, 2px, 2py, and 2pz). In the Lewis structure there is a triple bond between the nitrogen atoms and a non-bonding pair of electrons on each. This is consistent with the physical properties of N2.

Molecular Orbitals

Because there are 4 different valence orbitals on each nitrogen atom, need to see which orbitals have the same symmetry to overlap in a sigma sense.

Consider the two atoms along the z axis. The 2s orbital on one atom has the symmetry to overlap with the 2s orbital and with the 2pz orbital on the other atom. The 2pz orbital on the first atom also has the symmetry to overlap with the 2s and the 2pz orbital on the second atom.

There are 4 atomic orbitals with the correct symmetry to overlap. These must make 4 sigma symmetry molecular orbitals with an average energy equal to the average energy of the 4 atomic orbitals.
In N2 and in most other diatomic molecules (NO, NS, CO, CS) there are 4 sigma symmetry molecular orbitals made from a mixing of the 2s and 2pz atomic orbitals on each atom.

In both molecules the pi symmetry molecular orbitals are the same. The 2px orbitals on each atom combine to make a pi bonding and a pi antibonding molecular orbital in the xz plane.

Perpendicular to these in the yz plane, the 2py orbitals on each atom combine to make a pi bonding and a pi antibonding molecular orbital.

Here is the full molecular orbital diagram for N2.

Now we add the 10 electrons, 5 from each nitrogen atom. Note that the bottom sigma symmetry orbital is strongly bonding, the top one is strongly antibonding, and the 2 in the middle are only weakly bonding and antibonding, respectively. The 2 and 3 orbitals correspond to the non-bonding electron pairs in the Lewis structure. There is one sigma bond and 2 pi bonds when the 10 electrons are added to the lowest energy molecular orbitals.

Molecules with Similar Molecular Orbital Diagrams

Molecules and ions formed from 2 boron atoms or from 2 carbon atoms have molecular orbitals diagrams of the same sort as N2.

Diatomic molecules made up of two different atoms also have molecular orbital diagrams very similar to that of N2. When the electronegativity of one atom is lower than the other, the more electronegative atom's orbitals are lower in energy.

The molecular orbital diagram of carbon monoxide, CO, is show below. On the left you can see all of the orbitals. On the right, the total valence electrons (4 from C, 6 from O) have been added to the orbitals.

Professor Patricia Shapley, University of Illinois, 2012