We have now come to a very important aspect in covalent bonding, known as hybridization. This term arose when chemists tried to explain the unusual bonding behavior for some atoms.
Before we come to hybridization, lets look at CH4, which is a methane molecule. Carbon has four valence electrons and needs four more to achieve an octet. Likewise, each hydrogen atom needs an electron, which it shares with the carbon atom to be stable. However, if we examine the following orbital configuration diagram of carbon, we can see that carbon has two half-empty orbitals and one empty orbital.
These half-filled orbitals suggest that carbon can form two covalent bonds by merging with half-filled orbitals of other atoms. However, experiments showed that carbon formed four identical covalent bonds in methane. It is therefore evident that the carbon atom acts as though it has four unpaired electrons. For this, Linus Pauling invented the concept of hybridization, which is the rearrangement of electrons within the valence orbitals of atoms.
To understand the hybridization of the carbon atom, assume that one of the atom's 2s electrons leaves its place and takes the place of the empty 2p orbital, thus being 'promoted.' Then, with the 2s orbital, now half-empty, and the 2p orbitals all having electrons with parallel spins, they merge together, forming four equal energy orbitals. Look at the following figure to clarify this.
The four half-filled orbitals have equal energy and are called sp3 hybrid orbitals. The s has a superscript of 1 and the p has a 3, thus meaning that there are four sp3 orbitals, all with the same amount of energy. The hydrogen atoms can now bond to these orbitals. Now it is clear to us how methane forms.
Hybridization also occurs in boron atoms, where it forms sp2 orbitals. In the diagram on the right, one electron from the 2s orbital gets 'promoted' to an empty 2p orbital, thus forming three sp2 hybrid orbitals.
Finally, the diagram below illustrates sp hybridization, which occurs in beryllium. In this example, one of the two electrons in the 2s orbital gets 'promoted' to an empty 2p orbital, thus forming two sp hybrid orbitals.
Before going on to the next couple of sections, it is very important that you understand hybridization. Thus, do not hesitate to go back over the material. When ready, click 'Next Section' to continue.
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