Animation showing transient hydrogen bonding between water molecules. Hydrogen bonds (white) are a relatively weak (compared to a chemical bond) attraction between the slightly negatively charged oxygen atoms (red) and the slightly positive hydrogen atoms (blue). Each water molecule (H2O) has two hydrogens and two lone pairs of electrons on the oxygen, meaning it can make up to four hydrogen bonds with neighbouring molecules. The motion of the molecules means these bonds are short-lived, but the overall effect is to dramatically increase the melting point, boiling point and viscosity of water, compared to that of similar substances (such as the chemical analogue hydrogen sulphide, H2S).