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About Iodine

Iodine Bohr

In the early nineteenth century, sodium carbonate, used in the production of saltpeter, was frequently extracted from seaweed. The process required drying and burning the seaweed, producing ash that was then washed with water. When this liquid was allowed to evaporate slowly, several useful compounds would precipitate out in sequence. The liquid that remained after the desired precipitates were collect was generally treated with sulfuric acid before disposal. In 1811, a Frenchman named Bernard Courtois accidentally added an excess of sulfuric acid in this final step, and was astonished when this produced a cloud of purple vapor which condensed to form a shiny crystalline substance on cold surfaces. Though he was employed in the production of saltpeter due to his financial circumstances, Courtois had had enough formal chemistry training to realize that his discovery was significant and suspected that he had produced a new element. Lacking the time or resources to study the material further, he passed the material on to two chemist friends, who continued his investigations and published their findings in 1813. More famous chemists quickly confirmed the nature of the substance as a new element, and one, Joseph Gay Lussac, suggested the name be derived from iodes, Greek for violet, due to the color of iodine’s vapor.

In some ways, iodine mirrors the properties of the lighter members of the halogen family: fluorine, chlorine, and bromine. Like the other halogens, iodine in elemental form it exists as a diatomic molecule, and its compound with hydrogen, hydriodic acid, is a strong acid that is a useful chemical reagent, particularly notable for its role in the industrial production of acetic acid. Hydroiodic acid is additionally used to produce other useful iodine compounds, particularly alkyl halides, which are important in organic synthesis. Silver iodides, like the other silver halides, are light-sensitive, a property exploited in film photography. Both bromine and iodine can be used in metal halide and halogen lamps, though designs using iodine are more common.

Unlike the other halogens, the heavier iodine is solid at room temperature, and, being less electronegative, is less reactive. This lower reactivity plays a role in making it less toxic in elemental form than the lighter halides--while fluorine, chlorine, and bromine cause burns upon contact with tissue, elemental iodine is considered an irritant, and requires prolonged contact with skin to cause significant damage. This allows the use of iodine solutions as topical disinfectants, often used to clean skin prior to surgery. Elemental iodine is not particularly soluble in water, so these solutions typically include solubilizing agents in addition to iodine. An additional unique use of iodine is in various analytical chemistry procedures, particularly the detection of glucose polymers such as starch. Iodine-impregnated polymer films are used as extremely cost-effective light polarizing optical filters found in products such as LCD screens, sunglasses, and optical microscopes.

The other unique applications of iodine relate to its role as an essential nutrient. Iodine is a necessary component of the thyroid hormones T3 and T4, which regulate metabolic rate. Iodine deficiency causes enlargement of the thyroid gland, a condition known as goiter, as well as the myriad symptoms of hypothyroidism. Many populations lack access to sufficient dietary iodine, and many nations now mandate that table salt be treated with iodine salts in order to prevent endemic goiter. This is generally considered one of the simplest and most effective public health measures, as iodine deficiency is a leading cause of intellectual and developmental disabilities.

Iodine salts are common in nature, but relatively few sources of iodine are useful commercially. The most common source is brines that collect in used oil and gas wells, which may be purified and treated to produce iodides. The iodides are then reacted with chlorine to produce the pure element. The only other commercial source of iodine is caliche mineral formations in Chile; these are primarily mined for the extraction of sodium nitrate, but iodates and iodides are recovered as byproducts.

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Iodine forms compounds with many elements, but is less reactive than the other halogens. Iodine is only slightly soluble in water. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulfide to form purple solutions. Iodine compounds are important in organic chemistry and very useful in medicine. Potassium iodide is used in photography.

Iodine Properties

Iodine(I) atomic and molecular weight, atomic number and elemental symbol

Iodine Bohr ModelIodine is a Block P, Group 17, Period 5 element. The number of electrons in each of Iodine's shells is 2, 8, 18, 18, 7 and its electron configuration is [Kr] 4d10 5s2 5p5. The iodine atom has a radius of and it's Van der Waals radius is Elemental IodineIn its elemental form, CAS 7553-56-2, iodine has a lustrous metallic gray appearance as a solid. As a gas it has a violet appearance. Iodine is found mainly as the water-soluble iodide I3-. Iodine was discovered and first isolated by Bernard Courtois in 1811.

Symbol: I
Atomic Number: 53
Atomic Weight: 126.9
Element Category: halogen
Group, Period, Block: 17 (halogens), 5, p
Color: violet-dark gray, lustrous/ bluish-black solid, purple vapor
Other Names: Jod, Iodio
Melting Point: 113.7°C, 236.66°F, 386.85 K
Boiling Point: 184.4°C, 363.92°F, 457.55 K
Density: 4953 kg·m3
Liquid Density @ Melting Point: N/A
Density @ 20°C: 4.93 g/cm3
Density of Solid: 4940 kg·m3
Specific Heat: N/A
Superconductivity Temperature: N/A
Triple Point: 386.65 K, 12.07 kPa
Critical Point: 819 K, 11.7 Mpa
Heat of Fusion (kJ·mol-1): 15.27
Heat of Vaporization (kJ·mol-1): 41.67
Heat of Atomization (kJ·mol-1): 107.24
Thermal Conductivity: 0.449 W·m-1·K-1
Thermal Expansion: N/A
Electrical Resistivity: (0 °C) 1.3×107nΩ·m
Tensile Strength: N/A
Molar Heat Capacity: (I2) 54.44 J·mol-1·K-1
Young's Modulus: N/A
Shear Modulus: N/A
Bulk Modulus: 7.7 GPa
Poisson Ratio: N/A
Mohs Hardness: N/A
Vickers Hardness: N/A
Brinell Hardness: N/A
Speed of Sound: N/A
Pauling Electronegativity: 2.66
Sanderson Electronegativity: 2.78
Allred Rochow Electronegativity: 2.21
Mulliken-Jaffe Electronegativity: 2.74 (14.3% s orbital)
Allen Electronegativity: 2.359
Pauling Electropositivity: 1.34
Reflectivity (%): N/A
Refractive Index: N/A
Electrons: 53
Protons: 53
Neutrons: 74
Electron Configuration: [Kr] 4d10 5s2 5p5
Atomic Radius: 140 pm
Atomic Radius,
non-bonded (Å):
Covalent Radius: 139±3 pm
Covalent Radius (Å): 1.36
Van der Waals Radius: 198 pm
Oxidation States: 5, 7, -1
Phase: Solid
Crystal Structure: orthorhombic
Magnetic Ordering: diamagnetic
Electron Affinity (kJ·mol-1) 295.149
1st Ionization Energy: 1008.4 kJ·mol-1
2nd Ionization Energy: 1845.8 kJ·mol-1
3rd Ionization Energy: 3184 kJ·mol-1
CAS Number: 7553-56-2
EC Number: 231-442-4
MDL Number: MFCD00011355
Beilstein Number: 3587194
SMILES Identifier: [I]
InChI Identifier: InChI=1S/I
PubChem CID: 807
ChemSpider ID: 4514549
Earth - Total: 13.6 ppb 
Mercury - Total: 0.16 ppb 
Venus - Total: 14.3 ppb
Earth - Seawater (Oceans), ppb by weight: 60
Earth - Seawater (Oceans), ppb by atoms: 2.9
Earth -  Crust (Crustal Rocks), ppb by weight: 490
Earth -  Crust (Crustal Rocks), ppb by atoms: 80
Sun - Total, ppb by weight: N/A
Sun - Total, ppb by atoms: N/A
Stream, ppb by weight: 5
Stream, ppb by atoms: 0.04
Meterorite (Carbonaceous), ppb by weight: 260
Meterorite (Carbonaceous), ppb by atoms: 30
Typical Human Body, ppb by weight: 200
Typical Human Body, ppb by atom: 10 atoms relative to C = 1000000
Universe, ppb by weight: 1
Universe, ppb by atom: 0.01
Discovered By: Bernard Courtois
Discovery Date: 1811
First Isolation: Bernard Courtois (1811)

Health, Safety & Transportation Information for Iodine

Iodine in large amounts is poisonous but in small doses is only slightly toxic. Safety data for Iodine and its compounds can vary widely depending on the form. For potential hazard information, toxicity, and road, sea and air transportation limitations, such as DOT Hazard Class, DOT Number, EU Number, NFPA Health rating and RTECS Class, please see the specific material or compound referenced in the Products tab. The below information applies to elemental (metallic) Iodine.

Safety Data
Material Safety Data Sheet MSDS
Signal Word Warning
Hazard Statements H312-H332-H400
Hazard Codes Xn,N
Risk Codes 20/21-50
Safety Precautions 23-25-61
RTECS Number NN1575000
Transport Information UN 1759 8/PG 2
WGK Germany 2
Globally Harmonized System of
Classification and Labelling (GHS)
Exclamation Mark-Acute Toxicity Environment-Hazardous to the aquatic environment

Iodine Isotopes

Iodine has one stable isotope: 127I.

Nuclide Isotopic Mass Half-Life Mode of Decay Nuclear Spin Magnetic Moment Binding Energy (MeV) Natural Abundance
(% by atom)
108I 107.94348(39)# 36(6) ms a to 104Sb; ß+ to 108Te; p to 107Te (1)# N/A 868.93 -
109I 108.93815(11) 103(5) µs p to 108Te; a to 105Sb (5/2+) N/A 886.32 -
110I 109.93524(33)# 650(20) ms ß+ to 110Te; a to 106Sb; ß+ + p to 109Sb; ß+ + a to 106Sn 1+# N/A 894.4 -
111I 110.93028(32)# 2.5(2) s ß+ to 111Te; a to 107Sb (5/2+)# N/A 902.48 -
112I 111.92797(23)# 3.42(11) s ß+ to 112Te; ß+ + p to 111Sb; ß+ + a to 108Sn; a to 108Sb N/A N/A 919.88 -
113I 112.92364(6) 6.6(2) s ß+ to 113Te; a to 109Sb; ß+ + a to 109Sn 5/2+# N/A 927.96 -
114I 113.92185(32)# 2.1(2) s ß+ to 114Te 1+ N/A 936.03 -
115I 114.91805(3) 1.3(2) min ß+ to 115Te (5/2+)# N/A 953.43 -
116I 115.91681(10) 2.91(15) s ß+ to 116Te 1+ N/A 961.51 -
117I 116.91365(3) 2.22(4) min ß+ to 117Te (5/2)+ N/A 969.59 -
118I 117.913074(21) 13.7(5) min ß+ to 118Te 2- N/A 977.66 -
119I 118.91007(3) 19.1(4) min ß+ to 119Te 5/2+ N/A 985.74 -
120I 119.910048(19) 81.6(2) min EC to 120Te 2- 1.23 993.82 -
121I 120.907367(11) 2.12(1) h EC to 121Te 5/2+ 2.3 1011.22 -
122I 121.907589(6) 3.63(6) min EC to 122Te 1+ 0.94 1019.3 -
123I 122.905589(4) 13.2235(19) h EC to 123Te 5/2+ 2.82 1027.37 -
124I 123.9062099(25) 4.1760(3) d EC to 124Te 2- 1.44 1035.45 -
125I 124.9046302(16) 59.400(10) d EC to 125Te 5/2+ 2.82 1043.53 -
126I 125.905624(4) 12.93(5) d EC to 126Te; ß- to 126Xe 2- 1.44 1051.61 -
127I 126.904473(4) STABLE - 5/2+ 2.81328 1059.69 100
128I 127.905809(4) 24.99(2) min EC to 128Te; ß- to 128Xe 1+ N/A 1067.77 -
129I 128.904988(3) 1.57(4)E+7 y ß- to 129Xe 7/2+ 2.621 1075.85 -
130I 129.906674(3) 12.36(1) h ß- to 130Xe 5+ 3.35 1083.93 -
131I 130.9061246(12) 8.02070(11) d ß- to 131Xe 7/2+ 2.742 1092 -
132I 131.907997(6) 2.295(13) h ß- to 132Xe 4+ N/A 1100.08 -
133I 132.907797(5) 20.8(1) h ß- to 133Xe 7/2+ 2.86 1108.16 -
134I 133.909744(9) 52.5(2) min ß- to 134Xe (4)+ N/A 1116.24 -
135I 134.910048(8) 6.57(2) h ß- to 135Xe 7/2+ N/A 1115 -
136I 135.91465(5) 83.4(10) s ß- to 136Xe (1-) N/A 1123.08 -
137I 136.917871(30) 24.13(12) s ß- to 136Xe; ß- + n to 135Xe (7/2+) N/A 1131.16 -
138I 137.92235(9) 6.23(3) s ß- to 137Xe; ß- + n to 136Xe (2-) N/A 1129.92 -
139I 138.92610(3) 2.282(10) s ß- to 138Xe; ß- + n to 139Xe 7/2+# N/A 1138 -
140I 139.93100(21)# 860(40) ms ß- to 139Xe; ß- + n to 140Xe (3)(-#) N/A 1136.76 -
141I 140.93503(21)# 430(20) ms ß- to 140Xe; ß- + n to 141Xe 7/2+# N/A 1144.84 -
142I 141.94018(43)# ~200 ms ß- to 141Xe; ß- + n to 142Xe 2-# N/A 1143.6 -
143I 142.94456(43)# 100# ms [>300 ns] ß- to 143Xe 7/2+# N/A 1151.68 -
144I 143.94999(54)# 50# ms [>300 ns] ß- to 144Xe 1-# N/A 1159.76 -