Iodine, (Chemical Element, Symbol I, Atomic Number 53)




Iodine is an element in Group 7 of the Periodic Table. Members Group 7 are also known as Halogens.
(The other halogens - that is, members of the same group are Fluorine, Chlorine, Bromine and Astatine. These have some properties in common with iodine while there are also certain "trends" or patterns in the variation of other properties across this group of elements.)

Each iodine atom consists of 53 protons, 53 electrons plus a quantity of neutrons whose exact number* depends on the particular isotope of iodine (see below).
*The standard atomic mass of iodine is 126.90447 amu, which is often rounded to 127 in simple (e.g. school textbook) versions of the Periodic Table. Using a mass number of 127 for iodine, the number of neutrons in each atom would be 127 - 53 = 74.

History of Iodine / Discovery of Iodine

Iodine was discovered and isolated in its elemental form for the first time in 1811 by the French chemist Bernard Courtois. He discovered, described, and collected samples of a substance he first observed as a purple gas and then also as a dark crystalline solid which was later confirmed as a new element and named "iodine" (in English) after the Greek word for the colour "violet" - which was chosen due to the purple colour of iodine vapour.

More about Courtois's discovery of iodine:

Bernard Courtois's father was a manufacturer of saltpeter (the mineral form of potassium nitrate, KNO3), an important chemical used in the 19th century for the production of gunpowder and so in great demand for the Napoleonic Wars. Courtois was involved in the process of extracting saltpeter from its naturally occuring form as a colourless-to-white mineral typically found encrusted in caves/caverns in formations called "niter beds". This required the use of sodium carbonate (Na2CO3), which could be obtained from seaweed washed up on the coasts of northern France or plucked from the surrounding sea, which is still done today. In order to extract the sodium carbonate, seaweed was burned and the resulting ash washed with water. The remaining waste products were then destroyed by adding sulphuric acid to them. While performing this process on one occasion Courtois added too much sulphuric acid and observed a cloud of purple vapour rise from the reactants. He noticed that the purple vapour crystallised on cold surfaces forming dark crystals. Although he suspected this was a new element Courtois lacked the resources to pursue his observations by conducting further research himself.

Bernard Courtois gave samples of the unidentified material he had collected to other scientists including:

  • French physicist and chemist Charles Bernard Desormes,
  • French physicist and chemist Nicolas Clément,
  • French physicist and chemist Joseph Louis Gay-Lussac (best known for his gas laws), and
  • French physicist and mathematician André-Marie Ampère (best known for contributing to the theory of electromagnetism).

In November 1813 Dersormes and Clément described the substance discovered by Courtois at a meeting of the Imperial Institute of France. A week later Gay-Lussac stated that the new substance was either an element or a compound of oxygen. He also suggested the name "iode", from the Greek word for violet (due to the colour of the vapour). Meanwhile, Ampère had given some of his sample to British chemist and inventor Sir Humphry Davy (who is also known for improving safety in mining by inventing the "Davy Lamp"). Davy had conducted some experiments on the sample, noted its similarity to chlorine, and wrote to the Royal Society of London stating that he had identified a new element. There followed a dispute between Davy and Gay-Lussac about which of them had been the first to identify iodine - though both acknowledged Courtois as the first to isolate this 'new' chemical element.

Elemental Iodine (under "standard conditions")

At room temperature and pressure elemental iodine is a non-metallic dark-grey / purple-black lustrous solid, hence the dark crystals observed by Bernard Courtois in 1811. Although iodine is solid under standard conditions its melting and boiling points are quite low (114 °C and 184 °C respectively) so purple iodine vapour has been widely observed since it was first described by Courtois in 1811.

Elemental iodine consists of molecules, rather than atoms, of iodine.
Two atoms of iodine form each iodine molecule, hence molecules of iodine are diatomic and have the chemical formula I2. Elemental iodine is toxic and its vapour irritates the eyes and lungs - see "Iodine and Health" below.

Iodine dissolves in some solvents e.g. carbon tetrachloride CCl4 and is also (but only slightly) soluble in water.

Iodine and Health

Elemental iodine is toxic. I2 vapour irritates the eyes and lungs.
Compounds of iodine (called "iodides") are also toxic if taken in excess.

Iodine forms part of thyroid hormones (e.g. thyrotrophin ) that are essential for growth, the nervous system and the metabolism.

Consequences of a shortage of iodine in the human diet can include slowing-down of the function of the thyroid gland and the thyroid gland itself starting to swell-up (increase in size). This phenomenon is called "struma" but is now rare in developed countries due, among other things, to the addition of iodine to standard table salt and some other common foodstuffs. Conversely, excess amounts of iodine in the diet can also be dangerous because the thyroid gland is then likely to operate too quickly, affecting many aspects of the body. Overdosage of iodine may lead to disturbed heartbeat and loss of weight.

Compounds of Iodine (general overview)

There are many categories of compounds of iodine.

  • Inorganic Iodides are compounds that include the element iodine together with at least one other element and are not "organic compounds" - meaning that they are not classified within organic chemistry because they do not include carbon-hydrogen bonds (though there are a few exceptions to that general definition of organic chemistry). Examples of important inorganic iodides include potassium iodide (KI) and iodine trichloride (I2Cl6).
  • Oxides of Iodine are compounds that include both the elements iodine and oxygen. Oxides of iodine are therefore a sub-set of inorganic iodides. Iodine can form several different oxides because iodine has more than one oxidation state. Examples of compounds formed from only iodine and oxygen include diiodine tetraoxide (I2O4), diiodine pentaoxide (I2O5) and tetraiodine nonoxide (I4O9). An iodate includes the iodate anion which consists of an iodine atom attached to three oxygen atoms and has the molecular formula is IO3. Iodate anions form ionic compounds in combination with appropriate cation(s). Examples of iodates include sodium iodate (NaIO3), silver iodate (AgIO3), and calcium iodate (Ca(IO3)2).
  • Organic Iodides (also known as Organoiodine Compounds) are compounds that include at least the elements iodine and carbon and often also include other elements, especially hydrogen as organic compounds generally include C-H bonds. Examples of organic iodides include haloalkanes that have at least one iodine atom, e.g. iodomethane, iodoethane, 1-iodopropane, 2-iodopropane and so on.

Isotopes of Iodine

Iodine has 37 known isotopes, of which only one Iodine-127 is considered stable.
Notes about some isotopes of iodine are listed below.


Content of Nucleus:



53 protons, 70 neutrons

Half life 13 hours. Used as nuclear imaging tracer to evaluate thyroid function.
Although most medical imaging with iodine isperformed with a standard gamma camera, gamma rays from iodine-123 (and iodine-131) can also be seen by single photon emission computed tomography imaging, which is also known as "SPECT imaging".


53 protons, 71 neutrons

Half-life 4.18 days. Modes of decay are: 74.4% electron capture and 25.6% positron emission.
Iodine-124 as the iodide salt can be used to directly image the thyroid using positron emission tomography (PET). It can also be used as a PET radiotracer with a usefully longer half-life compared with fluorine-18. In this use, the iodine-124 nuclide is chemically bonded to a pharmaceutical to form a positron-emitting radiopharmaceutical, then injected into the body, where it is imaged by PET scan.


53 protons, 72 neutrons

Half-life 59 days. Used as nuclear imaging tracer to evaluate thyroid function of the thyroid. Also used by radiation oncologists in low dose rate brachytherapy to treat cancer at sites other than the thyroid, e.g. prostate cancer.


53 protons, 75 neutrons

Half-life 25 min. No major medical or industrial uses due to short half-life.


53 protons, 76 neutrons

Half-life 15.7 million years. Used as an extinct radionuclide and as a long-lived marker for nuclear fission contamination.


53 protons, 78 neutrons

Half-life 8 days. Emits beta-particles. One of the radionuclides involved in atmospheric testing of nuclear weapons. Thought to increase the risk of cancer and possibly other diseases of the thyroid and those caused by thyroid hormonal deficiency.

Used in biology and medicine e.g. for treatment of some thyroid conditions. The high energy beta radiation from iodine-131 makes it the most carcinogenic of the iodine isotopes. It is thought to cause most of the excess in thyroid cancers seen after nuclear fission contamination (e.g. bomb fallout or severe nuclear reactor accidents)


53 protons, 82 neutrons

Half-life 6.6 hours.
Important for nuclear reactor physics and produced in plentiful quantities as a fission product.

Uses of Iodine (including iodine combined with other elements to form compounds of iodine)

  • Disinfectant (e.g. for Water Treatment):
    Iodine has been used to disinfect water for almost 100 years. Use of iodine for water purifcation has advantages and disadvantages compared with use of chlorine for disinfecting water, e.g. comparisons re. convenience, effect on the taste of the water and short/long-term safety. Neither of these chemicals kills all harmful bacteria and iodine should not be used to treat water for use by anyone with an allergy to iodine, with active thyroid disease or who is or may be pregnant. Examples of iodine-based preparations used to disinfect water incl. iodine topical solution, iodine tincture, Lugol's solution, povidone-iodine and tetraglycine hydroperiodide - some of which are better known by their commercial registered tradenames.
  • Medical / Pharmaceutical:
    Iodine has been used in topical disinfectant preparations for cleaning wounds, sterilizing skin before surgical/invasive procedures and similar for many years, e.g. issued to military personnel in WW1 and WW2. Iodine is still used in topical medical disinfectants in modern hospitals, though usually within commercially prepared products in order to control the concentrations of the chemicals involved. See also iodine used as an X-ray radiocontrast agent (below) as that is another important medical use of iodine.
  • X-ray Radiocontrast:
    Radiocontrast agents
    are chemicals used to improve the quality and hence usefulness of images - usually of internal bodily structures - obtained using X-ray based imaging techniques e.g. Computed Tomography (CT) or Radiography (X-ray imaging). Radiocontrast agents are usually compounds of either barium or iodine.
    Examples of iodine-based radiocontrast agents incl. iopamidol (Isovue 370), iohexol (Omnipaque 350), ioxilan (Oxilan 350), iopromide (Ultravist 370) and iodixanol (Visipaque 320).
  • Photographic Films:
    Iodine was used the manufacture of chemical compounds used in traditional photography (e.g. silver iodide which is a light sensitive material used in film). This use relates only to photographs taken using old-fashioned "film" techniques and not to modern digital photography e.g. as used to take photographs or save images using mobile telephones or webcams.
  • To test for starch:
    A standard test for starch uses iodine.
    Because iodine is not very soluble in water the first step is to form an iodine reagent by dissolving iodine in water in the presence of potassium iodide, resulting in a linear triiodide ion complex - which is soluble and yellow / orange in colour.
    To use this to test an unknown sample to find out if it contains starch simply add a drop ofthe orange triiodide ion complex to a small volume of the other sample / suspected starch (usually in solution in a test-tube or directly onto a moist surface e.g. of a potato). If starch is present in the sample it reacts with the triiodine complex to forming a product that has a deep blue/black colour. If no starch is present then there is no colour change so the yellow/organe linear triiodide ion complex is usually still visible.

Read about more iodine uses.

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