Because the oxygen atom in each water molecule contains eight protons and each hydrogen atom contains only one proton, the electron pair shared in each covalent bond is more strongly attracted to the oxygen nucleus than to either of the hydrogen nuclei. Although the electrons surround both the oxygen and hydrogen nuclei, the negatively charged electrons are far more likely to be found near the oxygen nucleus at a given moment than near one of the hydrogen nuclei. Because of this situation, the oxygen end of the water molecule has a partial negative charge. The hydrogen end has a partial positive charge. Molecules such as water that have opposite partial charges at different ends of the molecule are called polar molecules .
Water is one of the most polar molecules known.
The polarity of water contributes to its ability to attract other molecules and form special types of chemical bonds with them. In fact, water forms hydrogen bonds with other water molecules because the hydrogen atoms of some water molecules are attracted to the oxygen atoms of other water molecules. The ability of water molecules to form weak bonds among themselves and with other molecules is the reason for much of the organization and chemistry of living things. Although hydrogen bonds are weak, these bonds are constantly made and broken. It is the reason that water molecules can bear the weight of organisms such as the water strider shown to the right.