Hybridisation in methane.

This is a common structure of methane that we would normally draw using dot and cross diagram, however, this is a wrong structure.

As you all know, the electronic configuration of CH_4­ is 1s²2s²2p_x¹2p_y¹. This electronic configuration (the 1s² is not shown on the electron in box diagram) shows that only 2 electrons are available for sharing. So why is methane CH₄ and not CH₂?

When bonds are form, it is an exothermic reaction and as you all know, energy is released and the whole system is more stable. (Remember your notes? Less energy = more stable). When carbon forms 4 bonds (4 C-H bonds), the energy released is twice the energy released when carbon forming 2 bonds (2 C-H bonds). The energy gap between the 2s orbital and the 2p orbital is very small and so it pays the carbon to provide a small amount of energy to promote an electron from the 2s to the empty 2p to give 4 unpaired electrons. The extra energy released when the bonds form more than compensates for the initial input.

If you are unsure and curious about why the arrow is drawn upwards, look down.

Now that we've got 4 unpaired electrons ready for bonding, another problem arises. In methane all the carbon-hydrogen bonds are identical, but our electrons are in two different kinds of orbitals. You aren't going to get four identical bonds unless you start from four identical orbitals.

Now the fun part

Hybridisation

The electrons rearrange themselves again in a process called hybridisation. This reorganises the electrons into four identical hybrid orbitals called sp³ hybrids (because they are made from one s orbital and three p orbitals). You should read "sp3" as "s p three" - not as "s p cubed".

sp³ hybrid orbitals look a bit like half a p orbital, and they arrange themselves in space so that they are as far apart as possible. You can picture the nucleus as being at the centre of a tetrahedron (a triangularly based pyramid) with the orbitals pointing to the corners. This explains why methane is tetrahedral in shape.

What happens when the bonds are formed?

Remember that hydrogen's electron is in a 1s orbital - a spherically symmetric region of space surrounding the nucleus where there is some fixed chance (say 95%) of finding the electron. When a covalent bond is formed, the atomic orbitals (the orbitals in the individual atoms) merge to produce a new molecular orbital which contains the electron pair which creates the bond.

Four molecular orbitals are formed, looking rather like the original sp3 hybrids, but with a hydrogen nucleus embedded in each lobe. Each orbital holds the 2 electrons that we've previously drawn as a dot and a cross.

The principles involved - promotion of electrons if necessary, then hybridisation, followed by the formation of molecular orbitals - can be applied to any covalently-bound molecule.

So why are the arrows drawn all pointing upwards?

1. Because the arrow indicates the direction in which the electron spins, either clockwise or anticlockwise and the direction of the magnetic moment it creates when the charged electrons are spinning.

2. If the arrow in 2p­_z is pointing downwards, the charged particle will create a magnetic field that attracts another electron, but since the 2s electron has a lower energy level, it will attract the 2p_z electron and we will be back to square one.

Michael