Formation of bonds from Atomic Orbitals: Ethylene

The C=C bond in ethylene has two components: the σ bond, formed by bringing an sp2 hybrid AO from each carbon together head-to-head, and the π bond, formed by bringing the unhybridized pz AOs together side-by-side.

Animate each by clicking the button below each structure.



We see that on the left, the sp2 atomic orbitals have their large lobes pointed toward each other; as the groups come together these overlap and the largest area of the resultant bonding orbital lies around the C-C axis. Electrons in this orbital are attracted to both carbon nuclei and that attraction holds the two carbons together — this is the sigma bond.

On the right, we are looking at a side-on view (you can, of course, rotate this). The unhybridized p orbitals line up parallel to each other. As the groups move together, we again see overlap between the lobes, but now side-to-side rather than head-on. The area occupied by electrons in this resultant orbital lies above and below the plane of the molecule. There is a node along the molecular plane. Again, electrons are attracted to both nuclei, resulting in a bonding interaction, but because the electrons are further away than in the sigma orbital, the bond strength of this pi interaction is weaker.