Molecular Architecture - Hybrid Orbitals

Molecular Architecture - Hybrid Orbitals

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The mixing of orbitals of different energy levels gives rise to new orbital types of intermediate energy that are able to form stronger bonds.

The approach that we have taken so far has worked well with some simple molecules. their shapes are explained very nicely by the overlap of simple atomic orbitals. It does note take long to find molecules with shapes and bond angles that fail to fir the model that has been developed. For example, CH₄, has a shape that the VSEPR theory predicts to be tetrahedral. The H-C-H bond angles in this molecule are 109.5^o. No simple atomic orbitals are orientated at this bond angle. So what kinds of orbitals are CH₄ molecules using?

When some atoms form bonds, their simple s, p, and d orbitals often mix to form new atomic orbitals, called hybrid atomic orbitals. These new orbitals have new shapes and new directional properties. The reason for this mixing can be seen if we look at their shapes.

One kind of hybrid atomic orbital is formed by mixing a s orbital with a p orbital. This creates two new orbitals called sp hybrid orbitals (the sp is used to designate the kinds of orbitals from which the hybrid was formed).

Notice that each of the hybrid orbitals has the same shape - each has one large lobe and another much smaller lobe. The large lobe extends further from the nucleus than either the s or p orbital from which the hybrid orbital was formed. This allows the hybrid orbital to overlap more effectively with an orbital on another atom when a bond is formed. In general, the greater the overlap of two orbitals, the stronger the bond.

Another point to notice is that the large lobes of the two sp hybrid orbitals point in opposite directions - that is, they are 180^o apart.

Let's look at a specific example, the linear beryllium hydride molecule, BeH₂, as it would be formed in the gas phase.

The orbital diagram for the valence shell of beryllium is

Note that the 2s orbital is filled and the three 2p orbitals are empty. For bonds to form at a 180^o angle between beryllium and the two hydrogen atoms, two conditions must be met:(1) the two orbitals that beryllium uses to for the Be-H bonds must be aligned oppositely at 180^o, and (2) each of the beryllium orbitals must contain only one electron. The reason for the first requirement is obvious. The reason for the second is that each bond is that each bond must contain two electrons, one from the beryllium and one from the hydrogen. The net effect of all this is that when the Be-H bonds form, the electrons from the beryllium unpaired, and the resulting half-filled s and p atomic orbitals become hybridized.