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Experiment Naming Ionic and Molecular Compounds

 Note that permanganate, chromate, and dichromate each have a metal and a nonmetal.

Note: There are additional tables of polyatomic ions available online and in textbooks. This table includes only the most common ions encountered in general chemistry classes.

Here are a few helpful hints for naming polyatomic ions:

a. Suffixes: the name of the ion usually ends in “-ite” or “-ate.” A low oxidation state will have an ion ending in “-ite,” versus a higher oxidation state that ends in “-ate.” The oxidation state is dependent upon a calculation of the charges of the polyatomic ion. Figure 5 shows the calculations of the oxidation state for the sulfite ion (SO3

2-) and the sulfate ion (SO4

2-). A simple algebraic calculation is done to determine the oxidation state for each polyatomic ion by solving for “x” since we know that oxygen (O) carries a 2- charge:

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Experiment Naming Ionic and Molecular Compounds

Figure 5. Algebraic calculations for the oxidation state of the polyatomic ions sulfite and sulfate.

b. Prefixes: “hypo-” indicates the very lowest oxidation state and “per-” indicates the very highest oxidation state. Figure 6 shows the calculations of the oxidation state for the hypochlorite ion (ClO-) and the perchlorate ion (ClO4

-). A simple algebraic calculation is done to determine the oxidation state for each polyatomic ion by solving for “x” since we know that oxygen (O) carries a 2- charge:

Figure 6. Algebraic calculations for the oxidation state of the polyatomic ions hypochlorite and perchlorate.

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Experiment Naming Ionic and Molecular Compounds

c. Few polyatomic ions have positive charges. These ions have names ending in “-onium.” For example, ammonium (NH4

+) and hydronium (H3O +).

d. There are a few exceptions to these rules. The following polyatomic ions were once thought to be monatomic ions so they end in “-ide:” hydroxide (OH-), cyanide (CN-), and peroxide (O2

2-).

Naming Polyatomic Ion Compounds

Example 9: Writing a Metal + Polyatomic Ion Name: KNO2 1. Read the formula and look at the subscripts.

KNO2 2. Determine if the metal ion has a fixed charge or variable charge.

K = Group IA = fixed charge = 1+

3. Record the entire name of the cation.

K = potassium

4. Record the entire name of the anion.

NO2 = nitrite

5. Write the name of the ionic compound.

potassium nitrite

Example 10: Writing a Metal + Polyatomic Ion Formula: Gold(I) Nitrate