Barrons SAT Subject Test Chemistry, 13th Edition

You should memorize all the polyatomic ions in Table 6 to help you use them in formulas and

equations.

Polyatomic Ions Containing the –ite and –ate Forms of Sulfur

Sometimes an element combines with oxygen to form more than just two

polyatomic ions, such as ClO−, ClO 2 −, ClO 3 −, and ClO 4 −. When this occurs, the

prefix hypo- is used to name the polyatomic ion with the fewest oxygen ions and
the prefix per- to name the polyatomic ion with the most oxygen ions.

Polyatomic Ions Containing Chlorine and Oxygen

Ionic Formula

Name of the Ion

ClO −

Hypochlorite

ClO 2 −

Chlorite

ClO 3 −

Chlorate

ClO 4 −

Perchlorate

Writing Formulas for Compounds with Polyatomic Ions

When writing formulas using polyatomic anions, the rules do not change. Simply
treat the polyatomic ion as if it were a single anion. If the cation is from category
I, follow the rules for category I. If the cation is from category II, follow the rules
for category II. The crisscross method does not change, either.

Example 1

Use the crisscross method to write the formula for calcium sulfate, a category I
cation and a polyatomic ion.