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Iodide (I⁻)

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Last updated date: 17th Jul 2024
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What is Iodide?

Iodide is described as an anion having a valency of -1. The iodine compounds having an oxidation state of -1 are referred to as iodides. The chemical formula for iodide is given as I.

Iodine is an element that is a halogen. This compound tends to accept an electron and exists as an ion, which is negatively charged. Thus, the anion name iodide can be used interchangeably with iodine. The IUPAC name is given as Iodide.

In everyday life, iodide occurs most often as a component of iodine-added salts prescribed by many governments. Iodine deficiency affects 2 billion people worldwide and is the most common preventable cause of intellectual disability.

Structure of Inorganic Iodides

Triiodide ion 

I2  + I-  ➡I3-

Three center, four electron bonding [I-I-I]-

Lewis Structure


Chemical substance consisting of iodine combined with another element.

Iodide is given as the largest monatomic anion, and it is assigned with a radius of around 206 picometers. Additionally, in comparison, the lighter halides are considerably much smaller: chloride (181 pm), fluoride (133 pm), bromide (196 pm). In contrast, because of its size, iodide produces relatively weak bonds with most of the elements.

Most of the iodide salts are soluble in water, but they are often less related to the related bromides and chlorides. Being large, iodide is very less hydrophilic compared to the smaller anions. A consequence of this is given as sodium iodide is highly soluble in acetone, but sodium chloride is not. The low solubility of lead iodide and silver iodide represents the covalent nature of these iodide metals. The iodide ion presence test is given as the formation of yellow precipitates of these compounds after the treatment of either lead(II) nitrate or silver nitrate solution.

Also, the iodide salt's aqueous solutions dissolve iodine better than pure water. This effect is because of the formation of the triiodide ion, which is brown in color, which is chemically represented as follows:

I + I2 ⇌ I3

Characteristics of Iodide

Iodine has a moderate vapour pressure at room temperature and in an open vessel slowly sublimes to a deep violet vapour that is irritating to the eyes, nose, and throat. This chapter tabulates the physical properties of iodine in solid, liquid, and gas phases. Iodine dissolves easily in most organic solvents such as hexane, benzene, carbon tetrachloride, and chloroform owing to its lack of polarity, but it is only slightly soluble in water. However, the solubility of elemental iodine in water can be increased by the addition of sodium or potassium iodide. Iodine accepts electrons from the solvent molecule into its lowest unoccupied molecular orbital (LUMO). This reduces the transition energy of the iodine atom from the highest occupied molecular orbital (HOMO)  to  LUMO,  changing the characteristic purple to brown color. 

 Physical Properties of Iodide

The physical characteristics of iodine are those that can be observed without changing one substance to another. Physical characteristics are things that we can perceive, such as color, luster, freezing point, boiling point, melting point, density, hardness, and odor. The physical characteristics of iodine are as follows.



Molecular Weight or Molar Mass

126.904 g/mol


Violet black. A grey solid that changes into purple vapours when heated


Has a shine or glow


Strong, harsh odour

Crystalline structure



3.13 g/cm3, solid



Specific heat



128 g/100 ml (6 °C)

Melting point

113.7 °C

Boiling Point

184.3 °C

Chemical Properties of Iodide

Chemical properties can only be observed during a chemical reaction. Reactions to substances can be caused by changes due to burning, rusting, heating, explosions, discoloration, etc. The chemical properties of iodine are:



Chemical Formula



Poisonous halogen


It does not combine directly with oxygen


With hydrogen, it forms hydrogen iodide, which in water solution becomes hydriodic acid. Its compounds are used in medicine and photography and in dyes


It is Highly corrosive

Reactivity with water

Dissolves only slightly in water

Reactivity with heat

Moves from the solid to the vapor state (sublimation)


  • The iodide compound's sodium salt reacts with lead nitrate and produces a yellow precipitate of sodium nitrate and lead iodide. The chemical equation for the same is given as follows.

Pb (NO3)2(aq) + 2 NaI (aq) → 2 NaNO3 (aq) + PbI2 (s) (which is a yellow precipitate)

  • Potassium iodide salt combines with chlorine by producing iodine and potassium chloride. Here, chlorine replaces iodine because chlorine is more reactive than iodine. The chemical equation for the same is given as follows.

2KI + Cl2 → 2KCl + I2

Redox, Including Antioxidant Properties

Iodide salts are defined as mild reducing agents, and several react with oxygen to form iodine. A reducing agent is described as a chemical term for an antioxidant. Its antioxidant properties are expressed as a redox potential quantitatively:

I ⇌ ​\[\frac {1} {2}\] I2 + e  E° = 0.54 volts.

Since the iodide is easily oxidized, a few enzymes readily transform it into electrophilic iodine agents as needed for the biosynthesis of myriad iodide-containing natural products. Also, iodide functions as an antioxidant reducing species that can destroy the species of reactive oxygen like hydrogen peroxide, where the chemical equation is represented as follows:

2 I + peroxidase + H2O2 + histidine, tyrosine, lipid,..... → iodide-compounds + H2O + 2 e (which are antioxidants). Iodide structure I- can be given as follows.

Uses of Iodide I

Let us look at the important uses of the iodide compound as listed below:

  • Iodide holds a disinfectant property. It is not readily affected because of the chlorine by organic content or water pH, but the cold water temperature markedly reduces iodide disinfectant activity.

  • Iodine preparations like povidone-iodine can be used to disinfect the skin before surgery. The allergic reactions to the iodine are more common and should be evaluated carefully since the resultant stain may mask the swelling and redness.

  • Potassium iodide can also be added as a nutrient to prevent goiter, a thyroid problem, which is caused by a lack of iodine and prevents a mental retardation form associated with the deficiency of iodine.

Valency of Iodide

Iodine valence is -1 because, in its last shell, it has 7 electrons and receives one electron from making it stable.

Health Hazard of Iodide

A few of the Health Hazard of iodide can be given as follows.

The major and primary effects of long-term oral exposure to the elevated amounts of inorganic iodide are given as paradoxically, hypothyroidism and hyperthyroidism. The excess intake of this iodide compound can inhibit the synthesis and release of thyroid hormone, resulting in goiter and hypothyroidism.

FAQs on Iodide (I⁻)

1. What is Iodide Used for?

Potassium iodide (with the chemical formula, KI) is described as a chemical compound used to shield the thyroid gland from potential radioactive iodine damage (radioiodine). A few radiological emergencies can also emit large amounts of radioiodine into the air.

2. Where can the Iodide be found?

Iodine is a compound, which is naturally found in the ocean and is preserved in its tissues by particular water plants and sea fish. Iodine can also be present in the water, soil, and air naturally. The seas are treated as the primary and major sources of natural iodine.

3. Are Iodized and Iodide the Same Thing?

Iodide and iodine are the only same product's different expressions. Whereas iodides only reflect healthy ingestion of iodine form. Iodides also contain the drawback of requiring additional body energy to sever the iodide bond to use the compound of iodine. 

4. What is Iodide Compound Good for?

Iodine compound lowers the thyroid hormone, and it can easily destroy microorganisms such as fungi, amoebas, bacteria, including others. A specific form of iodine, known as potassium iodide, can often be used to treat nuclear accident effects, but it does not help prevent them.

5. Are iodides soluble?

The chlorides, bromides, and iodides of all metals except lead, silver, and mercury (I) are soluble in water.  The sulfates of all metals except lead, mercury (I), barium, and calcium are soluble in water. Silver sulfate is slightly soluble. The water-insoluble sulfates are also insoluble in dilute acids.

6. Is iodide a solvent?

Hydrous hydrogen iodide is a poor solvent, able to dissolve only small molecular compounds such as nitrosyl chloride and phenol, or salts with very low lattice energies such as tetraalkylammonium halides.

7. Is iodide a metal?

Although it is technically non-metal, it exhibits some metallic qualities. Iodine is classified as a halogen — a subset of very chemically reactive elements (Group 17 on the periodic table) that exist in the environment as compounds rather than as pure elements.

8. Is iodide different from iodine?

There are two forms of iodine, elemental diatomic iodine (I2) and ionic monoatomic iodide (I-). Iodide is the ionic state of iodine, occurring when iodine forms a salt with another element, such as potassium. In this form, iodide can be ingested or applied topically (such as with povidone-iodine, an iodide).