Iodine (I) is a chemical element and a member of the halogen elements, belonging to Group 17 (Group VIIa) of the periodic table. It is a nonmetallic, nearly black solid at room temperature with a glittering, crystalline appearance. Iodine is essential for the production of thyroid hormones, which regulate heart action, nerve response, growth rate, and metabolism. The recommended daily intake is 150 micrograms for adults, 220 micrograms for pregnant women, and 290 micrograms for lactating women. It is commercially produced from iodine-containing brines and was once extracted from seaweed. Iodine and its compounds have various applications, including use as a disinfectant, antiseptic, in analytical chemistry, photography, and in the production of dyes. A small amount of iodine is also essential for the body to maintain normal metabolism in all the body’s cells.

What does iodine look like in its natural form?

In its natural form, iodine is a nonmetallic solid that is nearly black at room temperature and has a glittering, crystalline appearance. The molecular lattice of iodine contains discrete diatomic molecules, which are also present when iodine is molten or in a gaseous state. Iodine is never found in nature uncombined.

Where can iodine be found on the periodic table?

Iodine (I) is a chemical element that can be found in Group 17 (VIIa) of the periodic table , also known as the halogen group. It has an atomic number of 53 and an atomic weight of 126.9044. The halogen group includes fluorine, chlorine, bromine, iodine, astatine, and tennessine.

Why is iodine important for living things?

Iodine is vital for living organisms because it is a key component of thyroid hormones , like thyroxine, essential for maintaining normal metabolism in all body cells. The thyroid gland uses iodine to produce hormones that regulate heart action, nerve response, body growth rate, and metabolism. Iodine deficiency can lead to decreased thyroid hormone production, resulting in hypothyroidism, goitre, and, in severe cases, intellectual disability and cretinism in infants. Sources of iodine include seafood and iodized salt, with many countries mandating iodine additives to address natural deficits.

What are some common uses of iodine in everyday life?

Iodine is used in antiseptics, disinfectants, and analytical chemistry. It is essential for producing thyroxine in the thyroid gland, which helps maintain normal metabolism. A small amount of iodine is often added to table salt to prevent iodine deficiency, which can lead to thyroid issues. Silver iodide, an iodine compound, is used in photography and cloud seeding. The chemical industry uses iodine on a smaller scale, with Japan being the largest producer, extracting it from seaweed.

What are the health problems caused by a lack of iodine?

Iodine deficiency can cause several health problems because iodine directly affects thyroid gland secretions, which influence heart action, nerve response, growth rate, and metabolism. When iodine intake is low, thyroid hormone production decreases, leading to an increase in thyrotropin secretion, which causes the thyroid to absorb more iodine and enlarge, resulting in goitre. Mild hypothyroidism symptoms include dry skin, hair loss, a puffy face, weakness, weight gain, fatigue, and mental sluggishness. Severe iodine deficiency, especially during gestation and infancy, can result in cretinism, causing intellectual disability and growth retardation. Iodine deficiency is most common in mountainous areas where the soil has little iodine. It can be prevented by eating seafood or using iodized table salt.

iodine

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A labeled diagram highlights the periodic table entry for iodine, atomic number 53, symbol I, atomic weight 126.90447, and electron configuration. It shows iodine as a solid halogen with an orthorhombic crystal structure and strongly acidic higher-valence oxides, using color and shape keys for classification.

Two beakers show iodine dissolving in different solvents: on the left, iodine does not dissolve in water; on the right, iodine dissolves and turns purple in a benzene solution. Molecular models are shown above each beaker.

Explore an interactive periodic table of the elements

At a Glance

iodine summary

iodine

chemical element

Also known as: I

Written by

Karl ChristeAll

Fact-checked by

Britannica Editors

Last updated

Oct. 17, 2026 •History

Top Questions

What is iodine?
Iodine (I) is a chemical element and a member of the halogen elements, belonging to Group 17 (Group VIIa) of the periodic table. It is a nonmetallic, nearly black solid at room temperature with a glittering, crystalline appearance.

Iodine is essential for the production of thyroid hormones, which regulate heart action, nerve response, growth rate, and metabolism. The recommended daily intake is 150 micrograms for adults, 220 micrograms for pregnant women, and 290 micrograms for lactating women. It is commercially produced from iodine-containing brines and was once extracted from seaweed.

Iodine and its compounds have various applications, including use as a disinfectant, antiseptic, in analytical chemistry, photography, and in the production of dyes. A small amount of iodine is also essential for the body to maintain normal metabolism in all the body’s cells.
4 Britannica Sources
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What does iodine look like in its natural form?
In its natural form, iodine is a nonmetallic solid that is nearly black at room temperature and has a glittering, crystalline appearance. The molecular lattice of iodine contains discrete diatomic molecules, which are also present when iodine is molten or in a gaseous state.

Iodine is never found in nature uncombined.
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1. iodine: Physical and chemical properties
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Where can iodine be found on the periodic table?
Iodine (I) is a chemical element that can be found in Group 17 (VIIa) of the periodic table, also known as the halogen group. It has an atomic number of 53 and an atomic weight of 126.9044. The halogen group includes fluorine, chlorine, bromine, iodine, astatine, and tennessine.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why is iodine important for living things?
Iodine is vital for living organisms because it is a key component of thyroid hormones, like thyroxine, essential for maintaining normal metabolism in all body cells. The thyroid gland uses iodine to produce hormones that regulate heart action, nerve response, body growth rate, and metabolism.

Iodine deficiency can lead to decreased thyroid hormone production, resulting in hypothyroidism, goitre, and, in severe cases, intellectual disability and cretinism in infants. Sources of iodine include seafood and iodized salt, with many countries mandating iodine additives to address natural deficits.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some common uses of iodine in everyday life?
Iodine is used in antiseptics, disinfectants, and analytical chemistry. It is essential for producing thyroxine in the thyroid gland, which helps maintain normal metabolism. A small amount of iodine is often added to table salt to prevent iodine deficiency, which can lead to thyroid issues. Silver iodide, an iodine compound, is used in photography and cloud seeding. The chemical industry uses iodine on a smaller scale, with Japan being the largest producer, extracting it from seaweed.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are the health problems caused by a lack of iodine?
Iodine deficiency can cause several health problems because iodine directly affects thyroid gland secretions, which influence heart action, nerve response, growth rate, and metabolism.

When iodine intake is low, thyroid hormone production decreases, leading to an increase in thyrotropin secretion, which causes the thyroid to absorb more iodine and enlarge, resulting in goitre. Mild hypothyroidism symptoms include dry skin, hair loss, a puffy face, weakness, weight gain, fatigue, and mental sluggishness. Severe iodine deficiency, especially during gestation and infancy, can result in cretinism, causing intellectual disability and growth retardation.

Iodine deficiency is most common in mountainous areas where the soil has little iodine. It can be prevented by eating seafood or using iodized table salt.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

iodine (I), chemical element, a member of the halogen elements, or Group 17 (Group VIIa) of the periodic table.

Element Properties
atomic number	53
atomic weight	126.9044
melting point	113.5 °C (236 °F)
boiling point	184 °C (363 °F)
specific gravity	4.93 at 20 °C (68 °F)
oxidation states	−1, +1, +3, +5, +7
electron configuration	2-8-18-18-7 or (Kr)5s²4d¹⁰5p⁵

History

In 1811 the French chemist Bernard Courtois obtained a violet vapor by heating seaweed ashes with sulfuric acid as a by-product of the manufacture of saltpeter. This vapor condensed to a black crystalline substance, which he called “substance X.” In 1813 British chemist Sir Humphry Davy, who was passing through Paris on his way to Italy, recognized substance X as an element analogous to chlorine; he suggested the name iodine from the Greek word ioeides, “violet colored.”

Occurrence and distribution

Iodine is never found in nature uncombined, and it is not concentrated sufficiently to form independent minerals. It is present in seawater, but sparingly, as the iodide ion, I⁻, to the extent of approximately 50 mg per metric ton (0.0016 ounce per ton) of seawater. It is also formed in seaweeds, oysters, and cod livers. Sodium iodate (NaIO₃) is contained in crude Chile saltpeter (sodium nitrate, NaNO₃). The human body contains iodine in the compound thyroxine, which is produced in the thyroid gland.

The only naturally occurring isotope of iodine is stable iodine-127. An exceptionally useful radioactive isotope is iodine-131, which has a half-life of eight days. It is employed in medicine to monitor thyroid gland functioning, to treat goitre and thyroid cancer, and to locate tumors of the brain and of the liver. It is also used in investigations to trace the course of compounds in metabolism. Several iodine compounds are used as contrast mediums in diagnostic radiology. In aqueous solution even minute amounts of iodine in the presence of starch produce a blue-black color.

iodine

History

Occurrence and distribution

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