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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(g), O2(g) and I2(s).
Hydroxide Ions and Ammonium Ions
- A Hydroxide ion is made of one oxygen ion and one hydrogen ion: its chemical formula is (OH)-. It has a negative charge.
- An Ammonium ion is made up of one nitrogen atom and four hydrogen atoms: its Chemical Formula is (NH4)+. It has a positive charge.
Note that many of the common negatively-charged anionic polyatomic ions are conjugate bases of acids derived from the oxides of non-metallic elements. For example the sulfate or SO42- ion 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 reduced by 1. This reduction is a function of the hydrogen ion being a +1 charge. An alternate to the bi- prefix is to use the word hydrogen in its place.
carbonate becomes bicarbonate or hydrogen carbonate
CO32- becomes HCO3-
The second rule looks at the number of oxygens in an ion. Consider the chlorate 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).
The most common type of file to view ion representations is a PDB which refers to Protein Databank. There are also now other kinds becoming available including CIF.
Common polyatomic ions
- List of polyatomic ions
- A Beginner's Guide To Polyatomic Ions.
- Tables of Common Polyatomic Ions. Also a source for PDB files (PDB information)