Polyatomic Ions: Names, Formulas, and Ionic Compounds

Polyatomic Ions

Definition and General Properties

Polyatomic ions are essential components in many chemical compounds, especially in GOB Chemistry. They are groups of covalently bonded atoms that carry an overall ionic charge, allowing them to participate in ionic bonding with other ions.

• Polyatomic ions are a group of atoms with an overall charge.

• They often consist of a nonmetal (such as phosphorus, sulfur, carbon, or nitrogen) bonded to oxygen.

• Most polyatomic ions have a negative charge (anions), except for ammonium (NH4+), which is a cation.

• Common charges are 1−, 2−, or 3−.

Example: Sodium chlorite (NaClO2) is used in the processing and bleaching of pulp from wood fibers and recycled cardboard.

Common Polyatomic Ions in Everyday Products

Polyatomic ions are found in many household and industrial products, such as fertilizers and medical materials.

• SO42− (sulfate ion) in plaster casts

• NH4+ (ammonium ion) in fertilizers

• NO3− (nitrate ion) in fertilizers

Names and Formulas of Common Polyatomic Ions

Table: Common Polyatomic Ions

The following table summarizes the names, formulas, and nonmetal components of several important polyatomic ions.

Nonmetal	Formula of Ion	Name of Ion
Hydrogen	OH−	Hydroxide
Nitrogen	NH4+	Ammonium
Nitrogen	NO3−	Nitrate
Nitrogen	NO2−	Nitrite
Chlorine	ClO4−	Perchlorate
Chlorine	ClO3−	Chlorate
Chlorine	ClO2−	Chlorite
Chlorine	ClO−	Hypochlorite

Nonmetal	Formula of Ion	Name of Ion
Carbon	CO32−	Carbonate
Carbon	HCO3−	Hydrogen carbonate (bicarbonate)
Carbon	CN−	Cyanide
Carbon	C2H3O2−	Acetate
Sulfur	SO42−	Sulfate
Sulfur	HSO4−	Hydrogen sulfate (bisulfate)
Sulfur	SO32−	Sulfite
Sulfur	HSO3−	Hydrogen sulfite (bisulfite)
Phosphorus	PO43−	Phosphate
Phosphorus	HPO42−	Hydrogen phosphate
Phosphorus	H2PO4−	Dihydrogen phosphate
Phosphorus	PO33−	Phosphite

Naming Polyatomic Ions

General Naming Rules

The names of polyatomic ions follow specific patterns based on their composition and number of oxygen atoms.

• Most common polyatomic ions end in -ate (e.g., SO42− sulfate, PO43− phosphate, NO3− nitrate).

• If a related ion has one less oxygen atom, its name ends in -ite (e.g., SO32− sulfite, PO33− phosphite, NO2− nitrite).

Exceptions and Variations

• Some ions do not follow the -ate/-ite pattern, such as CN− (cyanide) and OH− (hydroxide).

• When a hydrogen ion (H+) is added to a polyatomic ion, the name changes and the charge increases by +1:

  • $ext{CO}_3^{2-} + ext{H}^+ = ext{HCO}_3^-$ (bicarbonate or hydrogen carbonate)

  • $ext{SO}_4^{2-} + ext{H}^+ = ext{HSO}_4^-$ (bisulfate or hydrogen sulfate)

Halogen Polyatomic Ions

Halogens (such as chlorine) form four polyatomic ions with oxygen, each with a -1 charge:

• ClO4−: perchlorate

• ClO3−: chlorate

• ClO2−: chlorite

• ClO−: hypochlorite

Writing Formulas for Compounds Containing Polyatomic Ions

Charge Balance and Formula Construction

When writing formulas for ionic compounds containing polyatomic ions, the same rules of charge balance apply as for simple ionic compounds. Parentheses are used around the polyatomic ion when more than one is needed.

• Identify the cation and anion.

• Balance the charges so the total positive and negative charges equal zero.

• Write the formula, placing the cation first and using subscripts to indicate the number of each ion.

Example: Magnesium nitrate

• Magnesium ion: Mg2+

• Nitrate ion: NO3−

• Charge balance: $2(+1) + 1(+2) = 0$

• Formula: Mg(NO3)2

Step-by-Step Guide to Writing Formulas

1. Identify the cation and anion.

2. Balance the charges.

3. Write the formula, cation first, using subscripts from the charge balance.

Example: Aluminum Bicarbonate

• Cation: Al3+

• Polyatomic ion: HCO3−

• Charge balance: $1(3+) + 3(1-) = 0$

• Formula: Al(HCO3)3

Example: Sodium Phosphate

• Cation: Na+

• Polyatomic ion: PO43−

• Charge balance: $3(1+) + 1(3-) = 0$

• Formula: Na3PO4

Naming Compounds with Polyatomic Ions

General Naming Rules

When naming ionic compounds containing polyatomic ions:

• Write the name of the cation (usually a metal) first.

• Write the name of the polyatomic ion second.

• No prefixes are used in the names of ionic compounds.

• If the cation can have more than one charge (transition metals), indicate its charge with a Roman numeral in parentheses.

Examples:

• Na2SO4: sodium sulfate

• FePO4: iron(III) phosphate

• Al2(CO3)3: aluminum carbonate

Step-by-Step Guide to Naming Compounds

1. Identify the cation and polyatomic ion (anion).

2. Name the cation, using a Roman numeral if needed.

3. Name the polyatomic ion.

4. Write the name for the compound: cation first, polyatomic ion second.

Summary Table: Polyatomic Ion Naming Patterns

Suffix	Oxygen Content	Example
-ate	More oxygen	SO42− (sulfate)
-ite	Less oxygen	SO32− (sulfite)
hydrogen/bis-	With H+ added	HSO4− (hydrogen sulfate)
Other	Special cases	CN− (cyanide), OH− (hydroxide)

Practice Problems

Write the formula for the following compounds:

• Aluminum nitrate: Al(NO3)3

• Copper(I) nitrate: CuNO3

• Iron(III) hydroxide: Fe(OH)3

• Tin(IV) hydroxide: Sn(OH)4

Name the following compounds:

• Ca(NO3)2: calcium nitrate

• FePO4: iron(III) phosphate

• MgSO3: magnesium sulfite

• Pb(PO3)2: lead(II) phosphite

Additional info: These notes expand on the original slides and text, providing full explanations, tables, and stepwise guides for GOB Chemistry students.