What is Iodine's atomic number?

Iodine has atomic number 53, meaning it has 53 protons in its nucleus. Its chemical symbol is I, from the Greek iodes, meaning violet-colored, a reference to its striking purple vapor.

Is Iodine a metal or nonmetal?

Iodine is a nonmetal and a halogen (group 17). It is a shiny blue-black solid at room temperature but sublimes easily to a purple-violet vapor. It does not conduct electricity and behaves as a typical nonmetal in its chemistry.

What is Iodine used for?

Key uses include antiseptics (povidone-iodine, Lugol's solution), thyroid hormone synthesis (thyroxine T4 and triiodothyronine T3), iodized salt to prevent goiter, iodinated contrast agents for X-ray and CT imaging, polarizing film in LCD screens, and catalysts in organic synthesis.

What is Iodine's electron configuration?

Iodine's electron configuration is [Kr] 4d10 5s2 5p5. With seven valence electrons it needs only one more to complete its octet, making -1 its most common oxidation state, though it also exhibits +1, +3, +5, and +7 in various compounds.

Is Iodine essential for human health?

Yes — iodine is a trace element essential for the synthesis of thyroid hormones. Deficiency causes goiter (enlarged thyroid) and, in severe cases, hypothyroidism or cretinism in developing fetuses. Global iodization of salt has largely eliminated iodine-deficiency disorders, though they remain a concern in some regions.

Who discovered Iodine?

Iodine was discovered in 1811 by French chemist Bernard Courtois while treating seaweed ash with sulfuric acid. The element was characterized and named by Joseph Louis Gay-Lussac and Humphry Davy, who proposed the name from the Greek iodes (violet) because of its vivid purple vapor.

Iodine Element Properties

Explore the complete physical and chemical properties of Iodine (I, element 53) — from its striking violet vapor and antiseptic uses to its essential role in thyroid health.

126.90

Iodine

Halogen · Period 5 · Group 17

Melting Point

113.7 °C

Boiling Point

184.3 °C

Density

4.933 g/cm³

Atomic Mass

126.904 u

Electron Shell Configuration

[Kr] 4d¹⁰ 5s² 5p⁵

Shells: 2, 8, 18, 18, 7

Filter Properties

Property	Value
Element Name	Iodine
Symbol	I
Atomic Number	53
Atomic Mass	126.904 u
Category	Halogen (Nonmetal)
Group	17 (VIIA / Halogens)
Period	5
Block	p-block
CAS Number	7553-56-2
Discovered	1811 by Bernard Courtois
Named By	Joseph Louis Gay-Lussac (1813)
Named After	Greek iodes, meaning violet-colored
Standard State	Solid at 25 °C
Appearance	Shiny blue-black solid; violet vapor on sublimation
Crystal Structure	Orthorhombic

Property	Value
Electron Configuration	[Kr] 4d10 5s2 5p5
Electrons Per Shell	2, 8, 18, 18, 7
Number of Protons	53
Number of Neutrons (I-127)	74
Number of Electrons	53
Valence Electrons	7
Oxidation States	-1, +1, +3, +5, +7
Electronegativity (Pauling)	2.66
Electron Affinity	295.2 kJ/mol
First Ionization Energy	1008.4 kJ/mol
Second Ionization Energy	1845.9 kJ/mol
Third Ionization Energy	3184 kJ/mol
Atomic Radius (calc.)	115 pm
Covalent Radius	139 pm
Van der Waals Radius	198 pm
Stable Isotopes	I-127 (100% — only stable isotope)

Property	Value
Melting Point	113.7 °C (236.7 °F / 386.85 K)
Boiling Point	184.3 °C (363.7 °F / 457.45 K)
Sublimation Point	Sublimes readily below boiling point at 1 atm
Density (solid, at 25 °C)	4.933 g/cm³
Density (liquid, at m.p.)	3.960 g/cm³
Heat of Fusion	15.52 kJ/mol
Heat of Vaporization	41.57 kJ/mol
Specific Heat Capacity	0.214 J/(g·K)
Thermal Conductivity	0.449 W/(m·K)
Electrical Resistivity	1.3×10⁷ Ω·m (at 0 °C)
Mohs Hardness	Soft — no Mohs value defined for I2
Crystal Structure	Orthorhombic (Cmce, I2 molecules)
Magnetic Ordering	Diamagnetic
Triple Point	113.6 °C / 12.1 kPa
Critical Point	546 °C / 11.7 MPa
Molar Volume	25.72 cm³/mol

Property	Value
Reactivity	Less reactive than lighter halogens; forms I2 diatomic molecules
Reaction with Metals	Reacts to form metal iodides (e.g., NaI, KI, FeI2)
Reaction with Water	Slight disproportionation: I2 + H2O = HI + HIO (aqueous)
Reaction with Alkenes	Addition across double bonds; used in starch-iodine test
Reaction with Starch	Forms deep blue-black inclusion complex (starch-iodine test)
Common Compounds	NaI, KI, HI, I2O5, AgI, CH3I, CF3I, povidone-iodine
Medical / Antiseptic Use	Povidone-iodine (Betadine), Lugol's solution for wound disinfection
Thyroid Use	Essential for thyroxine (T4) and triiodothyronine (T3) synthesis
Nutrition Use	Iodized table salt to prevent goiter and hypothyroidism
Imaging Use	Iodinated contrast agents for X-ray, CT, and fluoroscopy
LCD Use	Iodine-doped PVA film used as linear polarizer in LCD screens
Oxidation State Range	-1, +1, +3, +5, +7 (-1 most common in salts)
Radioactive Isotope I-131	Used in thyroid cancer therapy and diagnostic scans; t½ = 8.02 days
Toxicity	Elemental iodine is toxic and corrosive; iodide salts are safe at low doses
Natural Occurrence	Found in seawater, seaweed, brine wells, and caliche (Chile saltpeter)
Global Production	~30,000 tonnes/year; Chile and Japan are leading producers

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Summary

Iodine (I) is element 53 on the periodic table, a halogen in group 17 (VIIA), period 5. Discovered in 1811 by Bernard Courtois from seaweed ash, iodine is a shiny blue-black solid at room temperature that sublimes readily to a distinctive violet vapor. It is the heaviest stable halogen and the least reactive of the common halogens. Iodine is biologically essential — the thyroid gland requires it to synthesize hormones — and is widely used as an antiseptic, in photography, in organic synthesis, and in nuclear medicine. This reference tool presents all key physical, atomic, and chemical properties of Iodine in one organized view.

How it works

All Iodine property data is embedded directly in the page — no server call is needed.
Use the section tabs (Overview, Atomic, Physical, Chemical) to navigate different property groups.
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The electron configuration diagram shows the shell distribution visually.
Use the search box to filter properties by name or value.

Use cases

Quick reference for students studying periodic table halogens and group 17 trends.
Verify Iodine specs during pharmaceutical or biochemical research.
Teaching aid for illustrating halogen reactivity, sublimation, and diatomic molecules.
Look up Iodine data for antiseptic formulations or medical iodine contrast agents.
Cross-check Iodine toxicology data for lab safety and handling procedures.
Support chemistry homework and exam preparation on halogen chemistry.
Reference oxidation states for analytical or inorganic chemistry problems.
Explore Iodine isotope data for nuclear medicine and radiochemistry work.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu