- Iodine (I) is a nonmetallic halogen with atomic number 53, located in Group 17 of the periodic table alongside fluorine, chlorine, bromine, and astatine.
- It appears as a lustrous, grayish-black solid at room temperature, notable for sublimating into a violet-colored vapor when heated.
- Iodine has seven valence electrons—two in the 5s subshell and five in the 5p subshell—and typically exhibits an oxidation state of –1 in its ionic form, though it can also show +1, +3, +5, and +7 oxidation states in covalent compounds. Its atomic structure consists of fifty-three protons, usually seventy-four neutrons, and fifty-three electrons arranged in five shells.
- Naturally occurring iodine consists almost entirely of one stable isotope, iodine-127 (¹²⁷I), but it also has a significant radioactive isotope, iodine-129 (¹²⁹I), with a half-life of 15.7 million years, and the medically important iodine-131 (¹³¹I), which has an 8-day half-life.
- Iodine is relatively rare in the Earth’s crust, with an average abundance of about 0.45 parts per million. It is more concentrated in seawater, where it exists mainly as iodide ions, and in certain minerals such as caliche ore from Chile. Large deposits are also found in underground brines and natural gas fields. The leading producers of iodine include Chile, Japan, and the United States.
- The element was discovered in 1811 by French chemist Bernard Courtois, who accidentally produced violet vapors while extracting sodium and potassium compounds from seaweed ash using sulfuric acid. The vapors condensed into dark crystals, which were later confirmed to be a new element by Joseph Louis Gay-Lussac, who named it “iode” from the Greek ioeides, meaning “violet-colored.”
- In modern applications, iodine is widely used in medicine, industry, and nutrition. Iodine compounds, such as povidone-iodine and tincture of iodine, are important antiseptics. Radioactive iodine-131 is used in medical diagnostics and treatment, particularly for thyroid disorders, as the thyroid gland selectively absorbs iodine. Iodine is also added to table salt (iodized salt) as a dietary supplement to prevent iodine deficiency and goiter. In industry, iodine compounds serve as catalysts, photographic chemicals, polarizing filters, and components in specialized polymers.
- Chemically, iodine is less reactive than chlorine or bromine but still readily forms compounds with most elements. In aqueous solution, it can exist as molecular iodine (I₂), iodide (I⁻), triiodide (I₃⁻), and various oxyanions such as iodate (IO₃⁻) and periodate (IO₄⁻). It reacts with metals to form metal iodides and with hydrogen to form hydrogen iodide (HI), a strong acid.
- Biologically, iodine is essential for all vertebrates, as it is a key component of thyroid hormones (thyroxine, T₄, and triiodothyronine, T₃) that regulate metabolism, growth, and development. Iodine deficiency can lead to goiter, hypothyroidism, and developmental disorders, while excessive intake can cause thyroid dysfunction. The human body contains about 15–20 milligrams of iodine, mostly stored in the thyroid gland.
- From an environmental perspective, iodine naturally cycles between the ocean, atmosphere, and land. Industrial release of radioactive iodine, especially from nuclear accidents such as Chernobyl and Fukushima, poses significant health risks due to its rapid uptake by the thyroid gland. Preventive measures, such as potassium iodide tablets, can block radioactive iodine absorption in emergencies.
