A third kind of chemical bond, about a tenth as strong and stable as covalent bonds, is the hydrogen bond. The hydrogen bond is one of several “weak” bonds that do not form molecules, but do make temporary links between different molecules, or between different parts of one large molecule. Weak bonds come about because most covalent bonds leave at least a slight electrical imbalance among the participating atoms. Consider water, whose chemical formula is H2O. The oxygen atom has a greater hunger for electrons than the two hydrogen atoms, and so the shared electrons are held closer to the oxygen than to the hydrogens. As a result, there’s an overall negative charge in the vicinity of the oxygen, and an overall positive charge around the hydrogen atoms. This unequal distribution of charge, together with the geometry of the covalent bonds, results in a molecule with a positive end and a negative end. Such a molecule is called polar because it has two separate centers, or poles, of charge.