Everything about Polyatomic Ion totally explained
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.
Hydroxide ions and ammonium ions
- A hydroxide ion is made of one oxygen atom and one hydrogen atom: its chemical formula is −. It has a charge of −1.
- An ammonium ion is made up of one nitrogen atom and four hydrogen atoms: its chemical formula is +. It has charge of +1.
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 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,
2−, is derived from
H2SO4 which can be regarded as
SO3 +
H2O.
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 isn't affected.
It is important to note that these rules won't work with all polyatomic ions, but they do work with the most common ones (sulfate, phosphate, nitrate, chlorate).
List of polyatomic ions
Caution: chemists classify ions and molecules even when such species don't exist to any appreciable extent. For example, small ions with high charges are very rare, as illustrated by the fact that oxide, O
2-, hasn't been observed in solution and isn't considered as a component in
reaction mechanisms. Similarly,
orthosilicate, SiO
44- enjoys no status as an ion in aqueous solution, except perhaps under extreme temperatures. In general, ions that have charges greater than 2- don't exist in solution unless they're protonated.
Further Information
Get more info on 'Polyatomic Ion'.
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