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Writing a Metal + Nonmetal Formula: Gold(III) Chloride

1. Read the name of the compound.

gold(III) chloride

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

gold = Group IB = variable charge = 1+ or 3+

3. Write the first chemical symbol based on the cation in the compound, including the charge. The name tells us it is gold(III) meaning it carries a 3+ charge:

gold(III) = Au3+

4. Write the second chemical symbol based on the anion in the compound, including the charge.

chloride = chlorine = Cl-

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Experiment Naming Ionic and Molecular Compounds

5. Balance the charges to determine the correct number of atoms of each element to write the formula for a neutral compound.

6. Write the formula of the molecular compound.

gold(III) chloride = AuCl3

Polyatomic Ions

Polyatomic ions are a group of two or more covalently bonded atoms that function as a single ion. For example, oxygen carries a 2- charge and hydrogen carries a 1+ charge. When combined they form a covalent bond, producing a single anion with a 1- charge called hydroxide (OH-). The 1- charge of the hydroxide ion is a product of the 2- charge of the O with the 1+ charge of the H (-2 + 1 = -1). Polyatomic ions are the fundamental unit in the majority of ionic compounds; therefore, knowing polyatomic ion names, formulas, and charges is important. Common ions and their charges are listed in Table 2.

Table 2. Common polyatomic ions.

Name Formula Charge

Ammonium NH4 + 1+

Hydroxide OH- 1-

Cyanide CN- 1-

Nitrite** NO2 – 1-

Nitrate** NO3 – 1-

Sulfite** SO3 2- 2-

Sulfate** SO4 2- 2-

Hydrogen sulfite** HSO3 – 1-

Hydrogen sulfate** HSO4 – 1-

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