Types of Covalent Bonds-Sigma (s) and Pi (p) Bonds

Depending upon the type of overlapping, the covalent bonds are mainly of two types.

Sigma (s) and Pi (p) Bonds

When a bond is formed between two atoms by the overlap of their orbitals along the intern clear axis the resulting bond is called sigma (s) bond. Such type of overlap is also known as end or head on overlap. It is a strong bond and symmetrical. The overlapping along the inter nuclear axis can take place in any of the following ways;

1.     s-s Overlapping:

This type of overlap takes place between atoms having half-filled s-orbital in their outer most energy shell. For example, in the formation of hydrogen molecule, 1s orbital of one hydrogen atom overlap with 1s orbital of other hydrogen atom thus forming a sigma bond.

2.     s-p Overlapping:

In this case, half-filled s-orbital of one atom overlaps with the half-filled p-orbital of another atom. A simple example of this type is the formation of hydrogen fluoride. Here 1s orbital of hydrogen overlaps with 2p_z­ orbital of fluoride.

3.     p-p Overlapping:

This type of overlapping occurs when p-orbital of one atom overlaps with the p-orbital of the other as in case fluoride molecule. The molecule of fluorine is produced by the overlapping of 2p_z orbitals of the two fluorine atoms.

Pi (p) Bonds

 Pi-bond is formed by lateral or side wise overlapping of p-orbitals. Sideways overlap means overlapping of p-orbital in a direction perpendicular to the inter nuclear axis. A p-bond in not formed between two bonded atoms unless the two are held together with a s-bond. It is relative a weaker bond since the electrons are not strongly attracted by the nuclear nuclear of bonding atoms.

Bonding in Molecules Explained by Valence Bond Theory

H₂ Molecule:

The electron configuration of hydrogen atom in the ground state in 1s¹. In the formation of hydrogen molecule, two half filled 1s orbital of hydrogen atoms overlap along the inter-nuclear axis and thus by forming a s_s-s bond.

The formation of H₂

 In this diagram, H_A and H_B both have one electron the can be account for. It is known that the electron on H_A belongs to H_A and the electron on H_B belongs to H_B . However, When these two hydrogen atoms bond together, it is impossible to know which electron belongs to H_A or H_B.  Also, since these two molecules are the same, they have equal attraction on the electrons they share. This is because their orbitals overlap and they now share the electrons. The electron are allowed to spin in their respective orbitals.

Cl₂ Molecule:

 The electron configuration of Cl atom in the ground state is [Ne]3s² 3p_x² 3p_y² 3p_z¹ . The two half filled 3p_z atomic orbitals of two chlorine atoms overlap along the inter-nuclear axis and thus by forming a s_p-p bond.