The bonds between adjacent water molecules are called hydrogen bonds. The strength of a single hydrogen bond is only 5 percent that of a covalent bond; however, many hydrogen bonds together provide stability and impart certain physical characteristics to water.
A hydrogen bond occurs when the hydrogen of one molecule of water attracts the oxygen atom of another. This occurs because of the differences in electronegativity between hydrogen and oxygen. Oxygen is the more electronegative atom, which means it has a stronger attraction for electrons. Hydrogen has a much lower affinity for electrons.
When hydrogen and oxygen bond, the electrons that are shared between them orbit around the oxygen atom for longer periods than around the two hydrogen atoms. This unequal sharing causes hydrogen to take on a partial positive charge, whereas oxygen becomes slightly negative. Positive and negative charges attract; therefore, the positive hydrogen regions of water molecules bond to the negative oxygen areas of other water molecules, creating a lattice of water molecules that is quite strong. Hydrogen bonding makes water more viscous and gives it a higher boiling point as compared to other liquids.