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What Is an Anion?
Anion Definition and Examples
Chemistry Basics: What Is an Anion?
by Anne Marie Helmenstine, Ph.D.
An anion is an ionic species
having a negative charge.
The chemical species may be a
single atom or a group of atoms.
An anion is attracted to the
anode in electrolysis.
Anions are typically larger
than cations (positively charged ions) because they have extra electrons around
them.
The word anion [an-ahy-uh n] was
proposed by English polymath Rev. William Whewell in 1834, from
Greek anion "thing going up",
referring to the movement of anions during electrolysis.
Physicist Michael Faraday was
the first person to use the term anion in a publication.
Anion Examples
· Free chloride in an aqueous table
salt (NaCl) solution: Cl-
· Singlet oxygen: O-
· Superoxide: O2-
· Hydroxide ion: OH-
· Sulfate: SO42-
· Al(OH)4-
Anion Notation
When naming a chemical compound,
the cation is given first, followed by the anion. For example, the compound
sodium chloride is written NaCl, where Na+ is the cation and Cl-is
the anion.
The net electrical charge of
an anion is denoted using a superscript after the chemical species symbol. For
example, the phosphate ion PO43- has
a charge of 3-.
Since many elements display a
range of valences, determining the anion and cation in a chemical formula isn't
always clearcut.
In general, the difference in
electronegativity may be used to identify the cation and anion in a formula.
The more electronegative species
in a chemical bond is the anion.
Table
of Common Anions
Simple
Anions
|
Formula
|
Hydride
|
H-
|
Oxide
|
O2-
|
Fluoride
|
F-
|
Sulfide
|
S2-
|
Chloride
|
Cl-
|
Nitride
|
N3-
|
Bromide
|
Br-
|
Iodide
|
I-
|
|
|
Oxoanions
|
Formula
|
Arsenate
|
AsO43-
|
Phosphate
|
PO43-
|
Arsenite
|
AsO33-
|
Hydrogen Phosphate
|
HPO42-
|
Dihydrogen Phosphate
|
H2PO4-
|
Sulfate
|
SO42-
|
Nitrate
|
NO3-
|
Hydrogen Sulfate
|
HSO4-
|
Nitrite
|
NO2-
|
Thiosulfate
|
S2O32-
|
Sulfite
|
SO32-
|
Perchlorate
|
ClO4-
|
Iodate
|
IO3-
|
Chlorate
|
ClO3-
|
Bromate
|
BrO3-
|
Chlorite
|
ClO2-
|
Hypochlorite
|
OCl-
|
Hypobromite
|
OBr-
|
Carbonate
|
CO32-
|
Chromate
|
CrO42-
|
Hydrogen Carbonate or Bicarbonate
|
HCO3-
|
Dichromate
|
Cr2O72-
|
|
|
Anions
from Organic Acids
|
Formula
|
Acetate
|
CH3COO-
|
Formate
|
HCOO-
|
|
|
Other
Anions
|
Formula
|
Cyanide
|
CN-
|
Amide
|
NH2-
|
Cyanate
|
OCN-
|
Peroxide
|
O22-
|
Thiocyanate
|
SCN-
|
Oxalate
|
C2O42-
|
Hydroxide
|
OH-
|
Permanganate
|
MnO4-
|
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