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IONIC COMPOUND NAMES The metal hydroxides are named by identifying the
Ionic compounds formed by representative metal ions are metal first and the term hydroxide second. Thus, NaOH is
named by stating first the name of the metal (positive ion), named sodium hydroxide and KOH is potassium hydroxide.
then the name of the nonmetal (negative ion). Ionic com- The salts are similarly named, with the metal (or ammonium
pounds formed by variable-charge ions of the transition ion) identified first, then the name of the polyatomic ion.
elements have an additional rule to identify which variable- Thus, NaNO 3 is named sodium nitrate and NaNO 2 is sodium
charge ion is involved. There was an old way of identifying nitrite. Note that the suffix -ate means the polyatomic ion
the charge on the ion by adding either -ic or -ous to the name with one more oxygen atom than the -ite ion. For example,
–
–
of the metal. The suffix -ic meant the higher of two possible the chlorate ion is (ClO 3 ) and the chlorite ion is (ClO 2 ) .
charges, and the suffix -ous meant the lower of two possible Sometimes more than two possibilities exist, and more oxy-
charges. For example, iron has two possible charges, 2+ or 3+. gen atoms are identified with the prefix per- and fewer with
–
The old system used the Latin name for the root. The Latin the prefix hypo-. Thus, the perchlorate ion is (ClO 4 ) and the
–
name for iron is ferrum, so a higher charged iron ion (3+) was hypochlorite ion is (ClO) .
named a ferric ion. The lower charged iron ion (2+) was called
a ferrous ion.
You still hear the old names sometimes, but chemists now IONIC COMPOUND FORMULAS
have a better way to identify the variable-charge ion. The newer The formulas for ionic compounds are easy to write. There are
system uses the English name of the metal with Roman numer- two rules:
als in parentheses to indicate the charge number. Thus, an iron

ion with a charge of 2+ is called an iron(II) ion, and an iron ion 1. The symbols: Write first the symbol for the positive
with a charge of 3+ is an iron(III) ion. Table 9.6 gives some of element, followed by the symbol for the negative element
the modern names for variable-charge ions. These names are (same order as in the name).

used with the name of a nonmetal ending in -ide, just like the 2. The subscripts: Add subscripts to indicate the numbers of
single-charge ions in ionic compounds made up of two different ions needed to produce an electrically neutral compound.
elements. As an example, let us write the formula for the compound
Some ionic compounds contain three or more elements, calcium chloride. The name tells you that this compound con-
and so they are more complex than a combination of a metal sists of positive calcium ions and negative chlorine ions. The
ion and a nonmetal ion. This is possible because they have suffix -ide tells you there are only two elements present. Follow-
polyatomic ions, groups of two or more atoms that are bound ing rule 1, the symbols would be CaCl.
2+
together tightly and behave very much as a single monatomic For rule 2, note the calcium ion is Ca , and the chlorine ion
–
–
ion. For example, the OH ion is an oxygen atom bound to a is Cl . You know the calcium is +2 and chlorine is –1 by apply-
hydrogen atom with a net charge of 1–. This polyatomic ion is ing the atomic theory, knowing their positions in the periodic
called a hydroxide ion. The hydroxide compounds make up one table, or by using a table of ions and their charges. To be elec-
of the main groups of ionic compounds, the metal hydroxides. trically neutral, the compound must have an equal number of
A metal hydroxide is an ionic compound consisting of a metal pluses and minuses. Thus, you will need two negative chlorine
with the hydroxide ion. Another main group consists of the salts ions for every calcium ion with its 2+ charge. Therefore, the
with polyatomic ions. formula is CaCl 2 . The total charge of two chlorines is thus 2–,
which balances the 2+ charge on the calcium ion.
One easy way to write a formula showing that a compound
is electrically neutral is to cross over the absolute charge numbers
TABLE 9.6
(without plus or minus signs) and use them as subscripts. For ex-
Modern names of some variable-charge ions ample, the symbols for the calcium ion and the chlorine ion are
2+
Ion Name of Ion Ca Cl 1–
Fe 2+ Iron(II) ion Crossing the absolute numbers as subscripts as follows
Fe 3+ Iron(III) ion
Cu + Copper(I) ion
Cu 2+ Copper(II) ion Ca 1 2 Cl 2 1
Pb 2+ Lead(II) ion
Pb 4+ Lead(IV) ion
Sn 2+ Tin(II) ion and then dropping the charge numbers give
Sn 4+ Tin(IV) ion
Ca 1 Cl 2
Cr 2+ Chromium(II) ion
No subscript is written for 1; it is understood. The formula for
3+
Cr Chromium(III) ion
calcium chloride is thus
Cr 6+ Chromium(VI) ion
CaCl 2
9-13 CHAPTER 9 Chemical Bonds 241