Iodine – facts and properties
- Symbol
- I
- Atomic number
- 53
- Atomic mass
- 126.904 u
- Period
- 5
- Group
- 17
- Category
- halogen
- Electron configuration
- [Kr] 4d¹⁰ 5s² 5p⁵
- Electronegativity
- 2.66
- Atomic radius
- 115 pm
- Valence electrons
- 7
- State at standard conditions
- solid
- Melting point
- 113.7 °C
- Boiling point
- 184.4 °C
- Density
- 4.93 g/cm³
- Discovered
- 1811
Iodine is a halogen with atomic number 53. It forms dark, metallic-looking crystals that give off a violet vapour when warmed. The name comes from the Greek ioeides, meaning violet-coloured. The human body needs iodine for the thyroid gland, which is why table salt is often enriched with it. Iodine solutions are also used for disinfection and to detect starch.
Position in the periodic table
Occurrence and extraction
Iodine is rare in the Earth's crust. Seawater contains only traces, but brown algae such as kelp concentrate it strongly. Today it is obtained mainly in Chile, where it occurs as iodate in Chile saltpetre, and in Japan from iodine-rich brines produced together with natural gas.
The discovery was made in 1811 by the French saltpetre manufacturer Bernard Courtois. He extracted soda from seaweed ash; it was needed to make saltpetre, an ingredient of gunpowder. When he added sulfuric acid to the residues, a violet vapour rose that settled on cold surfaces as dark crystals. Courtois passed samples to other chemists, who soon recognised the substance as a new element.
Iodine enters the body through food. Sea fish, seafood and algae are particularly rich in it. Inland, especially in mountain regions, soil and drinking water contain little iodine.
Properties and reactions
Iodine is in period 5 and group 17, below bromine and above astatine. In the period, tellurium lies to its left and xenon to its right. Its electron configuration is [Kr] 4d¹⁰ 5s² 5p⁵ and its electronegativity is 2.66. The element has 7 valence electrons. The density is 4.93 g/cm³, the melting point 113.7 °C and the boiling point 184.4 °C.
Iodine consists of diatomic I₂ molecules. Among the stable halogens it is the heaviest and the least reactive. The crystals sublime even with moderate warmth: the solid partly turns into the violet vapour without a liquid stage, and on cold surfaces it crystallises again. The vapour irritates the eyes and airways. Iodine barely dissolves in water and turns it brown, while in non-polar solvents such as hexane it gives a violet solution. With potassium iodide, on the other hand, the readily soluble triiodide ion forms, which is why Lugol's solution is brown.
Iodine is an oxidising agent, but weaker than chlorine and bromine. The iodide ion, in contrast, is easily oxidised to iodine. Common oxidation numbers are minus one and plus five.
A classic detection reaction is based on starch. Iodine settles into the helical structure of amylose, and a deep blue colour appears. This iodine-starch test is used to detect starch or to follow reactions in the laboratory. Elemental iodine is harmful and stains the skin brown.
Uses in everyday life and technology
The body needs iodine for the hormones of the thyroid gland, thyroxine and triiodothyronine, which control metabolism. With iodine deficiency the thyroid gland enlarges, causing a goitre. It used to be widespread in the Alpine countries and in upland regions. This is why table salt is enriched with potassium iodide or potassium iodate, in Switzerland since the 1920s. Sea fish also supplies iodine.
For disinfection, people use iodine tincture, a solution of iodine in alcohol, and above all povidone-iodine, a brown solution in which iodine is bound to a water-soluble polymer (povidone). It is used on the skin before operations and on wounds. In thyroid disease it should be used only on medical advice.
Medicine has further uses. Iodine-containing contrast agents make vessels and organs visible in X-ray and computed tomography examinations. After a reactor accident, radioactive iodine can be released and accumulates in the thyroid gland. Potassium iodide tablets saturate the thyroid with stable iodine and so reduce the uptake of the radioactive iodine. They protect only against radioactive iodine, not against other radioactive substances, and may be taken only when the authorities instruct it.
Technically, iodine is used, among other things, in the polarising films of liquid-crystal displays. Silver iodide was a component of classic photographic film.
Molecules containing this element
- Iodine I2
- Copper(I) iodide CuI
- Potassium iodide KI
Quick question
In which group are the halogens located?
FAQ
Why is iodine important for humans?
The thyroid gland builds iodine into its hormones, which control metabolism and growth. With a deficiency it enlarges, causing a goitre. Iodised salt and sea fish secure the supply.
What happens when iodine is heated?
Iodine sublimes: it partly passes directly from solid to gas. The vapour is violet and irritates the eyes and airways. On cold surfaces, dark crystals form again.
How does the iodine-starch test work?
Iodine molecules settle into the spirals of amylose, a component of starch. A deep blue complex forms. The colour indicates starch.