Iodine · element 53
The purple vapour found in seaweed ash that your thyroid cannot live without
Key facts
Iodine is the heaviest element that the human body needs in significant amounts. Without it, the thyroid gland cannot make the hormones that set the pace of metabolism, and a shortage in childhood stunts both growth and brain development. A pinch of iodised salt is one of the cheapest public health measures ever devised.
As an element it is a dark, glossy, almost metallic-looking solid that turns straight into a violet gas when warmed. It is the least reactive of the common halogens, yet still reactive enough to kill microbes on skin, which is why the brown antiseptic in a first aid kit has smelt of iodine for more than a century.
Where it is found
Iodine is present in seawater at low concentrations, and marine life concentrates it. Kelp and other seaweeds can hold thousands of times the concentration in the surrounding water, which is why seaweed ash was the first commercial source. Fish and shellfish are rich dietary sources for the same reason.
Today the two main sources are quite different. In northern Chile, iodine is extracted from caliche, the nitrate-rich desert deposits, where it occurs as iodate. In Japan it is recovered from brines pumped up alongside natural gas. Together these two countries supply most of the world's iodine.
How it was discovered
Iodine was found in 1811 by Bernard Courtois, a French manufacturer of saltpetre for gunpowder during the Napoleonic wars. Saltpetre production needed potassium salts, and Courtois obtained these from the ash of burnt seaweed. Cleaning his copper vats with sulfuric acid one day, he added too much and saw a cloud of violet vapour rise and condense into dark crystals on the cold metal.
Courtois suspected a new element but lacked the means to prove it. He passed samples to chemists in Paris, and Joseph Louis Gay-Lussac confirmed it was an element and named it iode, from the Greek word for violet-coloured. Humphry Davy, visiting France on a special passport, also examined it and reached the same conclusion, leading to a brief priority dispute.
What we use it for
Nutrition and medicine dominate. Iodised table salt has nearly eliminated goitre, the swollen thyroid caused by deficiency, in countries that adopted it. Povidone-iodine and tincture of iodine disinfect skin before surgery and treat wounds. Iodine-containing contrast agents are injected into patients so that blood vessels and organs show up clearly on X-ray and CT scans; this is one of the largest uses by weight.
Radioactive iodine-131 is used both to image the thyroid and to destroy overactive or cancerous thyroid tissue, since the gland eagerly absorbs iodine. Potassium iodide tablets are stockpiled near nuclear power stations to saturate the thyroid and block uptake of radioactive iodine after an accident. Industrially, iodine compounds serve as catalysts in making acetic acid, as stabilisers in nylon, in LCD polarising films, and in animal feed.
Reading the numbers
Iodine melts at about 114 °C and boils at about 184 °C, so it is a solid at room temperature but not by a wide margin. Gentle heating turns it into vapour. Its density of 4.93 grams per cubic centimetre makes it nearly five times as dense as water, remarkably heavy for a non-metal.
The configuration [Kr] 4d10 5s2 5p5 shows seven outer electrons, one short of a full shell. That is the signature of group 17, the halogens, and it is why iodine forms the iodide ion so readily and pairs up into I2 molecules. Compared with fluorine, chlorine and bromine above it, iodine's outer electrons are further from the nucleus and more shielded, so it is the least electronegative of the four at 2.66 and the least reactive. Its large size also lets it form compounds in the +5 and +7 states, such as iodates and periodates, more comfortably than the lighter halogens.
Safety and the environment
Iodine is essential in tiny amounts, harmful in large ones. A few hundred micrograms a day is about right for an adult; grams of elemental iodine are toxic and the vapour irritates the eyes and lungs. Excess iodine can also disturb the thyroid, so supplements should be taken with care. Iodine deficiency remains a public health problem in some regions and is still the leading preventable cause of intellectual disability worldwide.
Radioactive iodine released from nuclear accidents, as at Chernobyl and Fukushima, is a particular worry because it concentrates in the thyroid, especially in children. Its short half-life means it decays within weeks, which is why prompt distribution of stable iodine tablets matters.
Did you know?
- Iodine was discovered by accident when a gunpowder manufacturer added too much acid to seaweed ash and saw violet vapour rise.
- The thyroid hormone thyroxine contains four iodine atoms per molecule, which is why it is abbreviated T4.
- A drop of iodine solution turns starch an intense blue-black, a test still used in school laboratories.
- Iodine deficiency is the most common preventable cause of intellectual disability in the world.
- Solid iodine is dark and shiny like a metal, but its vapour is a rich violet, which gave the element its name.
More in the halogens
These share a column with iodine, so their outer electrons are arranged in a similar way and they tend to behave alike.
See all 6 halogens on one page, or browse all 118 elements.
Compare Iodine with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Why is iodine added to salt?
The thyroid needs iodine to make the hormones that control metabolism, growth and brain development, and many inland soils and diets contain too little. Adding a tiny amount of potassium iodide or iodate to table salt is a cheap way to reach whole populations, and it has largely eliminated goitre where it is used.
Is iodine a metal?
No. Solid iodine has a metallic-looking dark sheen, but it is a non-metal in the halogen group alongside fluorine, chlorine and bromine. It is a poor conductor, forms molecules of two atoms, and sublimes into a violet vapour when warmed.
What does iodine do in the body?
Almost all of it goes to the thyroid gland, which builds it into the hormones thyroxine and triiodothyronine. These hormones regulate how fast cells use energy and are essential for normal growth and brain development, especially before birth and in early childhood.
Why are iodine tablets given after a nuclear accident?
Reactor accidents can release radioactive iodine-131, which the thyroid absorbs just as readily as ordinary iodine. Taking a dose of stable potassium iodide fills the gland so that little radioactive iodine can be taken up. The tablets protect only the thyroid and work best if taken shortly before or after exposure.
Read next
Guess the mystery element in six tries. A new one every day, free, no sign-up.
Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Iodine reference article. Spotted an error? Tell us.