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A polyatomic ion is a Molecule that bears Ion ic groups, that is, a molecule with a charge. The majority of biological compounds and inorganic species conform to this strict definition. Ordinarily however, the term refers to small collections of Atom s, 3 to perhaps 50 atoms, such as many Metal Complex es and Oxyanion s such as Sulfate . in Greek, the prefix poly- means "many," which to a chemist means three or more atoms. Hydroxide ions and ammonium ions
A polyatomic ion can often be considered as the Conjugate Acid or conjugate base of a neutral molecule. For example the Sulfate anion, 2−, 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 hydrogencarbonate. The second rule looks at the number of oxygens in an ion. Consider the Chlorine Oxoanion family: First, think of the -ate ion as being the "base" name, in which case the addition of a per- prefix adds an oxygen. Changing the -ate suffix to -ite will reduce the oxygens by one, and keeping the suffix -ite and adding the prefix hypo- reduces the number of oxygens by two. In all situations, the charge is not affected. It is important to note that these rules will not work with all polyatomic ions, but they do work with the most common ones (sulfate, phosphate, nitrate, chlorate). Polyatomic ions vs. radicals Although most polyatomic ions are Diamagnetic (all electrons are paired), some are Radical s. Some radicals are charged and as such are polyatomic ions, such as the radical anion of Naphthalene , C10H8-. Some can be used in decomposition reactions, providing spectacular results, such as , and iodine. LIST OF POLYATOMIC IONS Caution: chemists classify ions and molecules even when such species do not exist to any appreciable extent. For example, small ions with high charges are very rare, as illustrated by the fact that oxide, O2-, has not been observed in solution and is not considered as a component in Reaction Mechanism s. Similarly, Orthosilicate , SiO44- enjoys no status as an ion in aqueous solution, except perhaps under extreme temperatures. In general, ions that have charges greater than 2- do not exist in solution unless they are protonated. SEE ALSO EXTERNAL LINKS
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