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(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.

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 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:

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).

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

Common polyatomic ions
acetate CH3COO- or C2H3O2-
aluminate AlO2-, Al2O4-2
amide NH2-
ammonium NH4+
antimonate SbO4-3
antimonite SbO3-3
arsenate AsO4-3
arsenite AsO3-3
azide N3-
bicarbonate (hydrogen carbonate) HCO3-
borate BO3-3
metaborate BO2-
tetraborate B4O7-2
bromate BrO3-
bromite BrO2-
carbide C2-2
carbonate CO3-2
chlorate ClO3-
chlorite ClO2-
chromate CrO4-2
chromite CrO2-
chromyl CrO2+2
citrate C6H5O7-3
cyanate OCN-
cyanide CN-
dichromate Cr2O7-2
dihydrogen arsenate H2AsO4-
dihydrogen phosphate H2PO4-
dihydrogen phosphite H2PO3-
dioxygenyl O2+
disulfide S2-2
ferrate FeO4-2
ferricyanide Fe(CN)6-3
ferrocyanide Fe(CN)6-4
formate (formiate) HCO2-
fulminate CNO-
hydrazide N2H3-
hydrogen carbonate (bicarbonate) HCO3-
hydrogen arsenate HAsO4-2
hydrogen phosphate HPO4-2
hydrogen phosphite HPO3-2
hydrogen sulfate HSO4-
hydrogen sulfite HSO3-
hydronium H3O+
hydroxide OH-
hypobromite BrO-
hypochlorite ClO-
hypoiodite IO-
hypophosphite PO2-3
hyposulfite SO2-2
iodate IO3-
iodite IO2-
isocyanate NCO-
mercury(I) Hg2+2
manganate MnO4-2
molybdate MoO4-2
nitrate NO3-
nitrite NO2-
oxalate (COO)2-2
ozonide O3-
perbromate BrO4-
perchlorate ClO4-
periodate IO4-
permanganate MnO4-
peroxide O2-2
perrhenate ReO4-
peroxymonosulfate SO5-2
peroxydisulfate S2O8-2
pertechnetate TcO4-
phosphate PO4-3
phosphite PO3-3
plumbate PbO3-2
plumbite PbO2-2
pyridinium C5H5NH+
pyrophosphate P2O7-4
quaternary ammonium NR4+
selenate SeO4-2
selenite (ion) SeO3-2
silicate SixOy
disilicate Si2O5-2
metasilicate SiO3-2
orthosilicate SiO4-4
pyrosilicate Si2O7-6
stannate SnO3-2
stannite SnO2-2
sulfate SO4-2
sulfite SO3-2
sulfonium R3S+
superoxide O2-
tartrate (CH(OH)COO)2-2
metatellurate TeO4-2
orthotellurate TeO6-6
tellurite TeO3-2
thiocyanate SCN-
thiosulfate S2O3-2
tosylate (toluenesulfonate) CH3C6H4SO3-
triflate (trifluoromethanesulfonate) CF3SO3-
tungstate WO4-2
uranyl UO+2
vanadate VO3-
vanadyl(IV) VO+2
vanadyl(V) VO2+

See also

External links