Iodine (I)
Iodine (symbol I) is the chemical element with atomic number 53: a halogen, solid at 0 °C, with an atomic mass of 126.9 u. Discovered in 1811 (Bernard Courtois).
Iodine is an essential halogen: our thyroid uses it to make the hormones that regulate metabolism. Deficiency, which still affects hundreds of millions of people, is the world's leading preventable cause of intellectual disability, hence the iodine added to table salt. It is also used as a disinfectant and in X-ray contrast agents.
Key facts
- Atomic number
- 53
- Symbol
- I
- French name
- Iode
- Atomic mass
- 126.9 u
- Molar mass
- 126.9 g/mol
- Family
- Halogens
- Position
- period 5, group 17, p-block
- State at 0 °C
- Solid
- Melting point
- 113.7 °C
- Boiling point
- 184.3 °C
- Density
- 4.93 g/cm³
- Electronegativity (Pauling)
- 2.66
- First ionization energy
- 1,008 kJ/mol
- Atomic radius
- 140 pm
- Electron configuration
- [Kr] 4d¹⁰ 5s² 5p⁵
- Abundance in Earth's crust
- 0.45 ppm (63rd most abundant element)
- Indicative price
- 35 $/kg (order of magnitude)
- Radioactive
- no
- Discovery
- 1811, Bernard Courtois
Origin of the name
From the Greek iōdēs, violet, the color of its vapor.
What does iodine look like?
Gray-black crystals with a violet metallic sheen. Gently heated, it sublimes, meaning it turns straight into a spectacular violet vapor.
Where is iodine found?
About 0.45 ppm of Earth's crust. Seawater contains about 0.06 mg per liter, which some seaweeds concentrate thousands of times. The main deposits are the nitrates of the Atacama Desert (Chile) and underground brines in Japan.
Natural sources
- Caliche (Chilean nitrates)
- Lautarite
- Natural brines
- Seaweed
How is iodine produced?
In Chile, it is recovered by leaching caliche, a nitrate ore; in Japan, from brines pumped up with natural gas. World output is about 30,000 tonnes a year, nearly two-thirds of it in Chile.
Top producers
- Chile
- Japan
- United States
- Turkmenistan
What is iodine used for?
- Iodinated contrast agents for CT scans and X-rays.
- Antiseptics (povidone-iodine, tincture of iodine).
- Iodized table salt and dietary supplements.
- Polarizing films in LCD screens.
- Industrial catalysts (acetic acid production).
In everyday life
- Iodized salt
- Brown antiseptic (such as Betadine)
- CT scan with contrast injection
- Seaweed and seafood
Role in living things
Essential: it is part of the thyroid hormones (T3 and T4). An adult needs about 150 µg a day. Deficiency causes goiter (a swollen thyroid) and, in unborn children, severe developmental disorders. It is the heaviest element essential to humans.
Hazards of iodine
In large amounts, pure iodine is irritating and corrosive, and its vapors are toxic. Radioactive iodine-131, released in nuclear accidents, lodges in the thyroid: potassium iodide tablets are then handed out to saturate the gland with stable iodine.
Cosmic origin
- Neutron stars: Created when two neutron stars collide — the “forge” of gold.
History of iodine
In 1811, Parisian saltpeter maker Bernard Courtois was producing saltpeter for the gunpowder of Napoleon's armies from seaweed ash. When he added sulfuric acid, he saw a violet vapor rise and condense into dark crystals. Gay-Lussac named it iode (iodine), from the Greek iodes, “violet.”
Did you know?
- Switzerland was one of the first countries to iodize its salt, starting in 1922, to fight the goiter that was very common in the Alps.
- A drop of iodine turns starch blue-black: a classic test to reveal the starch in a potato.
- Courtois, the discoverer of iodine, died poor despite the importance of his discovery.
Isotopes of iodine
A single stable isotope, iodine-127. Iodine-131 is both a treatment for thyroid cancer and the most feared fallout in the days right after a nuclear accident.
- iodine-123: radioactive, half-life 13.2 h. Thyroid scans with a low dose to the patient.
- iodine-124: radioactive, half-life 4.18 days
- iodine-125: radioactive, half-life 59.4 days. Brachytherapy seeds and biological lab assays.
- iodine-127: 100% of natural iodine, stable
- iodine-129: radioactive, half-life 1.61 × 10⁷ years. Very long-lived waste, also used as a tracer of ocean currents.
- iodine-131: radioactive, half-life 8.03 days. Treats hyperthyroidism and thyroid cancer; hence the stable iodine tablets handed out after an accident.
Related elements
Same family (halogens): Fluorine (F), Chlorine (Cl), Bromine (Br), Astatine (At).
Same column (group 17): Fluorine (F), Chlorine (Cl), Bromine (Br), Astatine (At), Tennessine (Ts).