Iodine (I, Latin iodum) – a chemical element from the group of 17 – halogen in the periodic table. Its name comes from stgr. ἰοειδής ioeides – purple.
There is only one stable isotope 127I in nature. Radioactive isotopes: 123I, 125I, 129I, 131I.
Occurrence
Iodine is one of the elements that is not very common in nature. Larger amounts are found in seawater and brine, it also accompanies potassium nitrate, e.g. in Chile. It is also found in many everyday foods such as milk, eggs, vegetables and fish, as long as they come from areas where iodine is found in soil and water.Iodine compounds and reactivity

• Inorganic Chemistry
Iodine is very reactive, like other halogens, especially to metals with which it produces iodides. Forms numerous chemical compounds in which it occurs as one, three, five or seven-valued. Forms explosive nitrogen iodide in reaction with ammonia. The properties of iodine compounds are similar to those of bromine or chlorine. The most important inorganic iodine compounds are: potassium iodide, hydrogen iodide, iodates, periodates.
• Organic chemistry
The C-I bond is the least stable among halogen to carbon bonds (iodide anion is a fairly good leaving group) and is relatively easily substituted by nucleophiles. The reactivity of alkyl iodides is used in alkylation reactions, especially for methylation with methyl iodide CH3I (milder than dimethyl sulfate). At the same time, it is the cause of the highly toxic properties of these compounds.

Iodine can form complex compounds with non-ionic surfactants (so-called iodophores). In such complexes, iodine binds to the hydrogen of the hydroxyl group of the nonionic compound via a hydrogen bridge type coordination bond. This bond gives iodophores gentle skin properties with strong microbicidal and cleaning properties.

Iodine has mild oxidizing properties, e.g. towards organic phosphorus compounds:

PIII → PV

In an alkaline environment, the P-I bond in iodophosphoric acid (RO) 2P (O) I diesters is extremely reactive to nucleophiles, resulting in a variety of phosphoric acid derivatives (diesters, triesters, amidoesters, pyrophosphates).
In an alkaline environment, iodine reacts very easily with aldehydes and ketones having a methyl group in the α position to form iodoform (haloform reaction). This reaction is used in analytics, while iodoform itself is one of the first antiseptics.

Application
Iodine is used in the production of dyes and in photography. In medicine, it is used in the treatment and prevention of thyroid disease and as a disinfectant in the form of iodine. Radioactive isotopes 123I and 131I are used in the diagnosis and treatment of thyroid disease. Iodine is also used to detect starch in a chemical analysis (iodine test), in the form of iodine or Lugol’s liquid. In addition, iodine pairs are used to develop thin layer chromatography plates.

Biological Importance
Iodine is an essential trace element for humans. It is provided with food and water. Soils and water of coastal areas are the richest in iodine, the further from the sea the soils are becoming poorer in this element. The least iodine is in the soil and water of the mountain and foothill areas. A healthy human body contains 30-50 mg iodine. Most iodine occurs in the thyroid gland, which has the ability to actively accumulate this element. Without iodine, thyroxine (T4) and triiodothyronine (T3), necessary for proper functioning of all body cells, cannot be produced in thyroid follicles.
Iodine deficiency in food and water can lead to thyroid goiter after some time. Such threats occur mainly in mountainous areas. In the past, these areas were places of endemic goiter in adults and congenital cretinism in children. Iodine deficiency is now eliminated by iodizing table salt or adding iodine compounds to flour.
Iodine deficiency in children reduces learning ability, slows down growth and physical development during puberty. In adults, it can impair reproductive function, hinder pregnancy, lead to thyroid failure and, as a consequence, to inhibit the function of many organs and life processes of the body.
The daily demand for iodine is very small, around 200 micrograms, so you only need a few grams of iodine throughout your life. Some vegetables mainly from the brassica family (e.g. cabbage) contain rodates which, acting antagonistically to iodides, may worsen the course of some thyroid disorders.

Iodine content in sample products [µg / 100g]:
• cod, haddock 116
• tuna 20
• pork 7-12
• beef 15–19
• chicken 15-18
• milk 21-25
• white cheese 26
• egg approx. 25 pcs.
• rice 3

Daily demand for iodine in various periods of life:
• infants up to 1 year old 50 μg
• children 1-3 years 70 μg
• children up to 6 years old 90 μg
• up to 6-10 years of 120 μg
• adolescents 150 μg
• adults on average 200 μg
• pregnant women 230 μg
• nursing women 260 μg

From about 12 weeks, the fetus begins to produce its own thyroid hormones. She takes the needed iodine from her mother, also during breastfeeding the mother’s only source of iodine for her child. Therefore, especially during these periods, iodine supplementation in mother’s food is important.

Around 1.5 billion people in the world live in areas affected by iodine deficiency, and endemic goiter occurs in about 600 million people. The same problem occurs in Europe (except for the narrow Baltic coast belt), especially in the southern provinces.

Iodine in its pure form is toxic. Contact with skin causes severe irritation, wounds and necrotic changes. After ingestion, it causes dark-colored vomiting, stomach ache, dark patches in the mouth and tinnitus. In more severe cases, shock, delirium and stupor may occur. Large doses of iodine can damage the walls of the stomach and intestines, as well as the kidneys.

Prolonged use of increased doses of iodine leads to the development of iodine. The lethal dose for humans is 3-4 g.