Polyatomic ion

A polyatomic ion is an electrically charged group of two or more covalently bonded atoms that behaves as a unit in chemical reactions.

Covalent bond: A chemical bond formed when atoms share electron pairs. Shared electrons hold the atoms in a polyatomic ion together.
Electron affinity: The energy change when a neutral atom gains an electron. Electron gain can contribute to the negative charge of an anion, though bonding also matters.
Ionic compound: A substance composed of cations and anions held together by electrostatic attraction. Many ionic compounds contain polyatomic ions alongside simpler monatomic ions.
Sulfate: The polyatomic anion SO₄²⁻, composed of one sulfur atom and four oxygen atoms. Sulfate illustrates a common divalent oxyanion found in minerals and dissolved salts.
Formal charge: An electron-bookkeeping value assigned to an atom in a molecule or ion. Formal charges help track how a polyatomic ion’s total charge is distributed among its atoms.
Acid–base reaction: A reaction involving transfer of a proton or, more broadly, an electron pair. Proton transfer often converts one polyatomic ion into another.
Chemical nomenclature: The systematic naming of chemical substances according to agreed conventions. Ion names such as nitrate and ammonium are retained when naming their compounds.
Nitrate: The polyatomic anion NO₃⁻, composed of one nitrogen atom and three oxygen atoms. Nitrate shows how resonance can distribute bonding across equivalent oxygen atoms.
Lewis structure: A diagram showing valence electrons and bonds in a molecule or ion. Lewis structures represent the bonding and electron arrangement within a polyatomic ion.
Conjugate acid–base pair: Two species that differ by one proton in a Brønsted–Lowry acid–base reaction. Polyatomic ions such as sulfate and bisulfate form conjugate pairs by gaining or losing a proton.

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