CH 106 - Lesson 2
Home Up Hybridization Covalent Bonding Sigma Bonds Types of Hybridization Double Bonds Pi Bonds Triple Bonds Comparing Hybrid Types Valence

 

Sigma Bonds

This particular kind of covalent bond in which electrons are shared between atoms is called a sigma bond. 

Also, this particular kind of bonding orbital is called a sigma orbital. Sometimes the term molecular orbital is used rather than bonding orbital. That term emphasizes that this represents the space taken up by the electrons in a molecule. The distinguishing feature of a sigma bond (or sigma bonding orbital) is that the overlap region lies directly between the two nuclei.

In this diagram, there are quite a few examples of sigma bonds. Notice that it does not matter what shapes the orbitals have or what types they are. They can be s orbitals or p orbitals or hybrid orbitals. What makes each of these a sigma bond is that the orbital overlap occurs directly between the nuclei of the atoms. Those nuclei are represented by dots in this diagram.

Diagrams of several sigma bonds. [62orb03.JPG]

Sigma Bonding in Alkane Molecules

As you know, a carbon atom can form a single sigma bond to other carbon atoms, as well as to hydrogen atoms when alkane molecules are formed.

Let's look at the orbitals in some of those molecules. This diagram shows the sigma bond formed by the overlap of hybrid orbitals from two carbon atoms. Notice that in this drawing of the hybrid orbitals the smaller lobes have been left off. As usual, only the large lobes (where the electrons spend most of their time) are shown. Each atom has four such orbitals and can form four bonds. In this diagram, only one such bond is indicated so each carbon atom can form three more bonds. Diagram of overlapping sp3 orbitals. [62orb04.JPG]
If all of those bonds are to hydrogen atoms, the result would be a molecule like the one diagrammed here. The C's and H's show the location of the centers of the atoms and the shaded areas represent the bonding orbitals. The lines emphasize the tetrahedral shapes. They do not represent bonds or anything like that. Bonding orbitals in ethane. [62orb05.JPG]

 

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