In this post we shall begin by constructing MO diagrams. We shall begin with the simplest of all homonuclear diatomic molecules – The H2 molecule.
The rules of building a MO diagram are –
- The atomic orbitals are shown on the far left and far right of the diagram. The MOs are drawn between these two ends.
- The Y-axis shows energy of orbitals.
- Electrons get filled in MOs in accordance with Afbau principle -orbitals with lower energy get filled first.
- The molecular orbital which is occupied by electrons and has the highest energy is called HIGHEST OCCUPIED MOLECULAR ORBITAL(HOMO) and the orbital with lowest energy which is vacant is termed as LOWEST UNOCCUPIED MOLECULAR ORBITAL(LUMO).The energy difference between them is called HOMO-LUMO gap.
Now let us begin by constructing a MO diagram for H2 molecule.
H2 molecule –
As seen above, the atomic orbitals (AO) are drawn on the extreme right and extreme left of the diagram. There is one electron each in the 1s AO of hydrogen atom. These two AOs form two orbitals –
• Bonding orbital (σ) – This is drawn in the centre, between the two AOs and it lies below the two AOs of hydrogen.This is because it has lower energy than the two atomic orbitals.
• Anti bonding orbital (σ*) – This orbital again lies in between the two AOs but it is higher in energy than the two AOs and so it lies above them.
As we know , the electrons shall pair up and then occupy the MO with least energy(Afbau principe) , which is always the bonding MO. So, the two electrons sit in the BMO(σ1s) as shown in the above figure. Remember, the BMO is delocalized i.e it is spread on both the atoms and so the electrons are not on any one atom anymore. In this case, the ABMO is vacant.
In the above figure , σ is HOMO and σ* is LUMO.
∴Bond order = 1/2 ( 2-0) = 1. Thus, there is a single bond between two H- atoms in a H2 molecule.This is experimentally verifiable too .
Just as elements have a electronic configuration which tell us the whereabouts of an electron , molecules too have electronic configuration which give us a clear picture of the way electrons occupy MOs.
The electronic configuration of H2 molecule can be represented as – (σ2s)2
Now that we know how to construct the MO diagram, we shall study the symmetry of MOs in the next post.Till then ,
Be a perpetual student of life and keep learning..
Good day !