The matter is made up of different types of elements. These elements have constituent atoms, and these atoms are held together by force. In a chemical species, the constituents like atoms and ions are joined together by an attractive force called a chemical bond. 

Covalent Bond

Lewis’s postulations were refined and elaborated by Langmuir by introducing the term covalent bond. When two atoms are joined by mutually sharing one or more pairs of valence electrons, then they are said to have a covalent bond. These molecules with covalent bonds are called covalent compounds. These compounds achieve stability because their atoms either share the valence electrons with other elements or with the atoms of the same element to complete their octet configuration.

Octet Rule- It states that the atoms can combine with one another either by transferring the electrons from one atom to another or sharing the electrons in order to obtain the octet in their shells.

Covalent bond examples

1] Water- The water is made up of covalent bonds where hydrogen and oxygen bond together to form H2O. In a water molecule, the oxygen atoms share two electrons with hydrogen, and hydrogen shares its single electron in return with oxygen. Oxygen exhibits high electronegativity.

2] Chlorine- The formation of a chlorine molecule where two chlorine atoms come together and contribute one electron to the shared pair. In this process, the chlorine atom obtains its outer shell octet.

3] Diamond- Diamond has a covalent bond of carbon, and it contains four covalent bonds. Each carbon atom is joined by the mutual sharing of electrons with the other four carbon atoms. These covalent bonds are very strong, and thus diamond has very high melting and boiling point. 

Types of Covalent Bonds based on polarisation

Polarisation is a condition in which an atom has both positive and negative charges on it.

Polar covalent bonds

The polar covalent bonds are present in the molecule that has the atoms arranged in such a way that one end has a positive charge and the other has a negative charge. These polar molecules have unequal sharing of electrons, and thus there is more difference in the electronegativity in the atoms, and these atoms exhibit a strong pull for electrons.

If the difference in the electronegativity is greater than 0 and will be less than 2, then the bond is said to be polar. Thus, the greater the difference in electronegativity, the more polar is the bond. Polar molecules are only soluble or dissociate in polar solvents. 

Examples- 

• Fluorine and Hydrogen- Fluorine has more protons and has a stronger pull for electrons than hydrogen; thus, in the covalent bond between fluorine and hydrogen, the electrons are unequally shared.
• Ammonia (NH3)- Ammonia has one nitrogen atom with a negative charge on one side bonded with three hydrogen atoms that are electropositive. So, each end of ammonia has a positive and negative charge, respectively; thus, ammonia is a polar covalent molecule.
• Hydrogen Bromide (HBr)- The hydrogen bromide shares a polar covalent bond as the bromine is more electronegative than hydrogen and attracts the electrons from hydrogen. There is an electronegative difference between hydrogen and bromine.

Non-polar Covalent bonds

The atoms that share an equal number of valence electrons with each other, and thus the electronegativity difference between them is zero, are called no-polar covalent molecules. While sharing an equal number of electrons, these atoms are able to create an arrangement where they are more stable. There is the symmetrical distribution of electrons within the atoms in order to obtain the octet of their valence shells. The nonpolar molecules are only soluble in non-polar solvents.

For example- Oil is a nonpolar molecule, and it is not soluble in water which is a polar molecule.

Examples-

• Methane (CH4)- Methane has one carbon atom bonded to four hydrogen atoms. Both carbon and hydrogen atoms share an equal number of electrons and leave no charge on the molecule and make it non-polar.
• Carbon tetrachloride (CCl4)- The molecule is nonpolar because there is an asymmetrical sharing of electrons between the tetrahedral chlorine structure and the carbon atom. There is no electronegative difference between these atoms.
• Boron trifluoride- It contains one boron atom and three fluorine atoms. The trigonal planar structure of boron trifluoride eliminates the dipole moment and makes it equal to zero. 

Conclusion

A number of molecules that we see in our day-to-day life are made up of covalent bonds. These bonds are very strong. Covalent bonds play a major role in guiding various mechanisms in the human body. All the biomolecules like nucleic acids, proteins etc., use the covalence principle.

The conventional method for drug discovery is based on covalent bonds and interactions of molecules. These interactions have become the target for the study of bio functions and for various discovery approaches. The nonpolar covalent molecules have zero dipole moments and can be used as good surfactants. Noble gases are also considered non-polar molecules.