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 and iodine.

Hydroxide Ions and Ammonium Ions

A Hydroxide ion is made of one oxygen ion and one hydrogen ion: its chemical formula is ()^-. It has a negative charge.
An Ammonium ion is made up of one nitrogen atom and four hydrogen atoms: its Chemical Formula is ()⁺. 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 ^2- 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.

Example:

carbonate becomes bicarbonate or hydrogen carbonate

^2- becomes ^-

The second rule looks at the number of oxygens in an ion. Consider the chlorate family:

perchlorate ^-
chlorate ^-
chlorite ^-
hypochlorite ^-
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 replacing the -ate 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 C_{2_{H_{3_O₂^-}}}
aluminate AlO₂^-, Al₂O₄^-2
amide NH₂^-
ammonium NH₄⁺
arsenate AsO₄^-3
arsenite AsO₃^-3
azide N₃^-
bicarbonate (hydrogen carbonate) HCO₃^-
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 O_{2_⁺}
disulfide S₂^-2
ferricyanide Fe(CN)_{6_^-3}
ferrocyanide Fe(CN)_{6_^-4}
formate (formiate) HCO₂^-
fulminate CNO^-
hydrogen arsenate HAsO₄^-2
hydrogen carbonate (bicarbonate) HCO₃^-
hydrazide N₂H₃^-
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
iodate IO₃^-
iodite IO₂^-
isocyanate NCO^-
mercury(I) Hg₂⁺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
phosphate PO₄^-3
phosphite PO₃^-3
pyridinium C₅H₅NH⁺
pyrophosphate P₂O₇^-4
quaternary ammonium NR₄⁺
selenate SeO₄^-2
silicate Si_xO_y
sulfate SO₄^-2
sulfite SO₃^-2
sulfonium R₃S⁺
superoxide O₂^-
tartrate (CH(OH)COO)₂^-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) VO_{2_⁺}

Common polyatomic ions
acetate	CH₃COO^- or C_{2_{H_{3_O₂^-}}}
aluminate	AlO₂^-, Al₂O₄^-2
amide	NH₂^-
ammonium	NH₄⁺
arsenate	AsO₄^-3
arsenite	AsO₃^-3
azide	N₃^-
bicarbonate (hydrogen carbonate)	HCO₃^-
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	O_{2_⁺}
disulfide	S₂^-2
ferricyanide	Fe(CN)_{6_^-3}
ferrocyanide	Fe(CN)_{6_^-4}
formate (formiate)	HCO₂^-
fulminate	CNO^-
hydrogen arsenate	HAsO₄^-2
hydrogen carbonate (bicarbonate)	HCO₃^-
hydrazide	N₂H₃^-
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
iodate	IO₃^-
iodite	IO₂^-
isocyanate	NCO^-
mercury(I)	Hg₂⁺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
phosphate	PO₄^-3
phosphite	PO₃^-3
pyridinium	C₅H₅NH⁺
pyrophosphate	P₂O₇^-4
quaternary ammonium	NR₄⁺
selenate	SeO₄^-2
silicate	Si_xO_y
sulfate	SO₄^-2
sulfite	SO₃^-2
sulfonium	R₃S⁺
superoxide	O₂^-
tartrate	(CH(OH)COO)₂^-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)	VO_{2_⁺}

External links

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Polyatomic ion

Common polyatomic ions

See also

External links