Common Polyatomic Ions in General Chemistry

Polyatomic Ions

Introduction to Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, that together carry a net electrical charge. These ions are fundamental in general chemistry, especially in the study of ionic compounds, acid-base chemistry, and chemical nomenclature.

• Polyatomic ions can be either positively charged (cations) or negatively charged (anions), but most are anions.

• They often participate in the formation of salts and are commonly found in both inorganic and organic chemistry.

• Knowing the names, formulas, and charges of common polyatomic ions is essential for writing chemical formulas and equations.

Common Polyatomic Ions: Names, Formulas, and Charges

The following table summarizes some of the most frequently encountered polyatomic ions in general chemistry, including their names, chemical formulas, and charges.

Name	Formula	Name	Formula
Acetate	$\mathrm{C_2H_3O_2^-}$	Hypochlorite	$\mathrm{ClO^-}$
Carbonate	$\mathrm{CO_3^{2-}}$	Chlorite	$\mathrm{ClO_2^-}$
Hydrogen carbonate (bicarbonate)	$\mathrm{HCO_3^-}$	Chlorate	$\mathrm{ClO_3^-}$
Hydroxide	$\mathrm{OH^-}$	Perchlorate	$\mathrm{ClO_4^-}$
Nitrite	$\mathrm{NO_2^-}$	Permanganate	$\mathrm{MnO_4^-}$
Nitrate	$\mathrm{NO_3^-}$	Sulfite	$\mathrm{SO_3^{2-}}$
Chromate	$\mathrm{CrO_4^{2-}}$	Hydrogen sulfite (bisulfite)	$\mathrm{HSO_3^-}$
Dichromate	$\mathrm{Cr_2O_7^{2-}}$	Sulfate	$\mathrm{SO_4^{2-}}$
Phosphate	$\mathrm{PO_4^{3-}}$	Hydrogen sulfate (bisulfate)	$\mathrm{HSO_4^-}$
Hydrogen phosphate	$\mathrm{HPO_4^{2-}}$	Cyanide	$\mathrm{CN^-}$
Dihydrogen phosphate	$\mathrm{H_2PO_4^-}$	Peroxide	$\mathrm{O_2^{2-}}$
Ammonium	$\mathrm{NH_4^+}$

Key Features and Nomenclature of Polyatomic Ions

• Suffixes: The endings -ate and -ite indicate different numbers of oxygen atoms. For example, nitrate ($\mathrm{NO_3^-}$) has one more oxygen than nitrite ($\mathrm{NO_2^-}$).

• Prefixes: The prefixes per- and hypo- are used to indicate the highest and lowest number of oxygens, respectively, in a series (e.g., perchlorate $\mathrm{ClO_4^-}$ vs. hypochlorite $\mathrm{ClO^-}$).

• Hydrogen-containing ions: When a hydrogen is added to a polyatomic ion, the name often includes "hydrogen" or the prefix "bi-" (e.g., hydrogen carbonate or bicarbonate $\mathrm{HCO_3^-}$).

• Charge: The charge on the ion is indicated as a superscript (e.g., $\mathrm{SO_4^{2-}}$ for sulfate).

Examples and Applications

• Formation of Ionic Compounds: Polyatomic ions combine with cations or anions to form neutral compounds. For example, sodium sulfate is $\mathrm{Na_2SO_4}$, where two sodium ions ($\mathrm{Na^+}$) balance the charge of one sulfate ion ($\mathrm{SO_4^{2-}}$).

• Acid-Base Chemistry: Many polyatomic ions are conjugate bases of acids (e.g., $\mathrm{NO_3^-}$ is the conjugate base of nitric acid, $\mathrm{HNO_3}$).

• Redox Reactions: Ions like permanganate ($\mathrm{MnO_4^-}$) and dichromate ($\mathrm{Cr_2O_7^{2-}}$) are important oxidizing agents in redox chemistry.

Summary Table Purpose

The table above serves as a reference for the most common polyatomic ions encountered in general chemistry, aiding in the memorization and correct usage of these ions in chemical formulas and reactions.