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a.
In sodium chloride (salt), sodium loses The two ions are
an electron and becomes positively attracted to each other.
charged, and chlorine gains an electron
and becomes negatively charged.

Chloride ion
Sodium atom Chlorine atom Sodium ion purposes only - do not post.
–
+
(Na) (Cl) (Na ) (Cl )
Sodium chloride (NaCl)
b.
Sodium chloride (NaCl) dissolves in
solution because the sodium ions
+
–
– + + – (Na ) and chloride ions (Cl ) each
become surrounded by water
©2022 BFW Publishers. PAGES NOT FINAL. For review
Na
– Na molecules.
–
+ +
Cl –
+ +
NaCl H O
2
FIGURE. 1.9 An ionic bond
This figure shows how sodium chloride is formed and dissolved. (a) Sodium chloride is
+

formed when a sodium atom gives up an electron to a chloride atom, forming Na and

−

Cl ions. The two ions are then attracted to each other by their opposite charges. (b) In
solution, the polarity of water causes the ions to dissociate from each other and to become
surrounded by water molecules, with water’s negative (oxygen) ends surrounding the
−
+

Na ions and its positive (hydrogen) ends surrounding the Cl ions.

positive and negative ions. FIGURE 1.9 illustrates the for- In solution, sodium and chloride ions are completely sur-

mation of sodium chloride. Sodium loses an electron and rounded by water molecules. If the water is then removed from
becomes a positively charged cation, while chloride gains an this solution, ionic bonds will again form between the sodium
electron and becomes a negatively charged anion, as shown in and chloride ions. As the water evaporates, the concentrations
+
−

.

Figure 1.9a The two ions are then attracted to each other by of Na and Cl increase and the two ions come together to
their opposite charges. While covalent bonds are represented the point where they join and precipitate as salt crystals.
,
by lines like those shown in Figure 1.7 ionic bonds are indi-
+
−
cated by superscripts showing charge, such as Na Cl .

Figure 1.9b illustrates what happens when sodium chlo- Chemical Reactions
ride is placed in water. The negatively charged ends of water The chemical bonds that link atoms in molecules can

molecules are attracted to the positively charged sodium change in a chemical reaction, a process by which atoms

ions, and the positively charged ends of water molecules are or molecules are transformed into different molecules. The
attracted to the negatively charged chloride ions. The ions atoms or molecules that are changed in a chemical reaction
are pulled apart in the water and become surrounded by are called reactants. The molecules formed from the reac-

polar water molecules as sodium chloride dissolves in the tion are known as products. In biological systems, chemical

water. Chemicals that dissolve well in water tend to have reactions provide a way to build and break down molecules
polar or charged regions in the molecule. for use by the cell, as well as to harness energy.
MODULE 1 ELEMENTS OF LIFE 35
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