By Matthew Perdue – Updated Mar 24, 2022

A polyatomic ion is a charged group of at least two covalently bonded atoms. While most carry a negative charge, some, like ammonium, are positively charged cations. These ions form the backbone of many familiar inorganic compounds.

Sodium Hydroxide (NaOH)

NaOH consists of a sodium cation (Na⁺) and a hydroxide anion (OH^-). The hydroxide ion, with a single extra electron, donates that electron to the sodium ion, creating a neutral ionic compound that is widely used as a strong base.

Calcium Carbonate (CaCO₃)

CaCO₃ is a key component of limestone, shells, and dental enamel. It features a calcium cation (Ca²⁺) bonded to a carbonate anion (CO₃^2-), where the carbonate ion’s two extra electrons balance the calcium’s two positive charges.

Acids Containing Polyatomic Ions

Many acids—such as phosphoric (H₃PO₄), nitric (HNO₃), and sulfuric (H₂SO₄)—are built around polyatomic ions (PO₄^3-, NO₃^-, SO₄^2-). In aqueous solution these acids dissociate to release H⁺ ions, the concentration of which determines the pH.

Ammonium (NH₄⁺)

Ammonium is the most common positively charged polyatomic ion. Formed from a nitrogen atom bonded to four hydrogen atoms, it carries a +1 charge and pairs readily with anions like nitrate to form compounds such as ammonium nitrate (NH₄NO₃).

Understanding the behavior of polyatomic ions provides insight into the structure and reactivity of countless chemical substances encountered daily.