Atomic number 53

Iodine

I
+ phase
− phase

Shells Orbitals 4d 5s 5p

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53 electrons in 5 shells: 2, 8, 18, 18, 7.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Halogen
Period: 5
Group: 17
Block: p
Standard atomic weight: 126.90(1) Da
Electron configuration: [Kr] 4d¹⁰ 5s² 5p⁵
Phase at 25 °C: solid
Melting point: 113.7 °C
Boiling point: 184.3 °C
Density: 4.93 g/cm³
Electronegativity (Pauling): 2.66
Discovered: 1811, Bernard Courtois

Neighbours in the table

Te Xe

A crystal of iodine warmed gently skips the liquid state and passes straight into a deep violet vapour. Los Alamos calls the solid bluish-black and lustrous, OpenStax greyish-black; the name records the vapour. Iodes is Greek for violet, and the CIAAW text in PubChem’s record ties the name to those vapours.

The atom carries 53 electrons in five shells, 2, 8, 18, 18 and 7, written [Kr] 4d¹⁰ 5s² 5p⁵. The filled 4d subshell counts as core, OpenStax says, leaving seven valence electrons, the ones used in bonding, in 5s and 5p. Fluorine and chlorine carry seven too, which is why the group’s chemistry runs in parallel. One more electron would fill the 5p subshell; a single covalent bond supplies it, so iodine atoms pair into I₂.

Chlorine at room temperature is a gas, bromine a liquid and iodine a solid. OpenStax reads the sequence as London forces, the attractions between molecules, which grow with molecular mass and with polarisability, how readily a molecule’s electron cloud is distorted. Larger nonpolar molecules attract more strongly: I₂ melts at 113.70 °C, while carbon dioxide’s small molecules stay solid below −78 °C. Electronegativity, an atom’s pull on shared bonding electrons, drops down the group to iodine’s 2.66. OpenStax ranks it the halogens’ weakest oxidising agent, the one that takes electrons least readily.

Where it is found

PubChem’s record puts iodine at 0.45 milligrams per kilogram of crustal rock and 0.06 milligrams per litre of seawater. Across the oceans, the US Geological Survey says, that adds up to roughly 90 billion tons. Seaweeds of the Laminaria family draw it out of the water and hold up to 0.45 per cent iodine by dry weight. Los Alamos names kelp as the main natural source.

The element came out of seaweed ash. In Jefferson Lab’s account in PubChem’s record, Bernard Courtois was working up the ash in 1811 when a violet vapour rose from the mass. It condensed to crystals on nearby metal. Louis-Joseph Gay-Lussac proved it an element and named it in 1814. The seaweed trade stayed a major source until 1959, the Geological Survey notes, and remains a large resource.

Three places supply most of the world’s iodine: Chile’s desert nitrate deposits, Japan’s gas and oil fields, and brine wells in northwestern Oklahoma. OpenStax makes iodine the exception among the halogens, whose best sources are otherwise halide salts. The commercial route starts from sodium iodate, an impurity in Chile saltpetre, and reduces it with sodium hydrogen sulfite.

Why it matters

Every molecule of thyroid hormone carries iodine: three atoms in triiodothyronine, T₃, four in thyroxine, T₄. The thyroid’s follicle cells pump in iodide, and its concentration inside is many times the blood’s. There the ions are oxidised to iodine, and peroxidase enzymes attach it to tyrosine units in thyroglobulin, a protein. The hormones influence the basal metabolic rate, the body’s energy use at rest, and OpenStax calls them critical to nervous-system development before birth and in early childhood.

A shortage shows up as swelling. When T₃ and T₄ cannot be made, thyroid-stimulating hormone is secreted in growing amounts, thyroglobulin piles up in the follicles, and the enlarged gland is a goitre. OpenStax’s Anatomy and Physiology names iodine deficiency the leading cause of preventable intellectual disability worldwide. Crops carry what their soil holds, so in many countries the main dietary source is iodised salt, fortified in the United States since 1924.

Iodine-131 concentrates in the thyroid gland, which OpenStax says lets it monitor goitre and treat conditions such as Graves’ disease. It decays by beta emission to xenon-131 with a half-life of 8.0252 days; the IAEA evaluation lists no other decay branch. In the Geological Survey’s ranking of world uses by quantity consumed, X-ray contrast media come first, then pharmaceuticals and liquid crystal displays.

Properties

Properties of iodine Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	53
Standard atomic weight	126.90(1) Da	Da	IUPAC/CIAAW 2024
Melting point	113.7 °C	°C	Los Alamos: 113.7 °C
Boiling point	184.3 °C	°C	Los Alamos: 184.3 °C
Density	4.93 g/cm³	g/cm³
Electronegativity (Pauling)	2.66
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁵
Phase at 25 °C	solid

Safety

Elemental iodine is a harmful solid whose vapour, at 20 °C, contaminates the air rather quickly. The harmonised European classification calls it harmful in contact with the skin, harmful if inhaled and very toxic to aquatic life; its vapour severely irritates the eyes and airways. That concerns people who handle the crystals at work or in a laboratory; the iodide in iodised salt is a different substance. The authoritative sheets are PubChem’s Laboratory Chemical Safety Summary for iodine and the NIOSH Pocket Guide card for iodine crystals.

Questions

Does iodine dissolve in water?

Hardly at all. OpenStax says water takes up only a little iodine and turns brown with it, and NIOSH puts the solubility at 0.01 per cent. Iodide solutions take far more, because iodine and the iodide ion combine into the triiodide ion, which OpenStax makes responsible for that solubility. Chloroform, carbon tetrachloride and carbon disulfide dissolve it readily, Los Alamos says, giving purple solutions. Traces in water still show: with starch and iodide they turn deep blue.

Is iodine dangerous?

Yes, as the element. NIOSH's card for iodine crystals gives inhalation, ingestion and skin or eye contact as the exposure routes and lists the eyes, skin, respiratory system, central nervous system and cardiovascular system as target organs. Notifiers split on the signal word: the harmonised European entry says Warning, while 837 company reports aggregated by the European Chemicals Agency say Danger. Haz-Map's text in PubChem's safety data calls USP iodine tincture and povidone-iodine generally not corrosive.

Why is iodine the least reactive halogen?

It is the weakest oxidising agent of the group, in OpenStax's ranking. The standard reduction potentials, a measure of how readily each halogen takes up electrons, run chlorine 1.36 V, bromine 1.09 V, iodine 0.54 V, and the lighter halogens oxidise the heavier halide ions; Los Alamos puts it as the other halogens displacing iodine from iodides. Fluorine still takes it to 7+, and the larger iodine atom fits seven fluorines around it where chlorine and bromine reach only 5+.

Where does the world's iodine come from?

Chile, Japan and the United States, according to the US Geological Survey. Chile mined about 22,000 metric tons in 2024, two-thirds of a world total of about 33,000 tons that excludes the United States; Japan mined 9,300. The United States imports more than half of what it uses, 90 per cent of it from Chile over 2020–23. The Survey's reserves table puts Japan first, at 4.9 million tons.

Compounds containing iodine

KI Potassium iodide 2 atoms in one formula unit of an ionic lattice.
C₁₅H₁₁I₄NO₄ Thyroxine 35 atoms, 36 bonds.

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