Covalent bonding is the sharing of electrons between atoms in a molecule so that each atom acquires a stable outer shell. Electrons are shared in pairs called electrons pairs (one pair being a covalent bond).

Covalent bonds within a molecule are strong. Covalent compounds (compounds whose molecules have internal covalent bonds) are not normally so strongly held together. 

Substances with covalent bonding: H2,O2, N2, H2O y NH3

Lewis dot diagrams and Lewis structures of simple molecules

PROPERTIES OF COVALENT SUBSTANCES

• Most covalent compounds have low melting and boiling points, therefore they are usually liquids or gases at room temperature. It happens because the forces between their molecules are van der Waals' forces and they are very weak forces. Little energy is needed to overcome them.

• They do not conduct electricity because there are no ions present.