Polyatomic ion
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A polyatomic ion is an ion consisting of a molecule with covalently bonded atoms 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. 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. Some can be used in decomposition reactions, providing spectacular results, such as H2, O2 and iodine.
Hydroxide ions and ammonium ions
- A hydroxide ion is made of one oxygen atom and one hydrogen atom: its chemical formula is (OH)−. It has a charge of −1.
- An ammonium ion is made up of one nitrogen atom and four hydrogen atoms: its chemical formula is (NH4)+. It has charge of +1.
A large polyatomic ion will often be considered as the conjugate acid or conjugate base of a neutral molecule, for example the conjugate acid of adrenaline at the amine group.
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.
There are two "rules" that can be used for making the learning of polyatomic ions somewhat easier. First, when the prefix bi- is added to a name, a hydrogen is added to the ion's formula and its charge is reduced by 1. This reduction 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:
| perchlorate | ClO4− |
| chlorate | ClO3− |
| chlorite | ClO2− |
| hypochlorite | ClO− |
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).
