A polar covalent bond forms when two atoms share electrons unequally due to a difference in their electronegativities. Electronegativity is a measure of an atom's attraction for electrons in a chemical bond. The atom with the higher electronegativity draws the shared electrons closer to its nucleus, creating a partial negative charge on itself and a partial positive charge on the other atom. This unequal distribution of electrons creates a dipole moment, making the bond polar.