Common Polyatomic Ions: Formulas and Names

Common Polyatomic Ions

Introduction

Polyatomic ions are charged species composed of two or more atoms covalently bonded, acting as a single unit in chemical reactions. Recognizing their formulas and names is essential for understanding chemical compounds, nomenclature, and reactions in General Chemistry.

List of Common Polyatomic Ions

The following table summarizes the formulas and names of frequently encountered polyatomic ions. Mastery of these ions is crucial for writing chemical formulas, balancing equations, and predicting compound properties.

Formula	Name	Formula	Name
C2H3O2-	Acetate	PO43-	Phosphate
CO32-	Carbonate	HPO42-	Hydrogen phosphate
HCO3-	Bicarbonate (Hydrogen carbonate)	ClO-	Hypochlorite
NO2-	Nitrite	ClO2-	Chlorite
NO3-	Nitrate	ClO3-	Chlorate
CrO42-	Chromate	ClO4-	Perchlorate
Cr2O72-	Dichromate	OH-	Hydroxide
MnO4-	Permanganate	SO32-	Sulfite
HSO3-	Hydrogen sulfite (Bisulfite)	SO42-	Sulfate
HSO4-	Hydrogen sulfate (Bisulfate)	CN-	Cyanide
NH4+	Ammonium

Key Points

• Polyatomic ions often retain their identity in chemical reactions and are found in many common compounds.

• Names ending in -ate or -ite indicate different numbers of oxygen atoms (e.g., nitrate vs. nitrite).

• Prefixes such as hypo- and per- further distinguish ions with fewer or more oxygen atoms (e.g., hypochlorite, perchlorate).

• Some ions contain hydrogen, which is reflected in their names (e.g., bicarbonate, hydrogen sulfate).

• Ammonium is a common polyatomic cation, while most others listed are anions.

Examples and Applications

• Sodium sulfate: $\text{Na}_2\text{SO}_4$ contains the sulfate ion $\text{SO}_4^{2-}$.

• Ammonium nitrate: $\text{NH}_4\text{NO}_3$ contains both ammonium $\text{NH}_4^+$ and nitrate $\text{NO}_3^-$ ions.

• Calcium carbonate: $\text{CaCO}_3$ contains the carbonate ion $\text{CO}_3^{2-}$.

Formulas and Charges

• Polyatomic ions have specific charges that must be balanced when forming compounds.

• For example, the formula for calcium phosphate is $\text{Ca}_3(\text{PO}_4)_2$ because calcium is $\text{Ca}^{2+}$ and phosphate is $\text{PO}_4^{3-}$.

Additional info:

• Learning the names and formulas of these ions is foundational for success in chemical nomenclature, predicting compound formulas, and understanding chemical reactions.

• Some ions have alternative names (e.g., bicarbonate is also called hydrogen carbonate).