(Adapted from Kotz & Treichel, 4th Ed., Chemistry and Chemical Reactivity, p. 115)
Also see, section 4.3 (page 166) in your Chemistry: The Science in Context book (Gilbert, Kirss & Davies)
The name of an ionic compound is built from the names of the positive and negative ions in the compound. The name of the positive ion is given first, followed by the name of the negative ion.
With a few exceptions (such as NH4+), the positive ions described in this text are metal ions. Positive ions are named by the following rules:
1. For a monatomic positive ion, that is, a metal cation, the name is that of the metal plus
the word “ion.” For example, we have already referred to Al3+ as the aluminum ion.
2. Some cases occur, especially in the transition series, in which a metal can form more
Than one type of positive ion . The most common practice is to indicate the charge of the ion by a Roman numeral in parentheses immediately following the ion’s name. For example, Co2+ is the cobalt(II) ion, and Co3+ is the cobalt(III) ion. [An older naming system for ions uses the ending “-ous” for the ion of lower charge and “-ic” for the ion of higher charge. For example, there are cobaltous (Co2+) and cobaltic (Co3+) ions, and ferrous (Fe2+) and ferric (Fe3+) ions. We do not use this system in this book, but some chemical manufacturers continue to use it.]
Finally, you will encounter the ammonium cation, NH4+, many times in this book, in the
laboratory, and in your environment. Do not confuse the ammonium ion with the neutral ammonia molecule NH3 .
Two types of negative ions must be considered: those having only one atom (monatomic) and those having several atoms (polyatomic).
1. A monatomic negative ion is named by adding –ide to the stem of the name of the nonmetal element from which the ion is derived (Figure 3.10). As a group, the anions of the Group 7A elements, the halogens, are called halide ions.
2. Polyatomic negative ions are quite common, especially those containing oxygen (called oxoanions). The names of some of the most common oxoanions are given in Table 3.1. Although most of these names must simply be learned, some guidelines can help. For example, consider the following pairs of ions:
NO3- is the nitrate ion, whereas NO2- is the nitrite ion
SO42- is the sulfate ion, whereas SO32- is the sulfite ion.
The oxoanion which the greater number of oxygen atoms is given the suffix –ate, and the oxoanion with the smaller number of oxygen atoms has the suffix –ite. For a series of oxoanions with more than two members, the ion with the largest number of oxygen atoms has the prefix per- and the suffix –ate. The ion with the smallest number of oxygen atoms had the prefix hypo- and the suffix –ite.
The oxoanions containing chlorine are good examples.
ClO4- perchlorate ion
ClO3- chlorate ion
ClO2- chlorite ion
ClO- hypochlorite ion
Oxoanions that contain hydrogen are named by adding the word “hydrogen” before the name of the oxoanion. If two hydrogens are in the compound, we say dihydrogen. Many of these hydrogen-containing oxoanions have common names that are so often used that you should know them too. For example, the hydrogen carbonate ion, HCO3-, is often called the bicarbonate ion.
HPO4- dihydrogen phosphate ion
HCO3- hydrogen carbonate ion bicarbonate ion
HSO4- hydrogen sulfate ion bisulfate ion
HSO3- hydrogen sulfite ion bisulfite ion
When naming ionic compounds, the positive ion name is given first, followed by the name of the negative ion. Some examples are given in the table shown here, and others are shown in Figure 3.11.
CBr2 Ca2+ and 2 Br- calcium bromide
NaHSO4 Na+ and HSO4- sodium hydrogen sulfate
(NH4)2CO3 2 NH4+ and CO32- ammonium carbonate
Mg(OH)2 Mg2+ and 2 OH- magnesium hydroxide
TiCl2 Ti2+ and 2 Cl- titanium(II) chloride
Co2O3 2 Co3+ and 2 O2- cobalt(III) oxide
Thus far we have described naming ions and ionic compounds. Another kind of compound comes from the combination of two nonmetals and is composed of molecules. These “two-element” or binary, compounds or nonmetals, can also be named in a systematic way.
Hydrogen forms binary compounds with all the nonmetals (except the noble gases). For compounds of oxygen, sulfur, and the halogens, the H atom is generally written first in the formula and is named first. The other nonmetal is named as if it were a negative ion.
Compound Name
HF hydrogen fluoride
HCl hydrogen chloride
H2S hydrogen sulfide
Virtually all binary, nonmetal compounds are based on a combination of elements from Groups 4A-7A with one another or with hydrogen. The formula is generally written by putting the elements in order of increasing group number. When naming the compound, the number of atoms of a given type in the compound is designated with a prefix, such as “di,” “tri,” “tetra,” “penta,” and so on.
Compound Name
NF3 nitrogen trifluoride
NO nitrogen monoxide
NO2 nitrogen dioxide
N2O dinitrogen monoxide
N2O4 dinitrogen tetraoxide
PCl3 phosphorus trichloride
PCl5 phosphorus pentachloride
SF6 sulfur hexafluoride
S2F10 disulfur decafluoride
Finally, many of the binary compounds of nonmetals were discovered years ago and have names so common that they continue to be used. These names must simply be learned.
Compound Common Name
CH4 methane
C2H6 ethane
C3H8 propane
C4H10 butane
NH3 ammonia
N2H4 hydrazine
PH3 phosphine
NO nitric oxide
N2O nitrous oxide (“laughing gas”)
H2O water