4.9: Polyatomic Ions

Learning Objectives

• Recognize polyatomic ions in chemical formulas.
• Write the correct formula for ionic compounds with polyatomic ions.
• Use the rules for naming ionic compounds with polyatomic ions.

Polyatomic Ions

Some ions consist of groups of atoms bonded together and have an overall electric charge. Because these ions contain more than one atom, they are called polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For example, NO3 is the nitrate ion; it has one nitrogen atom and three oxygen atoms and an overall 1− charge. Table $$\PageIndex{1}$$ lists the most common polyatomic ions.

Table $$\PageIndex{1}$$ Some Polyatomic Ions
Name Formula
ammonium ion NH4+
acetate ion C2H3O2 (also written CH3CO2)
carbonate ion CO32−
chromate ion CrO42−
dichromate ion Cr2O72−
hydrogen carbonate ion (bicarbonate ion) HCO3
cyanide ion CN
hydroxide ion OH
nitrate ion NO3
nitrite ion NO2
permanganate ion MnO4
phosphate ion PO43−
hydrogen phosphate ion HPO42−
dihydrogen phosphate ion H2PO4
sulfate ion SO42−
hydrogen sulfate ion (bisulfate ion) HSO4
sulfite ion SO32−

The rule for constructing formulas for ionic compounds containing polyatomic ions is the same as for formulas containing monatomic (single-atom) ions: the positive and negative charges must balance. If more than one of a particular polyatomic ion is needed to balance the charge, the entire formula for the polyatomic ion must be enclosed in parentheses, and the numerical subscript is placed outside the parentheses. This is to show that the subscript applies to the entire polyatomic ion. An example is Ba(NO3)2.

Writing Formulas for Ionic Compounds Containing Polyatomic Ions

Writing a formula for ionic compounds containing polyatomic ions also involves the same steps as for a binary ionic compound. Write the symbol and charge of the cation followed by the symbol and charge of the anion.

Example $$\PageIndex{1}$$: Calcium Nitrate

Write the formula for calcium nitrate.

Solution

Criss Cross Method Write the formula for calcium nitrate
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second. $$\ce{Ca^{2+}} \: \: \: \: \: \ce{NO_3^-}$$
2. Transpose only the number of the positive charge to become the subscript of the anion and the number only of the negative charge to become the subscript of the cation.
3. Reduce to the lowest ratio.

$$\ce{Ca_1(NO_3)_2}$$

4. Write the final formula. Leave out all subscripts that are 1. If there is only 1 of the polyatomic ion, leave off parentheses. $$\ce{Ca(NO_3)_2}$$

Example $$\PageIndex{2}$$

Write the chemical formula for an ionic compound composed of the potassium ion and the sulfate ion

Solution

Potassium ions have a charge of 1+, while sulfate ions have a charge of 2−. We will need two potassium ions to balance the charge on the sulfate ion, so the proper chemical formula is K2SO4. $$K_2SO_4$$

Exercise $$\PageIndex{1}$$

Write the chemical formula for an ionic compound composed of each pair of ions.

1. the magnesium ion and the carbonate ion
2. the aluminum ion and the acetate ion
MgCO3
Al(CH3COO)3

Naming Ionic Compounds with Polyatomic Ions

The process of naming ionic compounds with polyatomic ions is the same as naming binary ionic compounds. The cation is named first, followed by the anion. One example is the ammonium Sulfate compound in Figure $$\PageIndex{6}$$.

Example $$\PageIndex{3}$$: Naming Ionic Compounds

Write the proper name for each ionic compound.

1. (NH4)2S
2. AlPO4,
3. Fe3(PO4)2

Solution

a. The ammonium ion has a 1+ charge and the sulfide ion has a 2− charge.

Two ammonium ions need to balance the charge on a single sulfide ion.

The compound’s name is ammonium sulfide.

ammonium sulfide

b. The ions have the same magnitude of charge, one of each (ion) is needed to balance the charges.

The name of the compound is aluminum phosphate.

aluminum phosphate

c. Neither charge is an exact multiple of the other, so we have to go to the least common multiple of 6.

To get 6+, three iron(II) ions are needed, and to get 6−, two phosphate ions are needed .

The compound’s name is iron(II) phosphate.

iron(II) phosphate

Exercise $$\PageIndex{2}$$

Write the proper name for each ionic compound.

1. (NH4)3PO4
2. Co(NO2)3
ammonium phosphate
cobalt(III) nitrite

Summary

• Formulas for ionic compounds contain the symbols and number of each atom and/or polyatomic ion present in a compound in the lowest whole number ratio.
• The process of naming ionic compounds with polyatomic ions are the same as binary ionic compounds.