Covalent bonds can be considered as either sigma(σ) or pi(π) bonds. The difference between these two types of covalent bonds is due to the way in which the atomic orbitals on each atom involved in forming the covalent bond overlap with each other during bond formation.
As an example consider what happens when two hydrogen atoms combine to form a molecule of hydrogen (H2). Hydrogen atoms contain a single electron and have an electronic configuation of 1s1, so when two hydrogen atoms react, the two 1s orbitals which contain the electron will overlap with each other and form a covalent bond between the two atoms. When the two 1s orbitals overlap or merge with each other they will form new molecular orbitals, which will contain the 2 electrons present in the hydrogen molecule. This is shown below.
The covalent bond formed between the two hydrogen atoms was formed by the head-on overlap of the 1s orbital. Covalent bonds which are formed by this head-on overlap of atomic orbitals are also called sigma(σ) bonds.
When a hydrogen molecule forms, the covalent bond between the 2 atoms was formed by the head-on overlap of two 1s-orbitals. This resulted in the formation of a sigma bond. However sigma bonds can also form when orbitals other than s-orbitals overlap. P-orbitals can also overlap in a head-on manner to form sigma bond. Recall that there are 3 p-orbitals, px, py and pz, with each orbital arranged along each of the x, y and z-axes as shown below:
Fluorine has an atomic number of 9 and an electronic configuration
of 1s22s22p5 or
As we have seen above the p-orbitals can overlap in a head-on manner to form a sigma bond. However there is another way in which the the p-orbitals can overlap to form a covalent bond. Instead of the p-orbitals overlapping in a head- manner they can overlap in a sideways manner, this sideways overlap results in the formation of a new type of covalent bond called a pi(π) bond. This is shown below: A pi(π) bond has a smaller area of overlap between the p-orbitals than a sigma bond which means a weaker covalent bond. In many molecules with double and even triple bonds you are likely to find that the covalent bonds between the atoms in the molecule will contain a mixture of sigma and pi bonds.
The bonding present in molecules with single covalent bonds is usually a sigma bond, where there is full head-on overlap of the atomic orbitals. However in molecules which have multiple bonds, such as double or triple bonds then the covalent bonds between the atoms in the molecule will be a mixture of sigma and pi bonds . This is shown below:
A possible way in which p-orbitals which contain only one electron could overlap and form sigma and pi bonds is shown below.