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Answer: A polyatomic ion, also known as a molecular ion, is a covalently bound set of two or more atoms, or a metal complex, that behaves as a single unit and has a net charge greater than zero. This chemical species is an ion, as opposed to a molecule, which has a net charge of zero. Although the prefix poly- means “many” in Greek, even ions with two atoms are frequently referred to as polyatomic.
A radical is a term used in older literature to refer to a polyatomic ion (or less commonly, as a radical group). The term “radical” now refers to a variety of free radicals, which are entities with an unpaired electron that do not need to be charged.
The hydroxide ion is a simple example of a polyatomic ion, consisting of one oxygen atom and one hydrogen atom with a net charge of -1; its chemical formula is OH. An ammonium ion, on the other hand, is made up of one nitrogen atom and four hydrogen atoms, with a charge of +1 and the chemical formula NH4+.
In acid–base chemistry and the production of salts, polyatomic ions are frequently used.
A polyatomic ion is frequently thought of as the conjugate acid or base of a neutral molecule. The polyatomic hydrogen sulphate anion (HSO4–) is the conjugate base of sulfuric acid (H2SO4), for example. The sulphate anion (SO42-) is formed when another hydrogen ion is removed.
Nomenclature of polyatomic anions:
For understanding the nomenclature of polyatomic anions, there are two “rules” to follow. When the prefix “bi” is added to a name, it adds a hydrogen to the ion’s formula and increases its charge by one, the latter being due to the +1 charge of the hydrogen ion. The anion generated from hydrogen and carbonate ion can be named either bicarbonate or hydrogen carbonate as an alternative to the bi- prefix.
