Iodine
Atomic Data
| Atomic Number | 53 |
| Symbol | I |
| Atomic Weight | 126.9 u |
| Density (STP) | 4.933 g/cm³ |
| Melting Point | 113.7 °C (386.85 K) |
| Boiling Point | 184.4 °C (457.55 K) |
| Electronegativity | 2.66 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5 |
| Oxidation States | −1, +1, +3, +5, +7 |
| Phase at STP | Solid |
| Category | Halogen |
| Period / Group | 5 / 17 |
| CAS Number | 7553-56-2 |
Electron Configuration
[Kr] 4d10 5s2 5p5
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| O | 5 | 5s | 2 | 48 |
| O | 5 | 5p | 5 | 53 |
| Total | 53 | 53 | ||
Isotopes of Iodine
Iodine has two naturally occurring stable isotopes. The most abundant is ¹²⁷I, comprising 100% of all naturally occurring Iodine.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Iodine-127 | ¹²⁷I | 53 | 74 | 100 | Stable |
| Iodine-131 | ¹³¹I | 53 | 78 | trace | Stable |
Abundance & Occurrence
Iodine is present in Earth's crust at approximately 0.45 ppm by mass and at approximately 1 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Iodine vapour: respiratory irritant: Iodine vapour is a respiratory and mucous membrane irritant; the TLV-ceiling is 0.1 ppm: above this level, iodine vapour causes eye, nose, and throat irritation, and at higher concentrations pulmonary oedema.
- Skin and eye contact: Solid iodine and concentrated iodine solutions cause chemical burns and staining; iodine is absorbed through skin in sufficient quantities to cause systemic toxicity at high exposures.
- Iodine-131: radiation hazard: I-131 (t½ = 8 days, beta/gamma emitter) used in thyroid treatment concentrates in the thyroid; medical handling requires radiation protection and patients must follow isolation protocols to protect others from contamination.
- Reactivity: Iodine reacts vigorously with ammonia and some organic compounds; it is a moderate oxidiser and should be kept away from flammable materials and strong reducing agents.
Iodine in the Real World
Real-World Uses
- Medical antiseptics: Povidone-iodine (Betadine) solution and iodine tincture are applied to skin and wounds as broad-spectrum antimicrobials that kill bacteria, fungi, and viruses by oxidative disruption of cellular proteins.
- Iodised salt and nutrition: Universal salt iodisation, in which potassium iodate or potassium iodide is added to table salt at 20–40 ppm, is a global public health programme that prevents iodine-deficiency disorders including goitre and cretinism.
- Thyroid medicine and imaging: Iodine-131 is used to treat hyperthyroidism and differentiated thyroid cancer, exploiting the thyroid gland's natural concentration of iodine; stable iodide tablets protect the thyroid in nuclear emergencies.
- LCD polarising filters: Sheets of polyvinyl alcohol (PVA) containing iodine-stained polyiodide chains serve as polarising films in LCD displays; the elongated iodine complexes absorb light polarised parallel to their orientation.
- Industrial catalysis: Hydroiodic acid and iodine are used as catalysts and reagents in the Monsanto/Cativa acetic acid processes and in the synthesis of organic iodide intermediates used in pharmaceuticals and agrochemicals.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is iodine used for?
Iodine has several important uses: as an antiseptic (iodine solution and povidone-iodine are used to clean wounds and skin before surgery), in thyroid medicine (radioactive iodine-131 treats thyroid cancer and hyperthyroidism), as a nutritional additive in iodised salt (preventing goitre and cretinism), in photography (silver iodide), and as a catalyst precursor in industrial chemical processes.
Is iodine essential for human health?
Yes, iodine is an essential trace element for humans and most animals. It is a key component of thyroid hormones (thyroxine, T4, and triiodothyronine, T3), which regulate metabolism, growth, and development. Iodine deficiency is the leading preventable cause of intellectual disability worldwide and causes goitre (enlarged thyroid). Most countries add iodine to table salt as a simple and effective public health measure.
How was iodine discovered?
Iodine was discovered in 1811 by French chemist Bernard Courtois while making saltpetre from seaweed ash for gunpowder production. When he added excess sulphuric acid to the seaweed ash liquor, he noticed violet vapours that condensed into dark crystalline solids. He shared samples with other chemists including Gay-Lussac, who confirmed it as a new element and named it iode from the Greek 'ioeides', meaning violet-coloured.
How does radioactive iodine treat thyroid disease?
The thyroid gland actively concentrates iodine from the blood to produce thyroid hormones. When a patient swallows radioactive iodine-131 (as a capsule or liquid), the thyroid preferentially absorbs it just as it would stable iodine. I-131 decays by emitting beta particles, which travel only a few millimetres in tissue, destroying the thyroid cells that have absorbed the iodine. This targeted destruction treats both hyperthyroidism and thyroid cancer with minimal radiation to the rest of the body.