A covalent bond becomes polar when there is an unequal sharing of electrons between the atoms involved in the bond. This unequal sharing occurs due to differences in the electronegativities of the atoms.
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 exerts a stronger pull on the shared electrons. As a result, the electron density in the bond is shifted towards the more electronegative atom.
This shift in electron density creates partial positive (+) and partial negative (-) charges on the atoms, resulting in a separation of charge or polarity within the bond. The more electronegative atom acquires a partial negative charge, denoted as δ-, while the less electronegative atom acquires a partial positive charge, denoted as δ+.
For example, in a molecule of hydrogen chloride (HCl), chlorine (Cl) is more electronegative than hydrogen (H). As a result, chlorine attracts the shared electron pair closer to itself, leading to a partial negative charge on the chlorine atom (δ-) and a partial positive charge on the hydrogen atom (δ+). Thus, the H-Cl bond becomes polar.
The polarity of a covalent bond is often represented using an arrow pointing towards the more electronegative atom, indicating the direction of electron density shift.
It's important to note that not all covalent bonds are polar. When two atoms have similar electronegativities, the electron density is shared equally, resulting in a nonpolar covalent bond.