A polyatomic ion is a charged chemical species consisting of two or more atoms that are covalently bonded and carry an overall electrical charge. Unlike simple ions (monatomic ions), which consist of single atoms with a charge, polyatomic ions are formed when a group of atoms collectively gains or loses electrons, resulting in a net charge.

Polyatomic ions can be either positively charged (cationic) or negatively charged (anionic) depending on whether they lose or gain electrons. These ions often have characteristic names and formulas that are used in chemical reactions and nomenclature.

Here are a few examples of common polyatomic ions:

1. Cationic Polyatomic Ions:
   • Ammonium ion: NH4+
   • Hydronium ion: H3O+
2. Anionic Polyatomic Ions:
   • Nitrate ion: NO3-
   • Sulfate ion: SO4^2-
   • Carbonate ion: CO3^2-
   • Phosphate ion: PO4^3-
   • Acetate ion: C2H3O2^- (also written as CH3COO^-)

These ions play a significant role in chemical reactions, especially in the context of acids, bases, and salts. For example, when an acid dissociates in water, it often produces hydronium ions (H3O+) or other cations. When a base dissociates, it may produce hydroxide ions (OH-) or other anions. In addition, many salts are formed by the combination of cations and anions, including polyatomic ions.

Understanding polyatomic ions is essential for comprehending various chemical processes, as they contribute to the overall charge balance and reactivity of compounds.