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slightly negative, while the other atom has less than its share, becoming slightly positive. Typically these polar covalent bonds occur where hydrogen bonds to oxygen or to nitrogen. Water molecules are polar and other polar molecules are soluble in water (fig. 2.6a).

The attraction of the positive hydrogen end of a polar molecule to the negative nitrogen or oxygen end of another polar molecule is called a hydrogen bond. Hydrogen bonds are weak bonds, particularly at body temperature. For example, at temperatures below 0° C, the hydrogen bonds between water molecules shown in figure 2.6b are strong enough to result in ice. As the temperature rises, increased molecular movement is sufficient to break the hydrogen bonds, and water becomes a liquid. Even at body temperature, hydrogen bonds are important in protein and nucleic acid structure. Clinical Application 2.2 examines how radiation that moves electrons can affect human health.

D What is an ion?

Describe two ways that atoms may combine with other atoms.

Distinguish between a molecule and a compound.

Distinguish between an ion and a polar molecule.

Usually atoms of each element form a specific number of chemical bonds. Hydrogen atoms form single bonds, oxygen atoms form two bonds, nitrogen atoms form three bonds, and carbon atoms form four bonds. Symbols and lines can be used to represent the bonding capacities of these atoms, as follows:

Slightly negative end

Slightly positive ends

Slightly negative end

Slightly positive ends

Hydrogen bonds
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