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( Polyatomic ion)
A polyatomic ion is a charged species (ion) composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix poly- means many in Greek, but even ions of two atoms are commonly referred to as polyatomic. Note, a polyatomic ion is also referred to in older works as a radical. In current usage the term radical refers to free radicals which are uncharged species with an unpaired electron. A polyatomic ion can often be considered as the conjugate acid or conjugate base of a neutral molecule. For example the sulfate anion, SO42-, is derived from H2SO4 which can be regarded as SO3 + H2O. There are two "rules" that can be used for the learning the nomenclature of polyatomic ions. First, when the prefix bi- is added to a name, a hydrogen is added to the ion's formula and its charge is increased by 1. It is a consequence of the hydrogen ion carrying a +1 charge. An alternate to the bi- prefix is to use the word hydrogen in its place the anion derived from H+ + CO32-, HCO3- can be called either bicarbonate or hydrogen carbonate. Note that many of the common polyatomic anions are conjugate bases of acids derived from the oxides of non-metallic elements. For example the sulfate anion, SO42-, is derived from H2SO4 which can be regarded as SO3 + H2O.
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