To get into the concept of hybridization, I want to start with the tetrahedral arrangement of electrons around carbon atoms (which was introduced in Lesson 9 of CH-104). Then we will move on to the historical development of the tetrahedron concept and the historical development of the orbital concept. Finally, we will look at how the concept of hybrid orbitals connects these developments.
We interpret this by saying that if there are four groups of electrons around the center atom which are all bonded to other atoms, then the shape of the molecule is tetrahedral because that shape allows each electron group to get as far away from the other groups as possible. Perhaps I should say that shape results from each electron group getting as far away from the other groups as possible.
Well, this is a nice geometric and electrostatic argument, but it is not really the way chemists figured out that methane molecules, CH4, had a tetrahedral shape. That was figured out even before much, if anything, was known about electrons.
Chemists in the 1800's determined that when one of the H atoms in methane was replaced by a Cl atom, it made no difference which H atom was replaced. They were all the same. Further, when a second H atom was replaced, it made no difference which one. Each of the three H atoms remaining after the first replacement are equivalent to one another. They have the same orientation with respect to the carbon and chlorine atoms. The only structure for methane and its derivatives that would allow this to happen is a tetrahedral structure with the C atom in the center and an H or other atom at each of the points of the tetrahedron.
Flat Molecule Options
Now there is no way to explain that kind of property using flat models or any other shape.
So, a flat arrangement for CH4 does not explain the property of being able to replace any two hydrogens with chlorine without it making any difference which ones are replaced.
In the late 1800's and early 1900's more and more was learned about electrons. You should remember that the number and arrangement of electrons in the atoms of the different elements was related to the shape of the periodic table and the location of the elements on it.
The answer quite simply is that it cannot, at least not directly. Nevertheless, they persisted and came up with a relationship or process that you need to know about. It is called hybridization.
First, let me set the stage. Just as you can alter your clothing (e.g. zip up a jacket and raise its hood, etc.) to respond to different situations, so an atom can alter the arrangement of its electrons to fit the conditions in which it finds itself. You know that electrons take up space and that the space they take up is referred to as an orbital. There are many kinds of orbitals. In the past we talked about s, p, d and f orbitals. These are all atomic orbitals. They represent the ways that electrons can arrange themselves in isolated, individual atoms. When atoms bond to one another, the electrons have to change their arrangement in order to accommodate influence of other atoms. The space taken up by bonded electrons is called a bonding orbital.
To review, atomic orbitals are used to explain the spectra of individual atoms and the shape of the periodic table. Bonding orbitals are used to explain the shape and properties of molecules consisting of atoms sharing electrons.
Enter hybridization and hybrid orbitals. Hybridization is the name we use to describe the process of change from atomic orbitals to bonding orbitals. We refer to the orbitals that have been changed as hybrid orbitals.
Here is the idea: when a carbon atom bonds to other atoms, the four orbitals in the second shell are somehow mixed together and rearranged to give four new orbitals. These four new orbitals are called hybrid orbitals. Each of the four hybrid orbitals is equivalent to the others and each contains one electron.
Naming hybrid orbitals has a tricky angle to it. If you think of hybridization having a "before" mode and an "after" mode, the hybrid orbitals are the "after" mode, but their name comes from the "before" mode. Because these hybrid orbitals resulted from the combination of an s orbital and three p orbitals, they are called sp3 hybrid orbitals or simply sp3 orbitals. A local parallel in this type of naming comes to mind. The "Berryhill" and "Holly Farm" shopping centers are named for what used to be in those locations. Similarly, hybrid orbitals are named for the atomic orbitals that the electrons used to occupy.
In fact, the smaller part of the orbital is often left out of drawings. You can see that in the previous diagram only the larger lobes (where the electrons spend most of their time) are shown.
Hopefully, this page has given you an awareness of the hybridization process. The following pages of this section will show how those hybrid orbitals are involved in the bonding in organic compounds.
E-mail instructor: Eden Francis
Clackamas Community College