Covalent bonds are formed between atoms when they share electrons. In the case of polar covalent bonds, the reason behind their formation lies in the electronegativity difference between the atoms involved.
Electronegativity is a measure of an atom's ability to attract electrons towards itself in a chemical bond. When two atoms with different electronegativities form a covalent bond, the more electronegative atom tends to attract the shared electrons more strongly than the less electronegative atom. As a result, the electron density in the bond becomes unevenly distributed, creating a partial negative charge near the more electronegative atom and a partial positive charge near the less electronegative atom.
This unequal sharing of electrons gives rise to a polar covalent bond. The atom with the higher electronegativity becomes partially negatively charged (δ-) as it attracts the electrons closer to itself, while the other atom becomes partially positively charged (δ+).
For example, in a water molecule (H2O), oxygen (O) is more electronegative than hydrogen (H). Oxygen attracts the shared electrons in the O-H bonds more strongly, creating a partial negative charge around the oxygen atom and partial positive charges around the hydrogen atoms. This polarity in the molecule enables water to form hydrogen bonds and gives it various unique properties.
In summary, the formation of polar covalent bonds is a result of the difference in electronegativity between atoms, causing an uneven distribution of electron density and resulting in partial charges within the bond.