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What is Iodine

A crystalline metalloid, iodine (pronounced EYE-eh-dine) belongs to the family of halogens. Denoted by the chemical symbol I, it undergoes sublimation when heated to form a purple gaseous vapor, sometimes called as Iodine gas whose molecular formula is I2. Although it is less reactive than the other members in its group, it forms compounds with other elements [1,2].


How Many Isotopes Does it Have

It has 34 isotopes with mass numbers ranging from 108 to 141 of which only I-127 has a stable half-life.

Where is it Found

Seawater contains around 0.05 parts per million of the metalloid, existing in the form of iodide, most of which would absorb seaweed. The other major sources are iodate minerals, natural brine deposits, and brackish water in oil or salt wells [1].


Origin of its Name: Its name is derived from the Greek word ‘iodes’ which means violet [1].

Who Discovered it: Bernard Courtois

When, Where, and How was Iodine Discovered

In 1811, when Courtois added sulfuric acid to seaweed ash to extract sodium and potassium, he observed some purple fumes that condensed into crystals exhibiting a metallic luster. As he suspected it to be a new metal, some amount of the sample was given to two other scientists Charles-Bernard Desormes and Nicole Clement for further investigation.

In 1813, November, the element was given for public display at the Imperial Institute, Paris after which Joseph gau-Lussac and Humphry Davy confirmed it to be a new addition to the periodic table. However, when Davy sent a report to the Royal Institution in London, he was mistakenly taken to be the original discover [1, 2].

Iodine Element


Atomic number 53 [1]
CAS number 7553-56-2 [1]
Position in the periodic table [1] Group Period Block
17 5 p

Classification, Properties and Characteristics of Iodine

General Properties

Relative atomic mass 126.904 [1]
Atomic mass/weight 126.904  atomic mass units [4]
Molar mass 253.808940 ± 0.000060 g/mol [8]
Mass number 127

Physical Properties

Color/physical appearance Shiny black [1]
Odor Strong [5]
Melting point/freezing point 113.7°C (236.7°F) [1]
Boiling point 184.4°C (363.9°F)  [1]
Density 4.933 g/cm3 [1]
Standard state at room temperature (solid/liquid/gas) Solid [1]
Solubility Soluble in iodide solutions [1]

Magnetic Properties [6]

Magnetic type Diamagnetic
Molar magnetic susceptibility -1.14×10-9 m3/mol
Mass magnetic susceptibility -4.5×10-9 m3/kg
Volume magnetic susceptibility -0.0000222

Chemical Properties

Flammability Non-flammable [7]
Oxidation states (numbers) 7,5, 1,-1 [1]

Atomic Data of Iodine (Element 53)

Valence electrons 7 [8]
Quantum numbers [11]
– n 5
– ℓ 1
– m 0
– ms -1/2
Electron configuration (noble gas configuration) [Kr] 4d105s25p5[1]
Atomic structure [8]
– Number of Electrons 53
– Number of Neutrons 74
– Number of Protons 53
Radius of atom
– Atomic radius 1.98 Å [1]
– Covalent radius 1.36 Å [1]
Ionization energy [1]


1st 2nd 3rd 4th 5th 6th 7th
1008.393 1845.89

Iodine Atomic Structure (Bohr Model)

What is it Used for

  • It is used in polarizing filters of LCD display screen for a high durability with no color distortion [1].
  • Its addition to edible salts helps in preventing thyroid-related disorders [1].
  • Radioactive iodine (I-131) is used for both scanning and treating the thyroid gland in cases like cancer where its radiation directly acts on the damaged thyroid cells [9].
  • It is an active ingredient in many antiseptic creams and liquids to treat minor wounds, cuts and scrapes [10]. Decolorized or white iodine also has good antifungal properties to treat toenail infection.
  • It is an important agent used to test starch in different chemical and biology experiments.

Iodine Gas

Does it Have Any Toxic Effects

Accidental ingestion of the element can lead to vomiting, nausea, headache, and severe gastrointestinal issues as result of poisoning. Its vapors may affect the lungs, causing edema. Excessive application of iodine solution can cause irritation and burns [11].

Interesting Facts

  • After Louis Daguerre, a French artist and photographer, discovered a method to use I on a metal sample to produce images in 1839, the metalloid was used commercially for photography [1].
  • The thyroid gland in an average human body constitutes about 20 milligrams of iodine [1].

Iodine Pictures

Iodine Cost

The price of pure iodine may vary between $8 and $9 per 100g.



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