Atoms, Molecules, and Ions
Interactive Student Tutorial
2.10 Naming Chemical Compounds
In order for chemists and computers around the world
to communicate, a system of rules has been devised for naming chemical
compounds. In this chapter we will consider some simple rules of chemical
Naming Binary Ionic Compounds
Binary ionic compounds are named by naming the two ions in the compound: cation and anion. Before we learn to name binary ionic compounds, we need some guidelines for naming the ions themselves.
Let's consider cations first. Metals tend to form cations,
and main-group metals form cations with a charge equal to the group number.
The name of the cation is metal ion.
Some main-group metals and transition metals form more than one kind of cation. We use Roman numerals to distinguish the different ions. Thus Cr2+ is chromium(II) ion, and Cr3+ is chromium(III) ion. An older naming system differentiates between the two ions using different endings: -ous indicates the ion with the lower charge, and -ic refers to the ion with the higher charge.
Now let's take a look at anions. Anions are formed
when nonmetals gain electrons. The main group nonmetals usually form anions
whose charge is equal to the group number minus eight. The name of the
anion is the root of the element name plus the suffix -ide.
Chemical compounds are neutral, so the charges of the cations must equal the charges of the anions. Because this is always true, we don't need to indicate how many of a particular ion are in a compound. The following are some examples of names and formulas for binary ionic compounds:
MgO: magnesium oxide (No Roman numeral necessary because Mg forms only one ion.)
CrCl2: chromium(II) chloride or chromous chloride (Chromium forms two ions.)
K2S: potassium sulfide
Now you try. Be sure to try your hand at predicting the names using the rules above before checking your answer.
Naming Ionic Compounds
1. Predict the name and formula for a combination of cation and anion.
2. Select the cation and anion to check your prediction.
Naming Binary Molecular Compounds
Recall from the Interactive Student Tutorial section 2.8 that molecular compounds form from nonmetalic elements. These compounds are named by assuming that one of the two elements is more like an anion, and one more like a cation. The cationlike element takes the name of the element itself, and the anionlike element takes an -ide ending. You can decide about cationlike and anionlike properties by looking at an element's location on the periodic table. The farther left and toward the bottom of the periodic table an element is located, the more likely it is to have cationic character; the farther right and toward the top, the more likely it is to have anionic character. Nonmetals form molecular compounds in different combinations, so numerical prefixes are used to indicate how many of each atom are present. Note that when a prefix ends in a or o, and the anion name begins with a vowel, the a or o is dropped.
The following are examples of names of binary molecular compounds:
CO carbon monoxide
CO2 carbon dioxide
N2O4 dinitrogen tetroxide
N2O dinitrogen oxide
Naming Compounds with Polyatomic Ions
Ionic compounds containing polyatomic ions are named in the same manner as other ionic compounds: cation and anion. In order to recognize and name this type of compound, you must memorize the names, formulas, and charges of the most common polyatomic ions.
There are some patterns in the names of polyatomic ions that may help you learn them. Most polyatomic anions end in -ate or -ite; only hydroxide (OH), cyanide (CN), and peroxide (O22) have the -ide ending.
There are several series of oxoanions, in which an atom of a given element is combined with different numbers of oxygen atoms. In a series of only two oxoanions, the ion with the fewer oxygens takes the -ite ending.
The prefixes hypo- and per- are used in the longer oxoanion series.
Several pairs of ions are related by the presence or absence of a hydrogen.
HCO32 hydrogen carbonate ion CO32 carbonate ion
When there is more than one of a particular polyatomic ion in a formula, the ion formula is enclosed in parentheses and the number of ions is indicated with a subscript, as shown below.
(NH4)2S ammonium sulfide
Naming Ionic Compounds
Predict the name and formula for a combination of cation and anion.
Select the cation and anion to check your prediction.
Most acids are oxoacids. Table 2.4 lists a number of oxoacids. Note the correlation of the -ite ending of the anion with the ous ending of the corresponding acid. Similarly the -ate anion is related to the -ic acid by the addition of one hydrogen ion.
Binary acids that do not contain oxygen are named hydro_____________ic acid. Thus the aqueous acid HBr is hydrobromic acid.