bei48482_FM

The Solid State 347

O^2 –

H+

H+

H+

H+

Region of high

electron probability

Figure 10.14In an H 2 O molecule, the four pairs of valence elec-

trons around the oxygen atom (six electrons contributed by the

O atom and one each by the H atoms) preferentially occupy four

regions that form a tetrahedral pattern. Each H 2 O molecule can

form hydrogen bonds with four other H 2 O molecules.

constitute ice crystals (Fig. 10.15). With only four nearest neighbors around each

molecule, instead of as many as twelve in other solids, ice crystals have extremely open

structures, which is why ice has a relatively low density.

Hydrogen bonds occur widely in biological materials. The peptide bonds that join

amino acids to form proteins are hydrogen bonds, for example, as are the bonds that

hold together the two strands of the double helix of DNA. The bonds in DNA are

strong enough for it to be a reliable store of genetic information but weak enough to

permit its strands to be unzipped temporarily for the information to be transcribed

ultimately into proteins and also permanently for DNA replication.

H

O

H

Figure 10.15The structure of an ice crystal, showing the open

hexagonal arrangement of the H 2 O molecules. There is less or-

der in liquid water, which allows the molecules to be closer to-

gether on the average than they are in ice. Thus the density of

ice is less than that of water, and ice floats.

The water molecules in a snowflake are held together by hydrogen

bonds.

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