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 electrical charge, allowing them to participate in ionic bonding.

• 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 with oxygen.

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

• Common charges are 1-, 2-, or 3-.

Examples and Applications

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

• Polyatomic ions are found in everyday products such as fertilizers (NH4NO3) and medical materials (plaster casts containing CaSO4).

Common Polyatomic Ions: Names and Formulas

Table of Common Polyatomic Ions

The following tables summarize the names, formulas, and charges of frequently encountered 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 Rules

The names of polyatomic ions follow systematic patterns based on their composition, especially the number of oxygen atoms present.

• Most common polyatomic ions end in -ate:

  • SO42-: sulfate

  • PO43-: phosphate

  • NO3-: nitrate

• If a related ion has one less oxygen atom, its name ends in -ite:

  • SO32-: sulfite

  • PO33-: phosphite

  • NO2-: nitrite

• Exceptions include:

  • CN-: cyanide

  • OH-: hydroxide

• Adding a hydrogen ion (H+) to a polyatomic ion increases its charge by +1 and changes its name:

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

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

• Halogens 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

Formulas for ionic compounds containing polyatomic ions are written using the same charge balance rules 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 ions needed for charge balance.

Example: Magnesium Nitrate

• Magnesium ion: Mg2+

• Nitrate ion: NO3-

• Charge balance: $2(+1) + 2(-1) = 0$

• Formula: Mg(NO3)2

Example: Aluminum Bicarbonate

• Cation: Al3+

• Anion: HCO3-

• Charge balance: $1(3+) + 3(1-) = 0$

• Formula: Al(HCO3)3

Example: Sodium Phosphate

• Cation: Na+

• Anion: PO43-

• Charge balance: $3(1+) + 1(3-) = 0$

• Formula: Na3PO4

Step-by-Step Guide to Writing Formulas with Polyatomic Ions

1. Identify the cation and anion.

2. Balance the charges.

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

Summary Table: Polyatomic Ion Naming Patterns

Suffix	Oxygen Content	Example
-ate	More oxygen	SO42- (sulfate)
-ite	Less oxygen	SO32- (sulfite)
hydrogen- or bi-	Contains H	HCO3- (bicarbonate)
Exceptions	Special cases	OH- (hydroxide), CN- (cyanide)

Key Points for GOB Chemistry Students

• Recognize polyatomic ions in chemical formulas to name compounds correctly.

• Use parentheses when more than one polyatomic ion is needed in a formula.

• No prefixes are used in naming ionic compounds with polyatomic ions.

• Always balance charges to ensure the compound is electrically neutral.

Example Application

Sodium chlorite (NaClO2) is used in industrial processes such as bleaching paper and cardboard, demonstrating the practical importance of polyatomic ions in everyday chemistry.