In the structure of propane, CH3-CH2-CH3, there are 10 total bonds. Two carbon-carbon single bonds, and eight carbon-hydrogen single bonds. Each of these single bonds is also known as a sigma bond. Every hydrogen atom in this molecule uses an s-orbital to form bonds.
Each carbon atom in this molecule forms four sigma bonds. The electrons of each carbon atom are found in one s-orbital and three p-orbitals. However, when forming sigma bonds, the carbon atoms combine the four atomic orbitals into four molecular orbitals. This results in each carbon now having four hydridized sp3 orbitals. Therefore, each carbon atom is sp3 hybridized.
Now, when forming a sigma bond, the hybrid orbitals are used in the bonding. So for each C-H bond in this structure, the sigma bond is composed of an sp3 orbital from the carbon, and an s-orbital from the hydrogen. In the case of the C-C bonds, each sigma bond is simply a combination of two sp3 orbitals, one from each carbon.
