Biology (Holt)

Differentiatebetween atoms and elements.

Describehow an atom differs from a molecule.

Distinguishbetween covalent bonds and ionic

bonds.

Identifythe type of weak bond that forms

between water molecules.

Critical ThinkingRecognizing Differences

Explain the difference between polar molecules

and nonpolar molecules. Give an example of a

polar molecule.

Sodium chloride is an

example of

Aa compound. Can isotope.

Ba molecule. Dan ion.

TAKS Test PrepTAKS Test Prep

Section 1 Review

Ionic Bonds

Sometimes atoms or molecules gain or lose electrons. An atom or

molecule that has gained or lost one or more electrons is called an

(IE ahn).Ions have an electrical charge because they contain an

unequal number of electrons and protons. An atom that has lost

electrons is positively charged, whereas an atom that has gained

electrons is negatively charged.

Ions of opposite charge may interact to form an ionic bond. For

example, an atom of sodium is unstable because it has only one elec-

tron in its outer level. Sodium readily gives up this electron to

become a stable, positively charged sodium ion, Na+. An atom of

chlorine is also unstable because it has seven electrons in its outer

level. Chlorine readily accepts an electron to become a stable, nega-

tively charged chloride ion, Cl–. The negative charge of a chloride

ion is attracted to the positive charge of a sodium ion. Thus, sodium

atoms and chlorine atoms readily form an ionic bond to become

sodium chloride, as shown in Figure 4.

ion

30 CHAPTER 2Chemistry of Life

Real Life

Cells contain water

and are vulnerable to

freezing.

Some species of fish can

swim in icy water because

their bodies contain chem-

icals similar to automotive

antifreeze.

Analyzing Information

Humans lack these

chemicals. Why then can

we tolerate short-term

exposure to subzero

temperatures?

Chloride ion, Cl–

Sodium ion, Na+

Ionic bonds between

sodium ions and

chloride ions

Salt crystal

Table salt (NaCl)

Ionic bonds in sodium chloride, NaCl, are formed by the interaction between

sodium ions, Na+, and chloride ions, Cl–.

Figure 4 Ionic bonds in sodium chloride