Page 100 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 6] INORGANIC NOMENCLATURE 89

Naming Monatomic Cations
Naming of the positive ion depends on whether the cation is monatomic (has only one atom). If not, the
special names given in the next subsection are used. If the cation is monatomic, the name depends on whether
the element forms more than one positive ion in its compounds. For example, sodium forms only one positive
ion in all its compounds—Na . Cobalt forms two positive ions—Co 2+ and Co . Cations of elements that form
3+
+
only one type of ion in all their compounds need not be further identified in the name. Thus, Na may simply be
+
called the sodium ion. Cations of metals that occur with two or more different charges must be further identified.
Co(NO 3 ) 2 and Co(NO 3 ) 3 occur with Co 2+ and Co 3+ ions, respectively. If we just call the ion the “cobalt ion,”
we will not know which one it is. Therefore, for such cations, we use a Roman numeral in parentheses attached to
the name to tell the charge on such ions. (Actually, oxidation numbers are used for this purpose, but if you have
not yet studied oxidation numbers—Chap. 14—follow the rules given next.) Thus, Co 2+ is called the cobalt(II)
ion, and Co 3+ is called the cobalt(III) ion.
The elements that form only one cation are the alkali metals (group IA), the alkaline earth metals (group
IIA), zinc, cadmium, aluminum, and most often silver. The charge on the ions that these elements form in their
compounds is always equal to their classical periodic table group number (or group number minus 10 for Ag,
Cd, Zn, and Al in the modern labeling system in the periodic table).

EXAMPLE 6.7. Name NiCl 2 and MgCl 2 .
Ans. Since Ni is not among the elements that always form ions of the same charge in all their compounds, the charge
must be stated. The name is nickel(II) chloride. Since Mg is an alkaline earth element, the charge in its compounds
is always 2+, so there is no need to mention the charge in the name. The compound is magnesium chloride.

EXAMPLE 6.8. Write formulas for (a) copper(I) sulfide and (b) copper(II) sulfide.
Ans. (a)Cu 2 S and (b) CuS. Note carefully that the Roman numerals in the names mean one thing—the charge on the
ion—and the Arabic numeral subscripts in the formulas mean another—number of atoms. Here the copper(I) has
a charge of 1+, and therefore two copper(I) ions are required to balance the 2− charge on one sulfide ion. The
copper(II) ion has a charge of 2+, and therefore one such ion is sufficient to balance the 2− charge on the sulfide
ion.

EXAMPLE 6.9. Name NF 3 .
Ans. Nitrogen trifluoride. (Although this is a binary compound of two nonmetals, it can be named with Roman numeral
designations. It is indeed possible to call this nitrogen(III) fluoride in the most modern usage, but most chemists do
not do that yet.)

Polyatomic Cations
Several cations that consist of more than one atom are important in general chemistry. There are few enough
of these important ions to learn them individually. They are presented in Table 6-3. There are several ions like
uranyl ion, also of limited importance in general chemistry. More will be said about these ions in the sections on
oxidation states (Chap. 14).

Table 6-3 Several Polyatomic Cations

+ Ammonium ion Very important
NH 4
H 3 O + Hydronium ion Very important in Chap. 17
Hg 2+ Mercury(I) ion or mercurous ion Somewhat important
2
2+
UO 2 Uranyl ion Not too important