Water is a good example of a hydrogen-bonded substance. The two hydrogen atoms in each water molecule are covalently bonded to the oxygen atom, but because of oxygen's strong attraction for the shared electrons, each hydrogen has a partial positive charge (see Figure 1A and 1B). Owing to this charge, a hydrogen atom in one water molecule is weakly attracted to a negatively charged oxygen atom in another nearby water molecule, forming a hydrogen bond (see Figure 1C).

Fig. 1. Hydrogen bonding. Hydrogen bonds form when hydrogen is covalently bonded to one electronegative atom and is simultaneously attracted to another electronegative atom. (A) Hydrogen bonding between two water molecules. In water, each hydrogen atom is covalently bonded to oxygen, an electronegative atom. Because of oxygen's strong attraction for electrons, the shared electrons in each bond spend more time around the oxygen atom. The oxygen atom has two partial negative charges and each hydrogen has a partial positive charge. Often, the symbols d⁺ and d^- are used to represent partial charges, where the d⁺ shows that the hydrogen atoms have partial positive charges, and the d^- symbol shows that the oxygen has two partial negative charges. The (partially) positively charged hydrogen atom is attracted to the (partially) negatively charged oxygen in another water molecule. This weak attraction (pink band) constitutes the hydrogen bond. (B) Hydrogen bonds often form when a hydrogen is shared between an oxygen and a nitrogen atom. Like oxygen, nitrogen is a strongly electronegative atom. (C) Like the central H₂O molecule shown here, each water molecule can form hydrogen bonds (pink bands) with four other water molecules, because there are four partial charges on each water molecule. The array then assumes the shape of a tetrahedron.

Since each of the hydrogens, while remaining covalently bonded to the oxygen atom of its own molecule, can form a weak attachment with the oxygen of another water molecule, and the oxygen can form a weak attachment with two external hydrogens, each water molecule has the potential for being simultaneously linked by hydrogen bonds to four other water molecules and each of these to four others, and each of these to four others, etc. In a sense, then, a volume of water is a continuous chemical entity, because of the hydrogen bonding between the individual water molecules. It is the polarity of water molecules and hydrogen bonds between them that gives water its special properties.

Though water molecules have the potential for forming hydrogen bonds with four other water molecules, the potential of each water molecule to form hydrogen bonds with four other water molecules is not fully realized because molecular motion prevents stabilization. The hydrogen bonds between water molecules are constantly breaking and reforming. As water is cooled, however, the water molecules move more slowly and the extent of hydrogen bonding increases. Hydrogen bonding reaches its full potential when the water has frozen into ice. When all four possible bonds have formed, each is oriented in space with maximal divergence from the other three. The resulting three-dimensional lattice of water molecules in ice is rather open (see Figure 2); the packing of the molecules is not as tight as would be possible if they were less rigidly arranged, as they are in liquid water. When ice is warmed to the melting point, a few of the hydrogen bonds break, and the water molecules become less rigidly oriented. The resulting deformation of the lattice and tighter packing of the molecules make the water denser than ice. This means that ice floats, and that ponds and streams freeze from the top down instead of from the bottom up.

A

B

Fig. 2. Molecular structure of ice. Because of the tetrahedral arrangement around each water molecule, the lattice is an open one, with considerable space between molecules (A). In liquid water the arrangement is not quite so rigid, and the packing of molecules is therefore slightly denser; but the general lattice arrangement is nonetheless largely preserved. (Planes have been added to help show the three-dimensional disposition of the molecules.) Note the hexagons created by the hydrogen bonding in this bit of ice. This conformation is the basis of the hexagonal shape of most snow crystals (B). Each snowflake contains about 10¹⁶ water molecules.