POLAR COVALENT BONDING
All nuclei of a given type of atom (e.g., all carbon atoms) have equal affinity for electrons. But the nuclei of different types of atoms (e.g., oxygen and hydrogen) have very different affinities for electrons.

The nucleus of oxygen has a much stronger affinity for electrons than does the nucleus of hydrogen. This differential affinity for electrons is demonstrated when a covalent bond forms between an oxygen and a hydrogen atom. Since the oxygen has a higher affinity for electrons than does the hydrogen, the electrons in the covalent bond between the two atoms spend a greater fraction of their time close to the oxygen atom.

This results in diminished density of the electron cloud surrounding the nucleus of the hydrogen atom. The diminished density of the electron cloud lets the positive charge of the hydrogen nucleus "show through" the electron cloud, resulting in a partial positive charge (+). Since the elctron spends a greater fraction of time associated with the oxygen atom, there is a partial negative charge (-) associated with the oxygen atom.

The partial positive and partial negative charges of polar covalent bonds interact with other partial positive and partial negative charges associated with other molecules that have polar covalent bonds.