We start with the valence atomic orbitals of the nitrogen atom. The ground electronic state of the nitrogen atom is 2s2 2p3. The 2s orbital is filled, and the three p orbital are half filled. Thus, there are four atomic orbitals from each N atom. We will therefore end up with eight molecular orbitals. We begin with the overlap of the 2s atomic orbitals. Since there are two of these (one from each N atom), we will end up with two molecular orbitals. The first is created when the 2s orbitals overlap constructively, forming a bonding molecular orbital (i.e. one with electron density between the two N nuclei), which tends to bind the nuclei together in the N2 molecule. This MO is occupied by two of the four 2s electrons available. This is termed the s2s orbital (read "sigma 2 s"), and is the lowest energy molecular orbital in the N2 molecule. Use the mouse in the following figure to rotate the s2s orbital. Note the N nuclei (yellow dots).
The other two 2s electrons occupy the next MO, termed the s*2s (read "sigma star 2s"). This is an antibonding orbital. This means that there is a node between the two N nuclei. This type of orbital tends to push two N nuclei apart, hence the term "antibonding". Note the node in the figure below. Bear in mind that both the s2s and the s*2s orbitals are present in the same N2 molecule.
Now we have to take care of the three 2p electrons from each N atom. The 2p orbitals can overlap end to end, creating s orbitals (both bonding and antibonding), or sideways, creating p ("pi") orbitals. Since we start with 6 atomic orbitals, we must create 6 MOs.The first we examine is the s2p bonding orbital. Note in the figure below that this orbital contributes electron density between the nuclei, hence it is a bonding orbital. Note also the definite p-character, with lobes at each end.
As stated above, each orbital has a corresponding anti-bonding orbital. The antibonding s*2p is shown below. As it happens, this MO is not occupied by electrons in the N2 molecule, but it still looks great. Note the node in the centre.
There are two bonding and two antibonding MOs produced by overlap of the other four p orbitals. These are shown below. See if you can decide which is the bonding and which is the anti bonding.
Ü Widen your screen if necessary by dragging the border at the left further left
The one on the right is the antibonding one, of course. You can see the node between the two N nuclei.
On to part 2, Energy Levels of MOs