Oxyanion

An oxyanion is a negatively charged polyatomic ion containing oxygen bonded to one or more other elements. Its oxygen atoms are often bonded to a central atom.

Polyatomic ion: An ion composed of two or more covalently bonded atoms that carry a net charge. Oxyanions are the oxygen-containing members of this broader class.
VSEPR theory: A model that predicts molecular shape from repulsions between electron groups around a central atom. It predicts the geometry around many oxyanion central atoms.
Sulfate: The polyatomic ion SO₄²⁻, with sulfur bonded to four oxygen atoms. Sulfate is a widespread oxyanion in minerals, water, and biological systems.
Halide: A monatomic anion formed from a halogen element, such as chloride or bromide. Unlike oxyanions, halides contain no oxygen and consist of a single atom.
Formal charge: A bookkeeping charge assigned to an atom by assuming equal sharing of bonding electrons. Formal charges help track how negative charge is distributed across an oxyanion.
Molecular orbital theory: A theory describing electrons as occupying orbitals spread across molecules rather than confined to individual bonds. It gives a more complete account of delocalized bonding in oxyanions.
Nitrate: The polyatomic ion NO₃⁻, with nitrogen bonded to three oxygen atoms. Nitrate is an oxyanion central to fertilizer chemistry and the nitrogen cycle.
Hydroxide: The diatomic anion OH⁻, consisting of one oxygen atom bonded to hydrogen. Hydroxide contains oxygen but is not an oxyanion under the usual definition, which requires another element besides hydrogen.
Oxidation state: A number assigned to an atom representing its electron ownership under a set of chemical rules. Oxidation states help relate an oxyanion’s formula to its central atom’s bonding.
Brønsted–Lowry acid–base theory: A theory in which acids donate protons and bases accept them. Adding or removing protons interconverts many oxyanions and their conjugate acids.

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