The Foundations of Chemistry

The names of some common ions appear in Table 2-3. You will need to know the

names and formulas of these frequently encountered ions. They can be used to write the

formulas and names of many ionic compounds. We write the formula of an ionic com-

pound by adjusting the relative numbers of positive and negative ions so their total charges

cancel (i.e., add to zero). The name of an ionic compound is formed by giving the names

of the ions, with the positive ion named first.

56 CHAPTER 2: Chemical Formulas and Composition Stoichiometry

EXAMPLE 2-1 Formulas for Ionic Compounds

Write the formulas for the following ionic compounds: (a) sodium fluoride, (b) calcium fluor-

ide, (c) iron(II) sulfate, (d) zinc phosphate.

Plan

In each case, we identify the chemical formulas of the ions from Table 2-3. These ions must

be present in the simplest whole-number ratio that gives the compound no net charge.Recall

that the formulas and names of ionic compounds are written by giving the positively charged

ion first.

Solution

(a) The formula for the sodium ion is Na, and the formula for the fluoride ion is F(Table

2-3). Because the charges on these two ions are equal in magnitude, the ions must be present

in equal numbers, or in a 1
1 ratio. Thus, the formula for sodium fluoride is NaF.

(b) The formula for the calcium ion is Ca^2 and the formula for the fluoride ion is F. Now

each positive ion (Ca^2 ) provides twice as much charge as each negative ion (F). So there

must be twice as many Fions as Ca^2 ions to equalize the charge. This means that the ratio

of calcium to fluoride ions is 1
2. So the formula for calcium fluoride is CaF 2.

(c) The iron(II) ion is Fe^2 , and the sulfate ion is SO 42 . As in part (a), the equal magnitudes

of positive and negative charges tell us that the ions must be present in equal numbers, or in

a 1:1 ratio. The formula for iron(II) sulfate is FeSO 4.

(d) The zinc ion is Zn^2 , and the phosphate ion is PO 43 . Now it will take threeZn^2 ions to

account for as much charge (6total) as would be present in twoPO 43 ions (6total). So

the formula for zinc phosphate is Zn 3 (PO 4 ) 2.

You should now work Exercises 14 and 21.

EXAMPLE 2-2 Names for Ionic Compounds

Name the following ionic compounds: (a) (NH 4 ) 2 S, (b) Cu(NO 3 ) 2 , (c) ZnCl 2 , (d) Fe 2 (CO 3 ) 3.

Plan

In naming ionic compounds, it is helpful to inspect the formula for atoms or groups of atoms

that we recognize as representing familiar ions.

Problem-Solving Tip:Where to Start in Learning to Name

Compounds

You may not be sure of the best point to start learning the naming of compounds. It has

been found that before rules for naming can make much sense or before we can expand

our knowledge to more complex compounds, we need to know the names and formulas

in Tables 2-2 and 2-3. If you are unsure of your ability to recall a name or a formula in

Tables 2-2 and 2-3 when given the other, prepare flash cards, lists, and so on that you

can use to learn these tables.

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 3.13, Naming Ionic
Compounds.