Common Polyatomic Anions: Names, Formulas, and Charges

Common Polyatomic Anions

Introduction

Polyatomic anions are ions composed of two or more atoms covalently bonded, carrying an overall negative charge. Recognizing their names, formulas, and charges is essential for understanding chemical nomenclature and predicting compound formulas in General Chemistry.

List of Common Polyatomic Anions

The following table summarizes the most frequently encountered polyatomic anions, their chemical formulas, and their systematic names. Mastery of these ions is crucial for naming ionic compounds and writing chemical equations.

Anion	Name
N3-	Azide
NO2-	Nitrite
NO3-	Nitrate
PO33-	Phosphite
PO43-	Phosphate
HPO32-	Hydrogen Phosphite
H2PO3-	Dihydrogen Phosphite
HPO42-	Hydrogen Phosphate
H2PO4-	Dihydrogen Phosphate
AsO43-	Arsenate
HS-	Hydrogen Sulfide
OH-	Hydroxide
O22-	Peroxide
CN-	Cyanide
C2H3O2-	Acetate (also written as CH3COO-)
C2O42-	Oxalate
HC2O4-	Hydrogen Oxalate
CO32-	Carbonate
HCO3-	Hydrogen Carbonate (Bicarbonate)
SCN-	Thiocyanate
SO32-	Sulfite
HSO3-	Hydrogen Sulfite (Bisulfite)
SO42-	Sulfate
HSO4-	Hydrogen Sulfate (Bisulfate)
S2O32-	Thiosulfate
ClO-	Hypochlorite
ClO2-	Chlorite
ClO3-	Chlorate
ClO4-	Perchlorate
BrO-	Hypobromite
BrO2-	Bromite
BrO3-	Bromate
BrO4-	Perybromate
IO-	Hypoiodite
IO2-	Iodite
IO3-	Iodate
IO4-	Periodate

Key Points and Nomenclature Rules

• Polyatomic ions are ions composed of more than one atom, usually of different elements, that act as a single charged entity.

• The suffixes -ite and -ate indicate ions with fewer or more oxygen atoms, respectively (e.g., nitrite NO2- vs. nitrate NO3-).

• Prefixes such as hypo- (less oxygen) and per- (more oxygen) are used for series of oxyanions (e.g., hypochlorite ClO-, chlorite ClO2-, chlorate ClO3-, perchlorate ClO4-).

• When a hydrogen atom is added to a polyatomic ion, the prefix hydrogen- or dihydrogen- is used (e.g., hydrogen carbonate HCO3-).

• Some ions have common names in addition to their systematic names (e.g., bicarbonate for hydrogen carbonate).

Examples and Applications

• Example 1: The formula for sodium sulfate is Na2SO4, where SO42- is the sulfate ion.

• Example 2: The formula for ammonium nitrate is NH4NO3, where NO3- is the nitrate ion.

• Example 3: The formula for calcium hydrogen phosphate is CaHPO4, where HPO42- is the hydrogen phosphate ion.

Summary Table: Oxyanion Series (Chlorine Example)

The following table illustrates the naming pattern for oxyanions of chlorine, which is similar for bromine and iodine:

Formula	Name
ClO-	Hypochlorite
ClO2-	Chlorite
ClO3-	Chlorate
ClO4-	Perchlorate

Additional info:

• Learning the names, formulas, and charges of these ions is foundational for success in General Chemistry, especially for writing and balancing chemical equations, predicting products, and naming compounds.

• Some ions, such as acetate, may be written in more than one way (e.g., C2H3O2- or CH3COO-).