Iodine

Iodine is the heaviest of the commonly encountered halogens and one of the most important elements for human health. Without it, the thyroid gland cannot make the hormones that control growth, metabolism and brain development. It is also the antiseptic in the brown liquid used to clean wounds, and one of the most striking substances in chemistry, a dark solid that turns straight to a vivid purple vapour without passing through a liquid.

• Atomic Number5353 protons, 53 electrons
• Atomic Mass126.9045 u53× heavier than hydrogen
• State at Room TempSolidSolid
• Density4.93 g/cm³
• Melting / Boiling113.7°C / 184.4°C
• Discovered1811

What is Iodine?

Iodine is a halogen in Group 17 of the periodic table, sitting below bromine. With 53 protons, iodine at room temperature is a dark grey-black solid with a slight metallic sheen, but it sublimes easily to produce a beautiful purple vapour. It is the only common halogen that is a solid at room temperature. Iodine dissolves in alcohol to form the brown tincture of iodine antiseptic, and it turns starch a deep blue-black, a very sensitive colour test used in biology laboratories.

Iodine gets its name from the ancient Greek word ioeides meaning violet or purple, referring to the vivid purple colour of its vapour. It was discovered in 1811 by the French chemist Bernard Courtois, who found it in the ash of seaweed used in making sodium nitrate. Gay-Lussac and Humphry Davy investigated it and established its elemental nature, with Davy proposing the name iodin (later iodine). The symbol I comes from the name.

Fact The iodine-starch test is one of chemistry's most dramatic colour reactions. Add a drop of iodine solution to starch and it turns a deep blue-black, almost instantly. The iodine molecules (I₃⁻ ions) thread themselves inside the helical coils of the starch polysaccharide, and the electron interactions within this complex produce the intense colour. This test can detect starch in amounts too small to taste.

Where you find Iodine

In space

Iodine is found in the Sun and is produced by stellar nucleosynthesis.

On Earth

Iodine is relatively rare in the crust at approx. 0.45 parts per million but is enriched in seawater and in some brine deposits.

• Caliche deposits. Sodium iodate occurs in the Atacama Desert of Chile alongside nitrate deposits. Chile is by far the world's largest iodine producer.
• Underground brines. Iodide-rich brines in Japan, China and the USA are a major source. Japan extracts iodine from deep natural gas well brines.
• Seaweed. Kelp and other seaweeds concentrate iodide from seawater. Before the 20th century, seaweed ash was the main iodine source.

How we use Iodine

• Nutrition.. Iodine is added to table salt (iodised salt) to prevent iodine deficiency, which causes goitre (enlarged thyroid) and, in pregnant women, can cause cretinism, severe intellectual disability in the child. Iodised salt is one of the most cost-effective public health interventions ever implemented.
• Antiseptic.. Iodine dissolved in alcohol (tincture of iodine) or in polyvinylpyrrolidone (Betadine) kills bacteria, fungi and viruses on skin and wounds.
• Medical imaging.. Iodine-based contrast agents are injected to make blood vessels and organs visible in CT scans and X-rays.
• Radiation protection.. Potassium iodide tablets are distributed to people living near nuclear power stations for use in emergencies: flooding the thyroid with stable iodine prevents it absorbing radioactive iodine-131 released in reactor accidents.

Did you know? About 2 billion people worldwide have insufficient iodine in their diets, making it the world's most common cause of preventable intellectual disability. Iodine deficiency during pregnancy impairs foetal brain development. The solution, adding a few cents' worth of potassium iodide to table salt, is one of the most effective public health interventions in history. In countries where iodised salt is universal, IQ scores have been shown to rise measurably.

How it was discovered

Iodine was discovered in 1811 by Bernard Courtois in Paris, who was making potassium nitrate from seaweed ash. Adding excess sulfuric acid, he noticed a purple vapour rising and condensing as dark, crystalline flakes. He distributed samples to scientists including Gay-Lussac and Humphry Davy, who competed to characterise the new element. Davy proposed the name iodine. Courtois, though the discoverer, received little recognition or financial reward.

Deeper dive: iodine chemistry and applications

The thyroid gland requires iodine to make its hormones, thyroxine (T4) and triiodothyronine (T3). These iodine-containing hormones regulate metabolism, heart rate, body temperature, growth and brain development throughout life. The thyroid concentrates iodide from the blood, it contains approx. 70-80% of all the iodine in the body. In areas with iodine-deficient soils and water, people develop goitre (an enlarged thyroid gland that grows attempting to capture more iodide) and hypothyroidism (underactive thyroid). In pregnant women, severe deficiency causes cretinism in the child, irreversible brain damage.

Potassium iodide (KI) tablets are stockpiled by governments near nuclear power stations. In a reactor accident, radioactive iodine-131 (half-life 8 days) is released and can be inhaled or ingested through milk. The thyroid concentrates any iodine, including radioactive iodine. Taking potassium iodide before or immediately after exposure floods the thyroid with stable iodine, blocking uptake of the radioactive form. This was done after the Chernobyl and Fukushima accidents. The tablet is only effective against thyroid uptake of radioactive iodine, it does not protect any other organ or against other radioactive releases.

Moving to 54 protons on the periodic table brings us to Xenon.