2.7 Naming Inorganic Compounds

To find information about a particular substance, you must know its chemical formula and name. The names and formulas of compounds are essential vocabulary in chemistry. The naming of substances is called chemical nomenclature from the Latin words nomen (name) and calare (to call).

There are now over 13 million known chemical substances. Naming them all would be a hopelessly complicated task if each had a special name independent of all others. Many important substances that have been known for a long time, such as water, H2O, and ammonia, NH3, do have individual, traditional names. For most substances, however, we rely on a systematic set of rules that lead to an informative and unique name for each substance, based on its composition.

The rules for chemical nomenclature are based on the division of substances into different categories. The major division is between organic compounds and inorganic ones. Organic compounds contain carbon, usually in combination with hydrogen, oxygen, nitrogen, or sulfur. All other compounds are called inorganic compounds. Early chemists associated organic compounds with plants and animals, and they associated inorganic compounds with the nonliving portion of our world. Although this distinction between living and nonliving matter is no longer pertinent, the classification between organic and inorganic compounds continues to be useful. In this section we consider the basic rules for naming inorganic compounds. Among inorganic compounds we will consider three categories of substances: ionic compounds, molecular compounds, and acids.

Names and Formulas of Ionic Compounds

Recall that ionic compounds usually consist of chemical combinations of metals and nonmetals. The metals form the positive ions, and the nonmetals form the negative ions. Let's examine the naming of positive ions, then the naming of negative ones. After that, we will consider how to put the names of the ions together to identify the complete ionic compound.

  1. Positive Ions (Cations)
    1. Cations formed from metal atoms have the same name as the metal:

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      Ions formed from a single atom are called monoatomic ions.

    2. If a metal can form cations of differing charges, the positive charge is given by a Roman numeral in parentheses following the name of the metal:

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      Ions with different charges exhibit different properties, such as color (Figure 2.21).

      Most of the metals that have variable charge are transition metals, elements that occur in the middle block of elements from 3B to 2B in the periodic table. The charges of these ions are indicated by Roman numerals. The common metal ions that do not have variable charges are the ions of group 1A (Li+, Na+, K+, and Cs+), those of 2A (Mg2+, Ca2+, Sr2+, and Ba2+), as well as Al3+ (group 3A) and two transition-metal ions: Ag+ (group 1B) and Zn2+ (group 2B). Charges are not shown explicitly when naming these ions. If there is any doubt in your mind whether a metal forms more than one type of cation, indicate the charge using Roman numerals. It is never wrong to do so, even though it may be unnecessary.

      An older method still widely used for distinguishing between two differently charged ions of a metal is to apply the ending -ous or -ic. These endings represent the lower and higher charged ions, respectively. They are added to the root of the element's Latin name:

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      Although we will not use these older names in this text, you might encounter them elsewhere.

    3. Cations formed from nonmetal atoms have names that end in -ium:

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      These two ions are the only ions of this kind that we will encounter frequently in the text. They are both polyatomic (composed of many atoms). The vast majority of cations are monatomic metal ions.

      The names and formulas of some of the most common cations are shown in Table 2.4 and are also included in a table of common ions that is placed in the back inside cover of the text. The ions listed on the left are the monatomic ions that do not have variable charges. Those listed on the right are either polyatomic cations or cations with variable charges. The Hg22+ ion is unusual because this metal ion is not monatomic. It is called the mercury(I) ion because it can be thought of as two Hg+ ions fused together.

  1. Negative Ions (Anions)
    1. Monatomic (one-atom) anions have names formed by dropping the ending of the name of the element and adding the ending -ide.

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      A few simple polyatomic anions also have names ending in -ide:

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    2. Polyatomic (many-atom) anions containing oxygen have names ending in -ate or -ite. These anions are called oxyanions. The ending -ate is used for the most common oxyanion of an element. The ending -ite is used for an oxyanion that has the same charge but one less O atom:

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      Prefixes are used when the series of oxyanions of an element extends to four members, as with the halogens. The prefix per- indicates one more O atom than the oxyanion ending in -ate; the prefix hypo- indicates one less O atom than the oxyanion ending in -ite:

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      Notice that if you learn the rules just indicated, you need to know only the name for one oxyanion in a series to deduce the names for the other members. These rules are summarized in Figure 2.22.

    Figure 2.22 A summary of the procedure for naming anions. The root of the name (such as "chlor" for chlorine) goes in the blank.

    1. Anions derived by adding H+ to an oxyanion are named by adding as a prefix the word hydrogen or dihydrogen, as appropriate:

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      Notice that each H+ reduces the negative charge of the parent anion by one. An older method for naming some of these ions is to use the prefix bi-. Thus, the HCO3 ion is commonly called the bicarbonate ion, and HSO4 is sometimes called the bisulfate ion.

      The names and formulas of the common anions are listed in Table 2.5 and on the back inside cover of the text. Those whose names end in -ide are listed on the left portion of the table, whereas those whose names end in -ate are listed on the right. The formulas of the ions whose names end with -ite can be derived from those ending in -ate by removing an O atom. Notice the location of these ions in the periodic table. The monatomic ions of group 7A always have a 1- charge (F, Cl, Br, and I), whereas those of group 6A have a 2- charge (O2– and S2–).

SAMPLE EXERCISE 2.10

The formula for the selenate ion is SeO42–. Write the formula for the selenite ion.

SOLUTION The selenite ion should have the same charge but one less oxygen than the selenate ion; hence, SeO32–.

PRACTICE EXERCISE

The formula for the bromate ion is BrO3. Write the formula for the hypobromite ion. Answer: BrO

  1. Ionic Compounds

    Names of ionic compounds are the cation name followed by the anion name:

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    Notice how parentheses are used in writing the chemical formulas for aluminum nitrate and copper(II) perchlorate. Parentheses followed by the appropriate subscript are used when a compound contains two or more polyatomic ions.

SAMPLE EXERCISE 2.11

Name the following compounds: (a) K2SO4; (b) Ba(OH)2; (c) FeCl3.

SOLUTION Each compound is ionic and is named using the guidelines we have already discussed. In naming ionic compounds, it is important to recognize polyatomic ions and also to determine the charge of cations with variable charge. (a) The cation in this compound is K+, and the anion is SO42–. (If you thought the compound contained S2– and O2– ions, you failed to recognize the polyatomic sulfate ion.) Putting together the names of the ions, we have the name of the compound, potassium sulfate. (b) In this case the compound is composed of Ba2+ and OH ions. Ba2+ is the barium ion and OH is the hydroxide ion. Thus, the compound is called barium hydroxide. (c) You must determine the charge of Fe in this compound because iron can have variable charges. Because the compound contains three Cl ions, the cation must be Fe3+, which is called the iron(III) or ferric ion. The Cl ion is called the chloride ion. Thus, the compound is iron(III) chloride or ferric chloride.

PRACTICE EXERCISE

Name the following compounds: (a) NH4Br; (b) Cr2O3; (c) Co(NO3)2. Answers: (a) ammonium bromide; (b) chromium(III) oxide; (c) cobalt(II) nitrate

SAMPLE EXERCISE 2.12

Write the chemical formulas for the following compounds: (a) potassium sulfide; (b) calcium hydrogen carbonate; (c) nickel(II) perchlorate.

SOLUTION In going from the name of an ionic compound to its chemical formula, it is necessary to know the charges of ions in order to determine subscripts. (a) The potassium ion is K+, and the sulfide ion is S2–. Because ionic compounds are electrically neutral, two K+ ions are required to balance the charge of one S2– ion, giving the empirical formula of the compound, K2S. (b) The calcium ion is Ca2+. The carbonate ion is CO32–, so the hydrogen carbonate ion is HCO3. Two HCO3 ions are needed to balance the positive charge of Ca2+, giving Ca(HCO3)2. (c) The nickel(II) ion is Ni2+. The perchlorate ion, which has one more O atom than the chlorate ion, is ClO4. Two ClO4 ions are required to balance the charge on one Ni2+ ion, giving Ni(ClO4)2.

PRACTICE EXERCISE

Give the chemical formula for (a) magnesium sulfate; (b) silver sulfide; (c) lead(II) nitrate. Answers: (a) MgSO4; (b) Ag2S; (c) Pb(NO3)2

Names and Formulas of Acids

Acids are an important class of hydrogen-containing compounds and are named a special way. For our present purposes, an acid is defined as a substance whose molecules yield hydrogen ions (H+) when dissolved in water. When we encounter the chemical formula for an acid at this stage of the course, it will be written with H as the first element, as in HCl and H2SO4.

We can consider an acid to be composed of an anion connected to enough H+ ions to totally neutralize or balance the anion's charge. Thus, the SO42– ion requires two H+ ions, forming H2SO4. The name of an acid is related to the name of its anion, as summarized in Figure 2.23.

Figure 2.23 Summary of the way in which anion names and acid names are related. Notice that the prefixes per- and hypo- are retained in going from the anion to the acid.

  1. Acids Based on Anions Whose Names End in -ide. Anions whose names end in -ide have associated acids that have the hydro- prefix and an -ic ending as in the following examples:

    Anion Corresponding Acid  Cl– (chloride) HCl (hy

  2. Acids Based on Anions Whose Names End in -ate or -ite. Anions whose names end in -ate have associated acids with an -ic ending, whereas anions whose names end in -ite have acids with an -ous ending. Prefixes in the name of the anion are retained in the name of the acid. These rules are illustrated by the oxyacids of chlorine:

    Anion Corresponding Acid  ClO– (hypochlorite) HC

SAMPLE EXERCISE 2.13

Name the following acids: (a) HCN; (b) HNO3; (c) H2SO4; (d) H2SO3.

SOLUTION (a) The anion from which this acid is derived is CN, the cyanide ion. Because this ion has an -ide ending, the acid is given a hydro- prefix and an -ic ending: hydrocyanic acid. Only water solutions of HCN are referred to as hydrocyanic acid: The pure compound is called hydrogen cyanide. (b) Because NO3 is the nitrate ion, HNO3 is called nitric acid (the -ate ending of the anion is replaced with an -ic ending in naming the acid). (c) Because SO42– is the sulfate ion, H2SO4 is called sulfuric acid. (d) Because SO32– is the sulfite ion, H2SO3 is sulfurous acid (the -ite ending of the anion is replaced with an -ous ending).

PRACTICE EXERCISE

Give the chemical formulas for (a) hydrobromic acid; (b) carbonic acid. Answers: (a) HBr; (b) H2CO3

Names and Formulas of Binary Molecular Compounds

The procedures used for naming binary (two-element) molecular compounds are similar to those used for naming ionic compounds:

  1. The name of the element farthest to the left in the periodic table is usually written first.

  2. If both elements are in the same group in the periodic table, the lower one is named first.
  3. The name of the second element is given an -ide ending.

  4. Greek prefixes (Table 2.6) are used to indicate the number of atoms of each element. The prefix mono- is never used with the first element. When the prefix ends in a or o and the name of the second element begins with a vowel (such as oxide), the a or o is often dropped.

The following examples illustrate these rules:

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It is important to realize that you cannot predict the formulas of most molecular substances in the same way that you predict the formulas of ionic compounds. That is why we name them using prefixes that explicitly indicate their composition. Compounds that contain hydrogen and one other element are an important exception, however. These compounds can be treated as if they contained H+ ions. Thus, HCl is hydrogen chloride (this is the name used for the pure compound; water solutions of HCl are called hydrochloric acid). Similarly, H2S is hydrogen sulfide.

SAMPLE EXERCISE 2.14

Name the following compounds: (a) SO2; (b) PCl5; (c) N2O3.

SOLUTION The compounds consist entirely of nonmetals, so you should expect that they are molecular rather than ionic. Using the prefixes in Table 2.6, we have (a) sulfur dioxide, (b) phosphorus pentachloride, and (c) dinitrogen trioxide.

PRACTICE EXERCISE

Give the chemical formula for (a) silicon tetrabromide; (b) disulfur dichloride. Answers: (a) SiBr4; (b) S2Cl2