What Is Iodine?

by Lucy Bell-Young

Iodine (I) is a chemical element that appears as a purple-black solid at room temperature. Classified as a halogen, it is best known as an essential mineral required by the body to make thyroid hormones. However, while its compound form has medical properties, iodine is very toxic in its elemental form. Having a comprehensive understanding this chemical may require a degree in chemistry, but this article will guide you through all the important information that you need to know about this element. 

What Are The Basic Properties Of Iodine?

Iodine is the heaviest element among the stable halogens. The halogens are five chemically related elements classified under group 17 of the periodic table. The other halogens include: 

  • Fluorine (F)
  • Chlorine (Cl)
  • Bromine (Br)
  • Astatine (At) 

Iodine is one of the rarest halogens to appear in its elemental form. In fact, it is ranked at 64 in terms of abundance when compared to other elements. 

Iodine has the atomic number 53 and a standard atomic weight of 127 u. Under standard temperature and pressure conditions, elemental iodine occurs as a non-metallic solid that is purple-black in colour. According to the International Union of Pure and Applied Chemistry (IUPAC), the standard conditions for elemental iodine are defined as:

  • 273.15 K (0 °C, 32 °F) temperature
  • 105 Pa (100 kPa, 1 bar) pressure

The physical properties of this chemical include a melting point of 114°C and a boiling point of 184°C. In addition, its density in solid form is 4.933 g/cm3, or about five times denser than water.

Owing to its large atomic radius, which is the largest of the halogens, iodine also has a very low ionisation energy and electron affinity. This means that compared to other halogens, it does not have high reactivity with many elements or compounds. Despite this, it has formed a variety of organic and inorganic compounds, which can be classified as:

  • Iodides‎
  • Iodates
  • Iodanes
  • Organoiodides‎
Iodine in the periodic table
Iodine is the heaviest element among the stable halogens and has the atomic number 53

What Is Iodine Used For?

Iodine was first chemically isolated from seaweed and identified by Bernard Courtois in 1811. Courtois originally intended to extract potassium compounds from the seaweed ash; but he accidentally discovered iodine when he further processed the ash by adding sulphuric acid to it, causing the mixture to produce a violet-coloured cloud.

  1. Medical Uses of Iodine

The first medical application of iodine was introduced in the 1820s as a treatment for syphilis. Since then, it has become mainly used as an antiseptic for first aid purposes as well as for surgical preparations. Iodine is a very potent antiseptic because it can kill microbial organisms, whether eukaryotes or prokaryotes. As such, it is a common product in the medicine cabinets of households, clinics, and hospitals.

Iodine’s medical uses are not limited to antiseptics, and actually go on to treat a range of medical conditions, from neurodevelopment during pregnancy to fibrocystic breast disease. This element is also crucial in maintaining the health of your thyroid, and it is prescribed by doctors to treat hyperthyroidism, hypothyroidism, goitres, and even thyroid cancer. 

An easy way to ensure that you are getting enough of this nutrient is by using iodised salt on your food. This is simply table salt that has been fortified with iodine in order to promote healthy cognitive function, as well as thyroid health. 

       2. Laboratory Analysis

Another common application of iodine is in chemical laboratory analysis, particularly in quantitative volumetric analysis, such as iodometry. One example of how it is used in this context is in the testing for the presence of starch: when iodine reacts with starch, the colour turns blue. This is a test that is still being used today to detect fake banknotes, which are commonly printed on paper materials containing starch. 

Iodine is also a good substance to use for diagnostic purposes, such as detecting and treating cancer:

  • Radioactive iodine, a.k.a. radioiodine, is commonly used for treating thyroid cancer
  • This radioactive counterpart now has the potential for diagnosing and treating other types of cancer, as well
  • The mechanism involves the synthesis of fatty acids and lipids associated with apoptosis and carcinogenesis

Another role iodine plays in laboratory analysis is in ammonia testing. A compound of iodine, potassium tetraiodomercurate(II), is an important chemical reagent known as Nessler’s reagent. This is what is used to test for ammonia. Another iodine compound, cuprous mercuric iodide, is similarly used as a reagent for testing for the presence of alkaloids.  

Bottle of open iodine
Iodine has a range of medical uses because of its bacteria-killing properties

What Colour Is Iodine?

In its solid elemental form, iodine is a glittering purple-black colour. Then, when it is subjected to a temperature of at least 114°C, it will melt into a deep violet liquid. It can also easily turn into a violet-colour gas through sublimation with the application of gentle heat.

The most common form of this element that many people are familiar with is the antiseptic tincture of iodine, which is yellowish brown. Household medicine cabinets would not be complete without a bottle of this stuff. This tincture is actually a mixture of different substances, including:

  • Iodine, 2.5% by density
  • Potassium iodide, 2.5% by density
  • Ethanol, 89% by volume
  • Purified water

What Does Iodine Do?

Depending on whether it’s in its elemental form or part of a compound chemical, iodine has a wide variety of possible chemical reactions. Although not as reactive as the other halogens, it still combines with other elements to form either inorganic or organic compounds. This allows it to do a range of things: 

  • It is an effective antibacterial because it kills the proteins that microorganisms need for cellular respiration, making energy, and maintaining their cell structure. Exactly how this happens is still a bit of a mystery, but it involves an oxidation-reduction reaction where iodine is reduced to iodide while proteins are oxidised, causing the dependent microorganisms to die. 
  • Iodine is an essential nutrient that is needed by the thyroid gland to synthesise thyroid hormones. These hormones are triiodothyronine (T3) and thyroxine (T4), and they have an important job in regulating metabolism and growth.
  • This element also has a crucial role in the health of neurons. Similar to the above, its role in maintaining thyroid health means that iodine also regulates how the thyroid hormone affects brain development. This is why deficiency in this nutrient can lead to some level of intellectual impairment, particularly among infants and children whose brains are still developing. 

Even though it can combine with other elements to do a variety of things, iodine is still not as highly reactive as other halogens. This is because its reactivity with other elements and compounds is affected by its atomic size, valence electrons, and electronegativity. For example, it does not readily react with oxygen in the air, but it does react with ozone, which is a highly reactive and unstable form of oxygen.

Person using a cotton swab to apply tincture of iodine to knee
Tincture of iodine is the most common form of iodine, and is recognised by its yellow-brown colour.

What Is The Chemical Symbol For Iodine?

Iodine is represented by the chemical symbol I. This element, though discovered by Bernard Courtois, was named by Joseph Louis Gay-Lussac as iode or iodis, derived from a Greek word (ἰώδης) meaning violet-coloured.

But element number 53 isn’t the only element whose name has Ancient Greek origin. Some other examples of elements discovered in the 1800s that have Greek or Latin etymology are the following:

  • Cerium, named after the asteroid Ceres
  • Rhodium, derived from the Greek word for rose
  • Lithium, derived from the Greek word ‘lithos,’ meaning stone
  • Niobium, named after Niobe, daughter of Tantalus in Greek mythology
  • Tantalum, from the Greek myth, Tantalos, father of Niobe. He was the king of Sipylus in Lydia
  • Iridium, the origin of which is the Latin word ‘iris,’ meaning rainbow

Iodine Structure

While iodine is an element, the molecular structure of its solid form is crystalline. The molecules are arranged in a face-centred cubic, which means that a cube of iodine molecules has a molecule that is located at the centre of each face.

The molecular structure of this element determines its polarity, which in turn determines its solubility in water. As such, it is only slightly soluble in water, but readily dissolves in organic solvents. Atomically, it has 53 protons and 74 neutrons which, when added together, indicate its atomic weight: 127.

Meanwhile, the electrons are distributed among the energy levels in the following manner:

  • 1st: 2 electrons
  • 2nd: 8 electrons
  • 3rd: 18 electrons
  • 4th: 18 electrons
  • 5th: 7 electrons
Diagrams of iodine's chemical structure
Iodine has 53 protons and 74 neutrons, and its molecules are arranged in a face-centred cubic.

Chemical Formula

Since it is an element, its chemical formula solely contains iodine. However, because each molecule is not a single atom, but rather a pair of atoms, it is represented as I2. When dissolved in an aqueous solution, iodine exists as ions, I ions, or as iodate, which is a salt of iodinic acid with IO3 anion.

The balanced equation for when it is dissolved in water is:

I2(l) + H2O(l) -> OI(aq)+ 2H+(aq)+ I(aq)


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