Polar molecules have partial charges because of an uneven distribution of electron density within the molecule. This uneven distribution arises due to differences in electronegativity between the atoms involved in the covalent bond.
Electronegativity is a measure of an atom's ability to attract electrons towards itself in a chemical bond. When atoms with different electronegativities form a covalent bond, the more electronegative atom attracts the shared electrons more strongly, creating a partial negative charge (δ-) on that atom. The other atom, which has a lower electronegativity, has a partial positive charge (δ+).
As a result, the electron cloud in the bond is shifted closer to the more electronegative atom, leading to an asymmetrical distribution of charge. This creates a separation of positive and negative charges, resulting in a dipole moment. The magnitude of the dipole moment is determined by the difference in electronegativity between the atoms and the overall molecular geometry.
For example, in a water molecule (H2O), oxygen (O) is more electronegative than hydrogen (H). Oxygen attracts the shared electrons more strongly, resulting in a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. This gives water its polar nature, with a net dipole moment.
Polar molecules are attracted to other polar molecules or ions through intermolecular forces such as dipole-dipole interactions or hydrogen bonding. These interactions play a crucial role in various biological and chemical processes.