Page 103 - Theory and Problems of BEGINNING CHEMISTRY

92 INORGANIC NOMENCLATURE [CHAP. 6

Special Anions
There are a few anions that seem rather unusual but are often used in general chemistry. Perhaps the best
way to remember them is to memorize them, but some hints to help do that will be given below. The special
anions are given in Table 6-6.

Table 6-6 Special Anions

ate endings ide endings

2− −
CrO 4 Chromate CN Cyanide
2− −
Cr 2 O 7 Dichromate OH Hydroxide
− 2−
MnO 4 Permanganate O 2 Peroxide
−
C 2 H 3 O 2 Acetate

Note that chromate and permanganate have central atoms in periodic groups with the same numbers as those
of sulfate and perchlorate, respectively, and they have analogous formulas to these ions. Dichromate has two Cr
atoms and seven O atoms. The cyanide and hydroxide ions have already been discussed. The acetate ion is really
the ion of an organic acid, which is why it looks so unusual. The peroxide ion has two oxygen atoms and a total
charge of 2−. (As usual, per- means one more O.)

Putting the Names of the Ions Together
Now that we know how to name the cations and anions, we merely have to put the two names together to get
the names of ionic compounds. The cation is named first, and the anion is named next. The number of cations
and anions per formula unit are not included in the name of the compound because anions have characteristic
charges, and the charge on the cation has already been established by its name. Use as many cations and anions
as needed to get a neutral compound with the lowest possible integral subscripts.

EXAMPLE 6.12. Name (a)Ca(NO 3 ) 2 and Cr(ClO 3 ) 3 .
Ans. (a) The cation is the calcium ion. The anion is the nitrate ion. The compound is calcium nitrate. Note that we
do not state anything to indicate the presence of two nitrate anions; we can deduce that from the fact that the
calcium ion has a 2+ charge and nitrate has a 1− charge. (b) The cation is chromium(III). We know that it is
chromium(III) because its charge must balance three chlorate ion charges, each 1−. The compound is chromium(III)
chlorate.

Writing Formulas for Ionic Compounds
Formulas of many ionic compounds are easily written by consideration of the charges on their ions. To do
this, the charges on the ions must be memorized. The charges on most common ions are given here in Sec. 6.3.
There are both positive and negative ions. The formula of a compound is written so that the ions are combined in
ratios such that there is a net charge of 0 on the compound as a whole. For example, to write the formula for sodium
chloride, one sodium ion (having a charge of 1+) is combined with one chloride ion (having a charge of 1−);
hence, the formula is NaCl. The algebraic total of the charges is 0. To write the formula of aluminum chloride,
an aluminum ion (having a charge of 3+) is combined with three chloride ions; the resulting formula is AlCl 3 .
Again, the total algebraic sum of the charges is 0. The formula for ammonium sulfate includes two ammonium
ions (each with a 1+ charge) and one sulfate ion (with a charge of 2−), written as (NH 4 ) 2 SO 4 . As a further
example, the formula for aluminum oxide requires the combination of aluminum ions (with a charge 3+) with
oxygen ions (with a charge of 2−) in such a manner that the net charge is 0. The formula is Al 2 O 3 , corresponding
to 2 × (3+) for aluminum plus 3 × (2−) for oxygen, a total of 0.