Iodine – History, Naming, Occurrence, Properties & Isotopes

Introduction to Iodine

Iodine is a member of the halogen family in group VII A of the periodic table placed at the fourth number.

  • Iodine contains 53 electrons and 53 protons in its neutral form.
  • The atomic mass of iodine is 127.
  • This tells us that it has 74 neutrons in its nucleus.
Naming and History

Iodine gets its name from the Greek word “Iodes” which means violet. This is because it has a violet color due to the reflecting wavelength of visible light.

Bernard Courtois in 1811 discovered iodine as a new element. He added sulphuric acid on seaweed and observed purple fumes which he guessed was a new element. For further systematic investigations, he sent this to Charles Bernard and Nicolas Clement who also confirmed this.


Occurrence of Iodine

Iodine is present in both Earth’s crust and oceans but in greater amounts is in oceans. It occurs in the form of iodine ions in seawater and seaweed. Some amount of iodine is also present near the underground reservoirs of oil and gas. It is also a part of some minerals and soils.

Properties of Iodine

Iodine is a bluish-black, non-metallic, least reactive lustrous solid in the halogen family. It is most electropositive. Although it is non-metal but it also shows some properties of metals.

The melting point of iodine is 113.7 ᵒC and the boiling point is 184.4 ᵒC. It has a density of 4.93 grams per cubic centimeter. It is the heaviest and least stable halogen.

As iodine belongs to the halogen family, it means it is salt-forming. On reaction with metals iodine forms salts such as sodium iodide, potassium iodide, magnesium iodide, etc.

Iodine has slight toxicity and can really be harmful in large amounts. It can cause irritation to the skin, form sores, and cause irritation in the eyes.


Iodine in biological systems

While iodine is not an essential nutrient for plant growth, fertilizing with iodine derivatives has been shown to aid in biomass production and increase the antioxidant levels in plants which help during drought and stress resistance.

It is also the part of some aquatic plants where it maintains the metabolism and antioxidant levels.

Iodine (I) is a necessary component of proteins produced by the thyroid. It is therefore essential for maintaining the appropriate performance of the thyroid gland. Iodine promotes the healthy growth and development of young animals.

Uses of Iodine

Today, iodine has numerous commercial uses. Iodide salts are utilized in pharmaceuticals and disinfectants, printing inks and dyes, drivers, animal feed supplements, and photographic chemicals.

Iodide is included in small amounts in salt, in order to avoid iodine deficiency affecting the thyroid gland. The radioactive isotope iodine-131 is in some cases utilized to deal with cancerous thyroid glands.

Iodine is used in medical treatment as cast and iodoform and is used in the preparation of certain drugs.

It is likewise an ingredient of water purification tablets that are utilized for drinking water preparation.

Potassium iodide (KI) is used to make photographic films and, when combined with iodine in alcohol, as an antibacterial for external injuries.

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Iodine is utilized as a test for starch and turns a deep blue when it can be found in contact with it.


There are 30 recognized isotopes of iodine, however only one is naturally occurring, the I-127. The radioactive isotope of iodine that is Iodine-131 has significance in the medical field.

It is utilized in medication to monitor thyroid gland working, to deal with goiter and thyroid cancer, and to locate tumors of the brain and of the liver. It is also utilized in examinations to trace the course of substances in metabolism.

MCQs on Iodine

  • What is the atomic mass of iodine?
    • A) 53
    • B) 74
    • C) 127
    • D) 181
    • Answer: C) 127
  • Who discovered iodine in 1811?
    • A) Charles Bernard
    • B) Nicolas Clement
    • C) Bernard Courtois
    • D) Iodine-131
    • Answer: C) Bernard Courtois
  • What does the name “iodine” mean in Greek?
    • A) Blue
    • B) Violet
    • C) Green
    • D) Yellow
    • Answer: B) Violet
  • In which form does iodine occur in seawater and seaweed?
    • A) Iodine gas
    • B) Iodine crystals
    • C) Iodine ions
    • D) Iodine molecules
    • Answer: C) Iodine ions
  • What is the melting point of iodine?
    • A) 84.2 ᵒC
    • B) 113.7 ᵒC
    • C) 150 ᵒC
    • D) 184.4 ᵒC
    • Answer: B) 113.7 ᵒC
  • Which family does iodine belong to in the periodic table?
    • A) Noble gases
    • B) Alkali metals
    • C) Halogens
    • D) Transition metals
    • Answer: C) Halogens
  • What is iodine’s density?
    • A) 2.56 grams per cubic centimeter
    • B) 4.93 grams per cubic centimeter
    • C) 6.71 grams per cubic centimeter
    • D) 8.19 grams per cubic centimeter
    • Answer: B) 4.93 grams per cubic centimeter
  • What type of reactions does iodine undergo with metals?
    • A) Acid-base reactions
    • B) Redox reactions
    • C) Salting reactions
    • D) Halogenation reactions
    • Answer: D) Halogenation reactions
  • What is a potential harm of iodine in large amounts?
    • A) Respiratory issues
    • B) Digestive problems
    • C) Skin irritation
    • D) Joint pain
    • Answer: C) Skin irritation
  • In biological systems, what is the role of iodine in plant growth?
    • A) Essential nutrient
    • B) Antioxidant production
    • C) Chlorophyll synthesis
    • D) Nitrogen fixation
    • Answer: B) Antioxidant production
  • What is the commercial use of potassium iodide (KI)?
    • A) Water purification
    • B) Animal feed supplements
    • C) Photographic films
    • D) Printing inks
    • Answer: C) Photographic films
  • How many recognized isotopes of iodine are there?
    • A) 15
    • B) 22
    • C) 30
    • D) 42
    • Answer: C) 30
  • Which is the naturally occurring isotope of iodine?
    • A) Iodine-127
    • B) Iodine-131
    • C) Iodine-150
    • D) Iodine-181
    • Answer: A) Iodine-127
  • What is the significance of Iodine-131 in the medical field?
    • A) Treatment of goiter
    • B) Brain and liver tumor detection
    • C) Antibacterial agent
    • D) Water purification
    • Answer: B) Brain and liver tumor detection
  • What is the role of iodine-131 in medication?
    • A) Treatment of cancerous thyroid glands
    • B) Water purification
    • C) Starch testing
    • D) Chlorophyll synthesis
    • Answer: A) Treatment of cancerous thyroid glands
  • What is iodine used for in water purification tablets?
    • A) Antibacterial agent
    • B) Antioxidant
    • C) Starch test
    • D) Disinfectant
    • Answer: A) Antibacterial agent
  • How many electrons and protons does iodine have in its neutral form?
    • A) 53
    • B) 74
    • C) 127
    • D) 181
    • Answer: A) 53
  • What color does iodine turn when it comes in contact with starch?
    • A) Red
    • B) Green
    • C) Blue
    • D) Yellow
    • Answer: C) Blue
  • What property makes iodine unique among halogens?
    • A) Least reactive
    • B) Most electropositive
    • C) Metallic conductivity
    • D) Noble gas characteristics
    • Answer: B) Most electropositive
  • What is the primary use of iodine-131 in medical examinations?
    • A) Treatment of cancerous thyroid glands
    • B) Brain and liver tumor detection
    • C) Monitoring thyroid gland functioning
    • D) Water purification
    • Answer: C) Monitoring thyroid gland functioning
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FAQs about Iodine – History, Naming, Occurrence, Properties & Isotopes

1. What is iodine, and where does it belong in the periodic table?

  • Iodine is a member of which family in the periodic table?

2. How did iodine get its name, and why is it called “violet”?

  • What is the etymology of the name “iodine,” and what property gives it a violet color?

3. Who discovered iodine, and how was it initially identified?

  • Provide details about the discovery of iodine and the process used by Bernard Courtois in 1811.

4. Where is iodine found in nature, and in what forms does it occur?

  • What is the distribution of iodine in Earth’s crust, oceans, and underground reservoirs?

5. Describe the key properties of iodine.

  • What are the physical characteristics and reactivity of iodine, especially in relation to the halogen family?

6. How does iodine interact with metals, and what compounds does it form?

  • Explain the salt-forming nature of iodine and provide examples of iodine compounds with metals.

7. What are the uses of iodine in biological systems?

  • Is iodine essential for plant growth, and how does it contribute to the development of young animals?

8. What are the commercial applications of iodine?

  • List and elaborate on the various commercial uses of iodine, including its presence in pharmaceuticals and photographic chemicals.

9. What are the potential harms of iodine, and why is it included in salt?

  • Discuss the toxicity of iodine and its inclusion in salt to prevent deficiencies affecting the thyroid gland.

10. How is iodine used in medical treatments and examinations? – Provide information about iodine’s role in medical treatments, including its use in cancerous thyroid glands and examinations.

11. How many isotopes of iodine are there, and which one is naturally occurring? – Detail the number of recognized isotopes of iodine and identify the naturally occurring isotope.

12. What is the significance of Iodine-131 in the medical field? – Explain the medical applications of Iodine-131, such as monitoring thyroid gland function and detecting tumors.

13. How is iodine utilized in water purification, and what other compounds is it a part of? – Describe iodine’s role in water purification tablets and its presence in compounds like iodoform.

14. What are the properties of iodine that make it unique among halogens? – Highlight the distinctive characteristics of iodine within the halogen family.

15. How does iodine function as a test for starch? – Explain the iodine starch test and the color change associated with it.



In this comprehensive tutorial on iodine, we explored various facets of this halogen, spanning its introduction, historical context, occurrence, properties, biological significance, uses, and isotopes.

Introduction to Iodine: Iodine, a member of the halogen family, occupies the seventh group (VII A) in the periodic table. With 53 electrons and protons, iodine’s neutral form has an atomic mass of 127, indicating the presence of 74 neutrons.

Naming and History: The name “iodine” originates from the Greek word “Iodes,” signifying violet. Discovered by Bernard Courtois in 1811 through the observation of purple fumes from seaweed treated with sulfuric acid, iodine’s existence was later confirmed by Charles Bernard and Nicolas Clement.

Further Reading:  Alkanes [Properties, Structure, Reactivity, and Uses of Alkanes]

Occurrence of Iodine: Iodine is distributed in Earth’s crust and oceans, with a higher concentration in oceans. Present as iodine ions in seawater and seaweed, it is also found near underground oil and gas reservoirs, minerals, and soils.

Properties of Iodine: Iodine exhibits characteristics of a bluish-black, non-metallic, and least reactive lustrous solid. As the heaviest and least stable halogen, it is salt-forming and reacts with metals to form salts like sodium iodide and potassium iodide.

Iodine in Biological Systems: While not essential for plant growth, iodine derivatives contribute to biomass production and antioxidant levels in plants. In aquatic plants, iodine aids in metabolism and maintains antioxidant levels. Iodine is crucial for proteins produced by the thyroid, ensuring the proper functioning and growth of the thyroid gland and young animals.

Uses of Iodine: Iodine finds extensive commercial applications, including pharmaceuticals, disinfectants, inks, dyes, animal feed supplements, and photographic chemicals. Iodide in salt prevents thyroid issues, and the radioactive isotope iodine-131 is employed in medical treatments for cancerous thyroid glands.

Isotopes: Thirty isotopes of iodine are recognized, with only one, I-127, occurring naturally. Iodine-131, a radioactive isotope, plays a vital role in medical applications, monitoring thyroid function, treating goiter and thyroid cancer, and locating brain and liver tumors.

This tutorial provides a comprehensive understanding of iodine, encompassing its diverse applications and significance in various fields, from chemistry to medicine.