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Compounds That Contain Ions
Objective: To describe how ions combine to form neutral compounds.
C
hemists have good reasons to believe that many chemical compounds
contain ions. For instance, consider some of the properties of common
table salt, sodium chloride (NaCl). It must be heated to about 800 °C to melt
and to almost 1500 °C to boil (compare to water, which boils at 100 °C). As
a solid, salt will not conduct an electric current, but when melted it is a very
good conductor. Pure water will not conduct electricity (will not allow an
electric current to flow), but when salt is dissolved in water, the resulting solution readily conducts electricity (see Figure 3.20).
Source of
electric power
Figure 3.20
(a) Pure water does not conduct
a current, so the circuit is not
complete and the bulb does not
light. (b) Water containing a dissolved salt conducts electricity
and the bulb lights.
Source of
electric power
Pure
water
Salt
dissolved
in water
(a)
C H E M I S T RY
Melting means that the solid,
where the ions are locked into
place, is changed to a liquid,
where the ions can move.
Chemists have come to realize that we can best explain these properties
of sodium chloride (NaCl) by picturing it as containing Na ions and Cl
ions packed together as shown in Figure 3.21. Because the positive and negative charges attract each other very strongly, salt must be heated to a very
high temperature (800 °C) before it melts.
Figure 3.21
(a) The arrangement of sodium
ions (Na) and chloride ions (Cl)
in the ionic compound sodium
chloride. (b) Solid sodium chloride highly magnified.
(a)
C H E M I S T RY
A substance containing ions
that can move can conduct an
electric current.
(b)
Cl–
Na+
Cl–
Na+
Cl–
Na+
Cl–
Na+
Cl–
Na+
Cl–
Na+
(b)
To explore further the significance of the electrical conductivity results,
we need to discuss briefly the nature of electric currents. An electric current
can travel along a metal wire because electrons are free to move through the
wire; the moving electrons carry the current. In ionic substances the ions
carry the current. Thus substances that contain ions can conduct an electric
current only if the ions can move—the current travels by the movement of the
3.11
Compounds That Contain Ions
75
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C H E M I S T RY
Dissolving NaCl causes the ions
to be randomly dispersed in the
water, allowing them to move
freely. Dissolving is not the
same as melting, but both
processes free the ions to
move.
C H E M I S T RY
An ionic compound cannot contain only anions or only cations,
because the net charge of a
compound must be zero.
charged ions. In solid NaCl the ions are tightly held and cannot move. When
the solid is melted and changed to a liquid, however, the structure is disrupted and the ions can move. As a result, an electric current can travel
through the melted salt.
The same reasoning applies to NaCl dissolved in water. When the solid
dissolves, the ions are dispersed throughout the water and can move around
in the water, allowing it to conduct a current.
Thus we recognize substances that contain ions by their characteristic
properties. They often have very high melting points, and they conduct an
electric current when melted or when dissolved in water.
Many substances contain ions. In fact, whenever a compound forms between a metal and a nonmetal, it can be expected to contain ions. We call
these substances ionic compounds.
One fact very important to remember is that a chemical compound must
have a net charge of zero. This means that if a compound contains ions, then
1. There must be both positive ions (cations) and negative ions (anions)
present.
2. The numbers of cations and anions must be such that the net charge is
zero.
C H E M I S T RY
The net charge of a compound
(zero) is the sum of the positive
and negative charges.
For example, note that the formula for
sodium chloride is written NaCl, indicating one
of each type of these elements. This makes sense
because sodium chloride contains Na ions and
Cl ions. Each sodium ion has a 1 charge and
each chloride ion has a 1 charge, so they must
occur in equal numbers to give a net charge of
zero.
Na
Cl
NaCl
Charge: 1
Charge: 1
Net charge: 0
Na
Group
1
Cl
Group
7
And for any ionic compound,
Total charge of cations Total charge of anions Zero net charge
C H E M I S T RY
The subscript 1 in a formula is
not written.
Thomson and Rutherford helped
to show that atoms consist of
subatomic particles, two of
which are charged.
What if subatomic particles had
no charge? How would it affect
compounds formed between
metals and nonmetals?
Chapter 3
Cl
Mg2
W H AT I F ?
76
Consider an ionic compound that contains
the ions Mg2 and Cl. What combination of
these ions will give a net charge of zero? To balance the 2 charge on Mg2, we will need two
Cl ions to give a net charge of zero.
Cation charge:
2
Cl
Anion charge:
2 (1)
MgCl2
Compound net
charge: 0
This means that the formula of the compound
must be MgCl2. Remember that subscripts are
used to give the relative numbers of atoms (or
ions).
Chemical Foundations: Elements, Atoms, and Ions
Mg
Group
2
Cl
Group
7
Page 3 of 4
Li
Group
1
N
Group
5
Now consider an ionic compound that contains the ions Ba2 and O2.
What is the correct formula? These ions have charges of the same size (but
opposite sign), so they must occur in equal numbers to give a net charge of
zero. The formula of the compound is BaO, because (2) (2) 0.
Similarly, the formula of a compound that contains the ions Li and N3
is Li3N, because three Li cations are needed to balance the charge of the N3
anion.
Li
Li
Li
N3
Positive charge:
3 (1)
Li3N
Negative charge:
(3)
Net charge:
0
Example 3.6
Writing Formulas for Ionic Compounds
The pairs of ions contained in several ionic compounds are listed below.
Give the formula for each compound.
a. Ca2 and Cl
b. Na and S2
c. Ca2 and P3
Solution
a. Ca2 has a 2 charge, so two Cl ions (each with the charge 1)
will be needed.
Cl
Ca2
where
2
Cl
0
2(1)
The formula is CaCl2.
b. In this case S2, with its 2 charge, requires two Na ions to produce a zero net charge.
Na
where
Na
S2
2(1)
2
0
The formula is Na2S.
c. We have the ions Ca2 (charge 2) and P3 (charge 3). We must
figure out how many of each is needed to balance exactly the positive and negative charges. Let’s try two Ca2 and one P3.
Ca2
Ca2
P3
The resulting net charge is 2(2) (3) (4) (3) 1.
(continued)
3.11
Compounds That Contain Ions
77
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This doesn’t work because the net charge is not zero. We can obtain the same total positive and total negative charges by having
three Ca2 ions and two P3 ions.
(continued)
Ca2
where
Ca2
Ca2
3(2)
P3
P3
2(3)
0
Thus the formula must be Ca3P2.
Self-Check Exercise 3.6
Give the formulas for the compounds that contain the following pairs of ions.
a. K and I
b. Mg2 and N3
c. Al3 and O2
Focus Questions
Sections 3.8–3.11
1. Why are the elements in vertical groups on the periodic table called
“families”?
2. What does the “stair-step” line on the periodic table tell you?
3. Which elements contain diatomic molecules? Write their formulas.
4. Which elements are gases at 25 °C? Write their formulas.
5. How is an ion formed?
Show how F forms an anion.
Show how Li forms a cation.
78
Chapter 3
Chemical Foundations: Elements, Atoms, and Ions