Molecules of H₂O exhibit polarity, an uneven distribution of charge on different parts of the overall neutral molecule. The electrons involved in bonding the molecule together are attracted more strongly to the oxygen atom. Thus, the oxygen atom has a slightly negative charge and the hydrogen atoms have slightly positive charges.

In a group of water molecules, the slightly negative oxygen atoms and the slightly positive hydrogen atoms are attracted to each other. This attraction between water molecules is called hydrogen bonding.