bonding

The most common type of covalent bond occurs when the orbitals of two atoms overlap and share a pair of electrons. This is called a **sigma** bond. All of the bonds in methane, for example, are sigma bonds:



Follow this link to a 3D animation of methane and its sigma bonding.

Double and triple bonds contain one sigma bond each, but also contain another type of bond called a **pi bond**. The sp2 and sp hybridized carbon atoms contain unhybridized 2p orbitals which are responsible for pi bonding.

In the diagram below, two sp2 hybridized carbons are forming two types of bonds (called a double bond). The center bond in green is a sigma bond (orbitals overlap); the yellow unhybridized 2p orbitals form the pi bond, which has two regions, opposite the plane of the molecule.



If the bonding atoms have two sets of unhybridized 2p orbitals (an sp hybrid), then two sets of pi bonds can form:



Note the sigma bonds between the sp hybrid orbitals (green) and with the hydrogen 1s orbitals, and the two sets of pi orbitals (hence it forms a triple bond).


 * Properties**

The type of bond between two atoms will have an effect on the bond length and bond energy between the atoms. For example, look at the three types of carbon bonds:


 * C-C bond type: || Length (A) || Energy (kJ/mol) ||
 * single || 1.54 || 376 ||
 * double || 1.33 || 611 ||
 * triple || 1.20 || 835 ||

So, as the bond order increases, the bonds get shorter and stronger.

One exception to this is in benzene. It has alternating double bonds, but they are uniform in length and energy.



When two or more double or triple bonds are adjacent to one another in a carbon chain or ring, the pi bonds in the sp2 or sp hybridized carbons can interact across a sigma bond to create a phenomenon known as **pi conjugation**.

In order to understand this phenomenon, one must look at the orbital picture of benzene.

The diagram above represents an aryl ring (like benzene) that has alternating double bonds. This means that every carbon in the molecule is sp2 hybridized and has unhybridized 2p orbitals (seen here as the red figure-eight shaped lobes).

See this page for a quick history on benzene and x-ray crystallography.

Any time two or more of these sp2 hybridized orbitals are next to one another, they end up sharing electrons across the atoms, which if you recall, creates a pi bond between two atoms. But if there are more than two adjacent sp2 atoms, the pi bonding occurs throughout ALL of the sp2 atoms, which is called pi bond conjugation. This is shown in the diagram above as the two 'donut' shaped clouds above and below the plane of the molecule.

Some other molecules that have pi bond conjugation are: