PeriodicElementTable.com
53I126.9045
Halogen

Iodine

Iodine is a dark, lustrous solid halogen that turns directly into a striking violet vapor when heated, which is how it got its name. It's also an essential nutrient — the human thyroid gland needs a steady supply of iodine to make the hormones that regulate metabolism, which is why it's added to table salt in many countries.

Group · Period
17 · 5
At room temp
solid
Melts at
386.85 K
Density
4.93 g/cm³
Discovered
1811

Uses

Iodine’s most essential role is biological: the human body uses it to build thyroid hormones, so small amounts must come from diet, and iodized salt has become the main way many countries guard against deficiency. In medicine, iodine compounds are used as antiseptics for cleaning skin and wounds, and iodine-based contrast agents are injected before certain X-ray and CT scans to make blood vessels and soft tissue stand out more clearly on the resulting images.

Beyond health applications, silver iodide was historically central to photographic film, since it darkens predictably when exposed to light, and iodine compounds still see use today in some LCD polarizing filters and as a catalyst in various industrial chemical reactions.

History

Iodine was discovered in 1811 by French chemist Bernard Courtois, who was extracting saltpeter from seaweed ash for gunpowder production when he noticed that adding excess sulfuric acid produced a striking cloud of violet vapor. Intrigued but short on resources to investigate fully, Courtois shared samples with other chemists, and Joseph Louis Gay-Lussac confirmed it as a new element and gave it the name iodine, from the Greek word for “violet,” describing the color of its vapor.

Fun facts

  • Iodine sublimes straight from a solid into a vivid violet-colored vapor without passing through a liquid stage at normal pressure.
  • The human body needs only tiny amounts of iodine, but without it the thyroid gland can swell into a condition called goiter.
  • Iodized table salt was introduced in the 1920s specifically to prevent widespread iodine-deficiency goiter, and it remains common today.

Frequently asked questions

Why is salt iodized?

The thyroid gland needs iodine to make hormones that control metabolism, but many regions have soils and diets naturally low in it, which historically caused widespread goiter and related health problems. Adding a small, reliable amount of iodine to table salt turned out to be a cheap, effective way to make sure most people get enough, and the practice spread quickly after it was introduced in the 1920s.

What is iodine tincture used for?

Iodine dissolved in alcohol, known as tincture of iodine, has long been used to disinfect wounds and skin before medical procedures, since it kills a broad range of bacteria on contact. Related iodine compounds are still used today in hospital settings and as a contrast agent that makes soft tissue and blood vessels show up more clearly on X-ray and CT scans.

Why does iodine turn into a purple vapor?

Solid iodine has a relatively weak attraction holding its molecules together, so at normal atmospheric pressure it sublimes — meaning it passes directly from solid to gas without melting into a liquid first. The resulting vapor retains iodine's characteristic violet color, which is also the origin of its name, from the Greek word for "violet."

Compounds

6 notable compounds containing I

  • CsIsalt

    Cesium iodide

    A dense ionic salt that, when doped with thallium, forms crystals that scintillate brightly under radiation and are widely used in detectors.

    Used for: Scintillation crystals in radiation and medical imaging detectors

  • HgI2salt

    Mercury(II) iodide

    A bright scarlet solid that reversibly turns yellow above about 127°C as its crystal structure shifts, then reverts to red on cooling or when touched.

    Used for: Room-temperature radiation detector material and component of Nessler's reagent

  • KIsalt

    Potassium iodide

    A simple ionic salt that supplies iodide ions, commonly added to table salt to prevent iodine deficiency. It also protects the thyroid gland by saturating it with stable iodine.

    Used for: Iodized table salt and thyroid-blocking tablets after radiation exposure

  • SmI2reagent

    Samarium(II) iodide

    A deep blue-green solution reagent in which samarium sits in its unusual +2 oxidation state, making it a powerful single-electron reductant.

    Used for: Widely used as Kagan's reagent for radical cyclizations in organic synthesis

  • AgIsalt

    Silver iodide

    A pale yellow, extremely insoluble silver salt whose crystal structure closely resembles that of ice, letting it act as a nucleation point for ice crystals inside clouds.

    Used for: Cloud seeding to encourage rain or snow

  • TlIsalt

    Thallium(I) iodide

    A yellow crystalline solid that, unlike most thallium(I) halides, adopts a distorted layered structure rather than the simple rock-salt packing seen in TlCl and TlBr.

    Used for: Additive in metal-halide lamps to enhance green light output

Isotopes

40 known isotopes

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IsotopeNatural abundanceHalf-lifeDecay modeMass excessBinding energy / nucleonDecay width
I-127 stable 100% Stable — -88,983.22 keV 8,445.48 keV —
Show all 40 isotopes
I-108 — 36 ms Alpha decay (91%), Electron capture / beta-plus decay (9%), Proton emission (1%) -52,771 keV 8,176 keV 1.27 × 10-14 eV
I-109 — 92.8 us Proton emission (99.986%), Alpha decay (0.014%) -57,672.51 keV 8,220.02 keV 4.92 × 10-12 eV
I-110 — 0.664 s Electron capture / beta-plus decay (83%), Alpha decay (17%), ECP (11%) -60,467.84 keV 8,244.08 keV 6.87 × 10-16 eV
I-111 — 2.5 s Electron capture / beta-plus decay (99.9%), Alpha decay (0.1%) -64,953.8 keV 8,282.93 keV 1.82 × 10-16 eV
I-112 — 3.34 s Electron capture / beta-plus decay (100%), ECP (0.88%), ECA (0.104%) -67,063.34 keV 8,299.88 keV 1.37 × 10-16 eV
I-113 — 6.6 s Electron capture / beta-plus decay (100%), Alpha decay (3.31e-7%) -71,119.52 keV 8,333.75 keV 6.91 × 10-17 eV
I-114 — 2.1 s Electron capture / beta-plus decay (100%), Beta-plus, proton emission -72,638.94 keV 8,344.78 keV 2.17 × 10-16 eV
I-115 — 1.3 m Electron capture / beta-plus decay (100%) -76,337.81 keV 8,374.57 keV 5.85 × 10-18 eV
I-116 — 2.91 s Electron capture / beta-plus decay (100%) -77,420.65 keV 8,381.29 keV 1.57 × 10-16 eV
I-117 — 2.22 m Electron capture / beta-plus decay (100%), Beta-plus decay (77%) -80,438.57 keV 8,404.43 keV 3.43 × 10-18 eV
I-118 — 13.7 m Electron capture / beta-plus decay (100%) -80,971.06 keV 8,406.12 keV 5.55 × 10-19 eV
I-119 — 19.1 m Electron capture / beta-plus decay (100%), Beta-plus decay (51%), Electron capture (49%) -83,777.73 keV 8,426.89 keV 3.98 × 10-19 eV
I-120 — 81.6 m Electron capture / beta-plus decay (100%) -83,747.16 keV 8,423.67 keV 9.32 × 10-20 eV
I-121 — 2.12 h Electron capture / beta-plus decay (100%) -86,245.65 keV 8,441.41 keV 5.98 × 10-20 eV
I-122 — 3.63 m Electron capture / beta-plus decay (100%) -86,079.28 keV 8,437.01 keV 2.09 × 10-18 eV
I-123 — 13.2230 h Electron capture / beta-plus decay (100%) -87,942.59 keV 8,449.19 keV 9.58 × 10-21 eV
I-124 — 4.1760 d Electron capture / beta-plus decay (100%) -87,364.56 keV 8,441.48 keV 1.26 × 10-21 eV
I-125 — 59.407 d Electron capture (100%) -88,836.02 keV 8,450.29 keV 8.89 × 10-23 eV
I-126 — 12.93 d Electron capture / beta-plus decay (52.7%), Beta-minus decay (47.3%) -87,910.5 keV 8,439.94 keV 4.08 × 10-22 eV
I-128 — 24.99 m Beta-minus decay (93.1%), Electron capture / beta-plus decay (6.9%) -87,738.03 keV 8,432.83 keV 3.04 × 10-19 eV
I-129 — 1.57E+7 Y Beta-minus decay (100%) -88,507.18 keV 8,435.99 keV 9.21 × 10-31 eV
I-130 — 12.36 h Beta-minus decay (100%) -86,936.19 keV 8,421.1 keV 1.03 × 10-20 eV
I-131 — 8.0252 d Beta-minus decay (100%) -87,442.73 keV 8,422.3 keV 6.58 × 10-22 eV
I-132 — 2.295 h Beta-minus decay (100%) -85,703.5 keV 8,406.46 keV 5.52 × 10-20 eV
I-133 — 20.83 h Beta-minus decay (100%) -85,857.3 keV 8,405.1 keV 6.08 × 10-21 eV
I-134 — 52.5 m Beta-minus decay (100%) -84,043.44 keV 8,389.07 keV 1.45 × 10-19 eV
I-135 — 6.58 h Beta-minus decay (100%) -83,779.18 keV 8,384.76 keV 1.93 × 10-20 eV
I-136 — 83.4 s Beta-minus decay (100%) -79,545.23 keV 8,351.32 keV 5.47 × 10-18 eV
I-137 — 24.5 s Beta-minus decay (100%), Beta-minus, neutron emission (7.14%) -76,356.27 keV 8,326 keV 1.86 × 10-17 eV
I-138 — 6.26 s Beta-minus decay (100%), Beta-minus, neutron emission (5.44%) -71,979.91 keV 8,292.45 keV 7.29 × 10-17 eV
I-139 — 2.280 s Beta-minus decay (100%), Beta-minus, neutron emission (10%) -68,470.96 keV 8,265.61 keV 2.00 × 10-16 eV
I-140 — 0.86 s Beta-minus decay (100%), Beta-minus, neutron emission (7.6%) -63,606.22 keV 8,229.47 keV 5.31 × 10-16 eV
I-141 — 0.418 s Beta-minus decay (100%), Beta-minus, neutron emission (21.2%) -59,926.67 keV 8,202.26 keV 1.09 × 10-15 eV
I-142 — 222 ms Unknown -54,802.97 keV 8,165.25 keV 2.06 × 10-15 eV
I-143 — 130 ms Beta-minus decay -50,790 keV 8,137 keV 3.51 × 10-15 eV
I-144 — 300 ns Unknown -45,330 keV 8,098 keV 1.52 × 10-9 eV
I-145 — 407 ns Beta-minus, neutron emission, Beta-minus decay -41,130 keV 8,069 keV 1.12 × 10-9 eV
I-146 — 94 ms Beta-minus decay (100%), B-2N, Beta-minus, neutron emission -35,540 keV 8,031 keV 4.85 × 10-15 eV
I-147 — — Beta-minus decay (100%), Beta-minus, neutron emission, B-2N -31,200 keV 8,001 keV —