Iodide vs. Triiodide

What's the Difference?

Iodide and triiodide are both chemical compounds that contain iodine atoms. However, they differ in terms of their molecular structures and properties. Iodide refers to the ion form of iodine, which has a single negative charge (I-). It is commonly found in various salts and is essential for the synthesis of thyroid hormones in the human body. On the other hand, triiodide (I3-) is a polyatomic ion formed by the combination of three iodine atoms. It is a dark-colored compound and is often used as an oxidizing agent in chemical reactions. Triiodide is also known for its ability to form complex compounds with other substances, making it useful in analytical chemistry. Overall, while iodide is a simple ion, triiodide is a more complex and reactive species with distinct applications.


Chemical FormulaNaI, KI, etc.I3-
Number of Iodine Atoms13
Physical StateSolidSolid
SolubilityHighly soluble in waterSoluble in water
ColorWhiteDark brown
UsesMedicinal, photography, disinfectantsPhotography, analytical chemistry

Further Detail


Iodide and triiodide are chemical compounds that contain iodine, a halogen element. While both compounds consist of iodine atoms, they differ in their molecular structures and properties. In this article, we will explore the attributes of iodide and triiodide, including their physical appearance, chemical reactivity, and applications in various fields.

Physical Appearance

Iodide, also known as iodine ion, is an anion with a charge of -1. It is typically found in the form of salts, such as sodium iodide (NaI) or potassium iodide (KI). These salts are white crystalline solids that are soluble in water. Iodide ions are larger in size compared to iodine molecules, and they have a spherical shape.

On the other hand, triiodide is a polyatomic ion composed of three iodine atoms bonded together. It has a charge of -1 and is commonly found in compounds like potassium triiodide (KI3). Triiodide appears as dark brown or black crystals, and it is also soluble in water. The triiodide ion has a linear molecular structure, with the three iodine atoms arranged in a straight line.

Chemical Reactivity

Iodide ions are relatively stable and do not readily react with other substances. However, they can undergo oxidation reactions to form iodine molecules or other iodine compounds. For example, when iodide ions react with strong oxidizing agents like chlorine or hydrogen peroxide, they can be converted into iodine (I2) or iodate ions (IO3-).

Triiodide ions, on the other hand, are more reactive compared to iodide ions. They can act as both oxidizing and reducing agents, depending on the reaction conditions. Triiodide ions can be reduced to iodide ions by certain reducing agents, such as thiosulfate ions (S2O3^2-). Conversely, they can oxidize certain compounds, such as sulfite ions (SO3^2-), to form iodine molecules.


Iodide compounds, particularly potassium iodide, have various applications in different fields. In medicine, potassium iodide is used as a supplement to prevent iodine deficiency and to treat certain thyroid conditions. It is also utilized in the pharmaceutical industry for the synthesis of organic compounds and as a reagent in chemical reactions.

Triiodide compounds, such as potassium triiodide, find applications in analytical chemistry. They are often used as reagents for the determination of certain substances, including starch, which forms a blue complex with triiodide ions. Additionally, triiodide compounds can be employed in the synthesis of organic compounds, particularly in the field of organic chemistry.


In summary, iodide and triiodide are chemical compounds that contain iodine. Iodide ions are larger in size and have a spherical shape, while triiodide ions have a linear molecular structure. Iodide ions are relatively stable and can undergo oxidation reactions, while triiodide ions are more reactive and can act as both oxidizing and reducing agents. Iodide compounds have applications in medicine and the pharmaceutical industry, while triiodide compounds find uses in analytical chemistry and organic synthesis. Understanding the attributes of iodide and triiodide is crucial for their proper utilization in various fields.

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