Pre-Requisite Reading: Electronegativity
Classification of covalent bonds
A covalent bond is formed between two similar or dissimilar atoms (Ex: Carbon-Carbon and Carbon-Halogen).
Each atom contributes one electron to form a two-electron covalent bond. After the bond formation, the two electrons are not restricted to the parent atom's nucleus but are simultaneously shared by both nuclei.
However, if the atoms forming the covalent bond are different (Ex: Carbon-Halogen), their electronegativity changes, altering the electron distribution of the covalent bond.
Therefore, based on the combining atom's electronegativity difference, the bond is categorized as polar or nonpolar.
Nonpolar covalent bond
When atoms of similar electronegative strength participate in the covalent bond formation, the bond is called a nonpolar covalent bond. The difference in electronegativity between the atoms must be less than 0.5.
For example, in molecules like H2, Cl2, Br2, I2, CH4, etc., the bond is nonpolar covalent.
The electrons are equally distributed between the two bonded atoms.
Nonpolar covalent bonds are formed by the p-block elements present on the right side of the periodic table.
Polar Covalent Bond
Suppose the combining atoms’ electronegativity difference is between 0.5 to 1.7, the electron density of the covalent bond shifts to create two terminals- electron rich and electron poor.
The more electronegative atom pulls the bond electrons closer to itself, leaving the lesser electronegative atom with a small electron density.
The change is denoted in partial charges, with the atom having more electrons getting the δ- sign and leaving the other with a δ+ sign. Such a bond is called a polar covalent bond.
The polar covalent bond is formed between nonmetals.
Examples of Polar Covalent Bond: Carbon - Nitrogen (C-N), Carbon - Oxygen (C-O), Carbon - Halogen (C-X).
Note: The covalent bonds’ electronegativity difference ranges from 0 to 1.7; however, ionic bonds are formed if the electronegativity difference between two atoms is >1.7.
Ionic bonds are formed between nonmetals and metals. Ex- NaCl, LiBr.