An iodide ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 is the ion I. Compounds with iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 in formal oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine deficiency
Iodine deficiency
Iodine is an essential trace element; the thyroid hormones thyroxine and triiodotyronine contain iodine. In areas where there is little iodine in the diet—typically remote inlandareas where no marine foods are eaten—iodine deficiency gives rise to...

 affects two billion people and is the leading preventable cause of mental retardation
Mental retardation
Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...


Structure and characteristics of inorganic iodides

Iodide is one of the largest (monoatomic) anions. It is assigned a radius of around 220 picometers. For comparison, the lighter halides are considerably smaller: bromide
A bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...

 (196 pm), chloride (181 pm), and fluoride (133 pm). In part because of its size, iodine forms relatively weak bonds with most elements.

Most iodide salts are soluble in water, but often less so than the related chlorides and bromides. Iodide, being large, is less hydrophilic than are the smaller anions. One consequence of this is that sodium iodide is highly soluble in acetone, whereas sodium chloride is not. The low solubility of silver iodide
Silver iodide
Silver iodide is a yellow, inorganic, photosensitive iodide of silver used in photography, in medicine as an antiseptic, and in rainmaking for cloud seeding.-Crystal structure:...

 and lead iodide reflects the covalent character of these metal iodides. A test for the presence of iodide ions is the formation of yellow precipitates of these compounds upon treatment of a solution of silver nitrate
Silver nitrate
Silver nitrate is an inorganic compound with chemical formula . This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides...

 or lead(II) nitrate
Lead(II) nitrate
Lead nitrate is an inorganic compound with the chemical formula Pb2. It commonly occurs as a colourless crystal or white powder and, unlike most other lead salts, is soluble in water....


Aqueous solutions of iodide salts dissolve iodine better than pure water. This effect is due to the formation of the triiodide
In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, ...

 ion, which is brown:
I + I2 I3

Redox, including antioxidant properties

Iodide salts are mild reducing agent
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...

s and many react with oxygen to give iodine. A reducing agent is a chemical term for an antioxidant. Its antioxidant properties can be expressed quantitatively as a redox potential:
I 1/2 I2 + e (electrons) = - 0.54 Volt vs SHE
Standard hydrogen electrode
The standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...

Because iodide is easily oxidized, some enzymes readily convert it into electrophilic iodinating agents, as required for the biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...

 of myriad iodide-containing natural product
Natural product
A natural product is a chemical compound or substance produced by a living organism - found in nature that usually has a pharmacological or biological activity for use in pharmaceutical drug discovery and drug design...

s. Iodide can function as an antioxidant reducing
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 species that can destroy reactive oxygen species
Reactive oxygen species
Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....

 such as hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

2 I + Peroxidase + H2O2 + tyrosine, histidine, lipid, etc. → iodo-Compounds + H2O + 2 e (antioxidants).

Representative iodides

Compound Formula Appearance Use or occurrence
potassium iodide
Potassium iodide
Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...

KI white crystals iodine component of iodized salt
hydrogen iodide
Hydrogen iodide
Hydrogen iodide is a diatomic molecule. Aqueous solutions of HI are known as iohydroic acid or hydroiodic acid, a strong acid. Gas and aqueous solution are interconvertible...

HI colourless solution strong mineral acid
Silver iodide
Silver iodide
Silver iodide is a yellow, inorganic, photosensitive iodide of silver used in photography, in medicine as an antiseptic, and in rainmaking for cloud seeding.-Crystal structure:...

AgI yellow powder that darkens in light photoactive component of silver-based photographic film
Thyroxine, or 3,5,3',5'-tetraiodothyronine , a form of thyroid hormones, is the major hormone secreted by the follicular cells of the thyroid gland.-Synthesis and regulation:...

C15H11I4NO4 pale yellow solid hormone essential for human health
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