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Chemical element, symbol I and atomic number 53 / From Wikipedia, the free encyclopedia

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Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at 114 °C (237 °F), and boils to a violet gas at 184 °C (363 °F). The element was discovered by the French chemist Bernard Courtois in 1811 and was named two years later by Joseph Louis Gay-Lussac, after the Ancient Greek Ιώδης 'violet-coloured'.

Quick facts: Iodine, Pronunciation, Appearance, Standard a...
Iodine, 53I
Pronunciation/ˈədn, -dɪn, -dn/ (EYE-ə-dyne, -din, -deen)
Appearancelustrous metallic gray solid, black/violet liquid, violet gas
Standard atomic weight Ar°(I)
Iodine in the periodic table


Atomic number (Z)53
Groupgroup 17 (halogens)
Periodperiod 5
Block  p-block
Electron configuration[Kr] 4d10 5s2 5p5
Electrons per shell2, 8, 18, 18, 7
Physical properties
Phase at STPsolid
Melting point(I2) 386.85 K (113.7 °C, 236.66 °F)
Boiling point(I2) 457.4 K (184.3 °C, 363.7 °F)
Density (near r.t.)4.933 g/cm3
Triple point386.65 K, 12.1 kPa
Critical point819 K, 11.7 MPa
Heat of fusion(I2) 15.52 kJ/mol
Heat of vaporisation(I2) 41.57 kJ/mol
Molar heat capacity(I2) 54.44 J/(mol·K)
Vapour pressure (rhombic)
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 260 282 309 342 381 457
Atomic properties
Oxidation states−1, 0, +1, +2,[3] +3, +4, +5, +6, +7 (a strongly acidic oxide)
ElectronegativityPauling scale: 2.66
Ionisation energies
  • 1st: 1008.4 kJ/mol
  • 2nd: 1845.9 kJ/mol
  • 3rd: 3180 kJ/mol
Atomic radiusempirical: 140 pm
Covalent radius139±3 pm
Van der Waals radius198 pm
Color lines in a spectral range
Spectral lines of iodine
Other properties
Natural occurrenceprimordial
Crystal structure base-centered orthorhombic
Base-centered orthorhombic crystal structure for iodine
Thermal conductivity0.449 W/(m⋅K)
Electrical resistivity1.3×107 Ω⋅m (at 0 °C)
Magnetic orderingdiamagnetic[4]
Molar magnetic susceptibility−88.7×10−6 cm3/mol (298 K)[5]
Bulk modulus7.7 GPa
CAS Number7553-56-2
Discovery and first isolationBernard Courtois (1811)
Isotopes of iodine
Main isotopes Decay
abun­dance half-life (t1/2) mode pro­duct
123I synth 13 h β+100% 123Te
124I synth 4.176 d ε 124Te
125I synth 59.40 d ε 125Te
127I 100% stable
129I trace 1.57×107 y β 129Xe
131I synth 8.02070 d β100% 131Xe
135I synth 6.57 h β 135Xe
Symbol_category_class.svg Category: Iodine
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Iodine occurs in many oxidation states, including iodide (I), iodate (IO
), and the various periodate anions. It is the least abundant of the stable halogens, being the sixty-first most abundant element. As the heaviest essential mineral nutrient, iodine is required for the synthesis of thyroid hormones.[6] Iodine deficiency affects about two billion people and is the leading preventable cause of intellectual disabilities.[7]

The dominant producers of iodine today are Chile and Japan. Due to its high atomic number and ease of attachment to organic compounds, it has also found favour as a non-toxic radiocontrast material. Because of the specificity of its uptake by the human body, radioactive isotopes of iodine can also be used to treat thyroid cancer. Iodine is also used as a catalyst in the industrial production of acetic acid and some polymers.

It is on the World Health Organization's List of Essential Medicines.[8]

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