Electron promotion is part of hybridization; an example is methane, CH
4.
The diagram for the electron configuration of the carbon of the methane is as follows (shamelessly taken from Wikipedia):
In methane, all of the C-H bonds are equivalent. However, as we can see in the picture, there are only two orbitals with unpaired electrons at the moment. What happens is an electron from the 2s shell is excited and comes into the 2p
z shell. This leaves unpaired electrons in the 2s, 2p
x, 2p
y, and 2p
z shells, allowing for four bonds.
However, this is not the end of it. If bonds were formed now, the bond containing the 2s orbital would have a different energy than the other three. This, shown through experimentation, is untrue. Therefore, the four bonding orbitals must be the same. Hence, the one 2s orbital and the three 2p orbitals hybridize to form four sp
3 orbitals.
More information can be found on the Wikipedia page (
http://en.wikipedia.org/wiki/Orbital_hybridisation) for orbital hybridization.