Iodine Facts (Atomic Number 53 or I)

Iodine Facts
Iodine is a halogen with atomic number 53 and element symbol I.

Iodine is a chemical element with the symbol I and atomic number 53. This halogen occurs in the human diet, primarily in iodized salt, and is essential for human health in trace amounts. Iodine plays a crucial role in thyroid function, helping to regulate metabolism.


Iodine was discovered by the French chemist Bernard Courtois in 1811. Courtois was extracting sodium and potassium compounds from seaweed ash and discovered iodine as an accidental byproduct. He observed violet fumes, which condensed to form dark crystals, and these were later identified as iodine.


The name iodine derives from the Greek word ‘iodes’, meaning violet or purple, referring to the color of iodine vapor.


Iodine is a lustrous dark-gray to purple-black solid at room temperature. Although it is a nonmetallic element, it often has a metallic sheen. It sublimates easily, forming a beautiful violet gas. Its liquid state is a deep violet color.


Iodine is a member of the halogen group in the periodic table. Like other halogens, it forms compounds with many elements. It has relatively low reactivity compared to other halogens.

Iodine Isotopes

There is one stable isotope of iodine, which is iodine-127. This is the only isotope that occurs in nature. Iodine has several radioactive isotopes, with iodine-129 and iodine-131 being the most notable. Some iodine-129 forms from cosmic ray spallation of xenon in air and also from nuclear testing, but isotope is very rare. Iodine-131 finds use in medicine for treating thyroid disorders.

Abundance and Sources

Iodine is relatively rare in the Earth’s crust, with a concentration of about 0.5 parts per million. It is the least abundant stable halogen. The primary source of iodine is from underground brines associated with natural gas and oil deposits. Historically, kelp was the key source of this element.

Uses of Iodine

One of the most common uses of iodine is in medicine. It is an antiseptic for external use and a treatment for thyroid disorders. Additionally, iodine is important in making dyes, inks, and certain types of photographic chemicals. It is also a nutritional supplement in areas where iodine deficiency is common.

Oxidation States

Iodine commonly exhibits an oxidation state of -1 in its compounds, particularly in iodides. However, it also displays positive oxidation states, including +1, +3, +5, and +7. The positive oxidation states occur in various iodine oxides and iodate anions.

Biological Role and Toxicity

Iodine is an element that is essential for life, particularly for the synthesis of thyroid hormones, which regulate metabolism. However, excessive iodine can be harmful. Ingesting large amounts of iodine causes iodine toxicity, leading to thyroid problems, goiter, and other health issues.

Iodine Facts and Properties

Atomic Number53
Atomic Weight126.90447
Group17 (VIIA, halogens)
Electron Configuration[Kr] 4d10 5s2 5p5
Electrons per Shell2, 8, 18, 18, 7
Phase at Room TemperatureSolid
Melting Point113.7°C (236.7°F)
Boiling Point184.3°C (363.7°F)
Density4.93 g/cm³
Heat of Vaporization (I2)41.57 kJ/mol
Heat of Fusion (I2)15.52 kJ/mol
Oxidation States-1, 0, +1, +2, +3, +4, +5, +6, +7
Pauling Electronegativity2.66
Ionization Energies1st: 1008.4 kJ/mol
Atomic Radius140 pm
Crystal StructureOrthorhombic
Magnetic OrderingDiamagnetic

More Interesting Iodine Facts

  • Vivid Colors: Iodine forms compounds with a variety of rich and vibrant colors. The elemental iodine itself is violet, but its compounds are red, yellow, brown, green, or even colorless.
  • Test for Starch: Iodine is a classic chemical test for starch. Iodine solution turns starch a deep blue or blue-black color. This is because the iodine interacts with the coiled structure of starch molecules, creating an intensely colored complex.
  • First Antiseptic: Iodine was one of the first antiseptics. It found extensive use for wound cleaning and treatment during the American Civil War, significantly reducing the incidence and severity of infections and complications.
  • Rare Element: Iodine is one of the least abundant of the non-gaseous elements in the Earth’s crust. It is about as abundant as silver, which is considered a precious metal.
  • Therapeutic Radioisotope: Radioactive iodine (I-131) is a targeted radiation therapy for thyroid cancer. The thyroid gland naturally takes up iodine, so by using radioactive iodine, doctors selectively target cancerous thyroid cells for destruction with minimal impact on the rest of the body.
  • Radiation Exposure Treatment: Taking potassium iodide (KI) pills helps protect the thyroid from irradiation in nuclear reactor accidents. But, there is a narrow window of time when the treatment is effective. It is only protective when taken no more than two days before or 8 hours after exposure to iodine-131.
  • Role in Art: Iodine was used in the early days of photography. The first successful photograph used a bitumen-based process that used iodine vapor to enhance sensitivity to light.
  • Chemistry Education: Several chemistry demonstrations use iodine, including the elephant toothpaste reaction, Halloween clock reaction, oscillating clock, and nitrogen triiodide demonstration.
  • Letter “J” on the Periodic Table: While the international symbol for iodine is I, its symbol on Mendeleev’s periodic table for J for jod. “Jod” remains the name for iodine in some countries.


  • Davy, Humphry (1 January 1814). “Some Experiments and Observations on a New Substance Which Becomes a Violet Coloured Gas by Heat”. Phil. Trans. R. Soc. Lond. 104: 74. doi:10.1098/rstl.1814.0007
  • Emsley, John (2001). Nature’s Building Blocks (Hardcover, 1st ed.). Oxford University Press. ISBN 0-19-850340-7.
  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.
  • Zanzonico, P.B.; Becker, D.V. (2000). “Effects of time of administration and dietary iodine levels on potassium iodide (KI) blockade of thyroid irradiation by 131I from radioactive fallout”. Health Physics. 78 (6): 660–667. doi:10.1097/00004032-200006000-00008