Atomic / phase data
Reference values are evaluated.
I2 molecular solid
Elemental iodine is molecular I2; solid packing is represented schematically.
Iodide distinction
I-, I2 and I atom are explicitly separated.
Geography
Brine/caliche supply points are dated examples.
The lens describes evidence status, not confidence theatre. “Unknown” is kept unknown, and teaching schematics are not presented as direct measurements.
Iodine (I)
Iodine is element 53, a halogen that appears as a dark molecular solid and violet vapor. Its chemistry makes a crucial distinction among I atoms, I₂ molecules and iodide I⁻, while ¹²⁷I dominates natural iodine.
Iodine atomic number, mass, electron configuration and key properties
Iodine: quick answers
How many protons, neutrons and electrons does iodine have?
Iodine’s atomic number is 53, so every iodine atom has 53 protons, and a neutral atom also has 53 electrons. Its most common natural isotope, iodine-127, has 74 neutrons (other isotopes have different neutron counts).
What is the symbol for iodine?
The chemical symbol for iodine is I.
Is iodine a solid, liquid or gas at room temperature?
Iodine is a solid at room temperature (about 25 °C).
What family (group) is iodine in?
Iodine is a halogen, in group 17, period 5 of the periodic table.
How many valence electrons does iodine have?
Iodine has 7 valence electrons, the electrons in its outer shell, which matches its position in group 17.
What is the electron configuration of iodine?
The ground-state electron configuration of iodine is [Kr] 4d¹⁰ 5s² 5p⁵.
From atomic number to chemistry
Read these as a chain of causes, not as isolated facts. Each step links to the concept hub if you want the underlying idea explained.
53 protons define iodine.
The ground-state configuration provides the starting point for its chemistry.
Periodic position organizes recurring chemistry and trends.
The teaching nucleus is one isotope, not the relative atomic mass.
Phase boundaries are condition-dependent and evidence-labelled.
Iodine in its period and family
Iodine is the Period 5 halogen in Group 17. Its seven outer electrons explain the strong tendency to gain one electron in iodide chemistry, while heavier-halogen polarizability permits positive oxidation states in compounds.
Iodine Visual Lab
Switch among I atom, I₂ and I⁻, inspect the 5p orbital, rotate an orthorhombic I₂ packing model, and explore sublimation/vapor color, brine/caliche supply and isotope context.
Every mark points to one exact feature
The numbered markers explain the same information system used throughout Element Lookup. Unknown or predicted fields remain visibly labelled rather than being replaced with guesses.
Iodine in one minute
Atomic number 53 means 53 protons.
Neutral iodine has seven valence electrons and often forms iodide I⁻.
Elemental iodine at ordinary conditions is molecular I₂, not isolated I atoms.
Solid iodine is dark and lustrous; iodine vapor is violet.
¹²⁷I is the only stable natural iodine isotope.
Shell rings organize electron counts. They are not electron trajectories or orbital shapes.
2 · 8 · 18 · 18 · 7 electrons
The 5p probability distributions belong to an isolated iodine atom. Elemental iodine is I₂ and solid iodine is a molecular crystal, so the atomic orbital is not a picture of the bulk solid.
Solid elemental iodine consists of I₂ molecules arranged in an orthorhombic molecular crystal. The viewer emphasizes molecular identity and packing rather than exact refined coordinates.. The viewer is evidence-aware: measured structures are identified as such; unknown bulk structures stay unknown.
Teaching visualization; not a literal finite sample or thermal trajectory.What this model does—and does not—show
The 5p probability distributions belong to an isolated iodine atom. Elemental iodine is I₂ and solid iodine is a molecular crystal, so the atomic orbital is not a picture of the bulk solid.
Where do I meet iodine?
Clickable learning cards connect the element to materials, environment, technology, biology or research - depending on what the evidence actually supports.
Nutrition as iodide
Iodine is an essential nutrient, but biology uses iodide/iodinated molecules rather than chunks of elemental I₂.
I atom, I₂ molecule and I⁻ ion are different
Iodine is a clear example of why element names must not blur atoms, elemental molecules and ions.
Neutral iodine atom
Seven valence electrons characterize the isolated atom.
Chlorine, bromine and iodine
Compare nearby elements to see which patterns repeat and which properties remain element-specific.
| Period | 3 |
|---|---|
| State 20°C | Gas |
| Period | 4 |
|---|---|
| State 20°C | Liquid |
| Period | 5 |
|---|---|
| State 20°C | Solid |
Iodine properties: atomic, physical, thermal and chemical
Categories follow the science of this element rather than a fixed decorative template. Each row carries condition/provenance context and an evidence label; unknown values stay unknown.
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Atomic number | 53 | Source-reviewed; see Sources below | Evaluated |
| Relative atomic mass | 126.90447 | Source-reviewed; see Sources below | Evaluated |
| Ground-state electron configuration | [Kr] 4d¹⁰ 5s² 5p⁵ | Source-reviewed; see Sources below | Evaluated |
| Group / period / block | Group 17 · Period 5 · p-block | Periodic-table placement | Evaluated |
| Electronegativity | 2.66 | Source-reviewed; see Sources below | Evaluated |
| Reference isotope | ¹²⁷I | Source-reviewed; see Sources below | Evaluated |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| State context | Bluish-black lustrous molecular solid at 20 °C | Source-reviewed; see Sources below | Evaluated |
| Density | 4.933 g/cm³ | Source-reviewed; see Sources below | Evaluated |
| Material / molecular structure | Solid iodine · orthorhombic molecular I₂ crystal | Solid elemental iodine consists of I₂ molecules arranged in an orthorhombic molecular crystal. The viewer emphasizes molecular identity and packing rather than exact refined coordinates. | Measured |
| Classification | Halogen | Periodic-table / chemistry classification | Evaluated |
| Structure-model scope | Solid elemental iodine consists of I₂ molecules arranged in an orthorhombic molecular crystal. The viewer emphasizes molecular identity and packing rather than exact refined coordinates. | Teaching visualization; exact crystallographic coordinates are not implied unless stated. | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Melting / transition reference | 386.9 K | Source-reviewed; see Sources below | Evaluated |
| Boiling / gas reference | 457.6 K | Source-reviewed; see Sources below | Evaluated |
| Phase-path context | At approximately standard pressure, elemental iodine is solid below about 386.9 K, liquid to about 457.6 K, and gaseous above. Iodine also has appreciable vapor pressure below its melting point, so sublimation/deposition can be observed without contradicting the phase boundaries. | Shared phase registry drives the slider, regions and markers. | Evaluated |
| Condition warning | Temperature and pressure define phase behavior; purity/allotropy may matter. | Teaching condition statement | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Common oxidation states | −1, +1, +3, +5, +7 | Source-reviewed; see Sources below | Evaluated |
| Ion / common ion context | I⁻ (iodide) | Source-reviewed; see Sources below | Evaluated |
| Periodic chemistry context | Iodine is the Period 5 halogen in Group 17. Its seven outer electrons explain the strong tendency to gain one electron in iodide chemistry, while heavier-halogen polarizability permits positive oxidation states in compounds. | Element-specific interpretation | Reviewed |
| Chemistry caution | Elemental form, ions and compounds are distinct chemical objects. | Interpretive teaching rule | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| ¹²⁷I | Stable · essentially all natural iodine | Reference teaching nucleus: 53 protons and 74 neutrons. | Evaluated |
| ¹²⁹I | Long-lived radionuclide | Important in environmental and nuclear-science tracing; it is not a stable natural isotope. | Evaluated |
| ¹³¹I | Short-lived medical radionuclide | Widely known in thyroid diagnostics/therapy and nuclear monitoring; use requires professional controls. | Evaluated |
| Teaching nucleus | ¹²⁷I · 53 protons + 74 neutrons | Reference isotope used in the nucleus model | Reviewed |
| Property | Value | Context / provenance | Evidence |
|---|---|---|---|
| Page evidence note | Atomic/phase values and I₂ molecular identity are evaluated/measured. Resource pins are dated supply examples and are kept separate from nutritional and medical chemistry. | Evidence summary for this guide | Reviewed |
| Structure evidence | Solid elemental iodine consists of I₂ molecules arranged in an orthorhombic molecular crystal. The viewer emphasizes molecular identity and packing rather than exact refined coordinates. | Measured structure, labelled schematic, prediction or explicit unknown as applicable. | Reviewed |
| Map evidence rule | Real pins are reviewed examples; conceptual layers are used when pins would mislead. | Geography Explorer 2.0 | Reviewed |
| Source set | 3 primary/reference links listed below | Open the Sources section for the actual references. | Reviewed |
Is Iodine a solid, liquid or gas? State at temperature
At approximately standard pressure, elemental iodine is solid below about 386.9 K, liquid to about 457.6 K, and gaseous above. Iodine also has appreciable vapor pressure below its melting point, so sublimation/deposition can be observed without contradicting the phase boundaries.
Where on Earth is Iodine found or produced?
Who discovered Iodine, and when?
Bernard Courtois discovered iodine while processing seaweed ash in France.
Gay-Lussac and Davy helped establish iodine as a new element.
Iodine compounds became important in medicine, photography and chemical synthesis.
Iodine supply comes mainly from specialized brines and caliche/nitrate deposits, while iodine chemistry remains important in health and industry.
From iodine-bearing brine or caliche to iodine products: high-level context
Iodine occurs mainly as iodide/iodate species in specialized brines and mineral deposits rather than as native I₂ accumulations.
Industrial recovery concentrates iodine-bearing streams from resource-specific operations.
Chemical conversion produces elemental iodine and iodine compounds for controlled commercial uses.
Products enter pharmaceutical, nutritional, industrial and analytical supply chains; radioactive iodine use is handled under specialized medical/nuclear controls.
What is iodine used for?
Nutrition
Iodized salt and other food systems supply iodine in safe chemical forms needed for thyroid hormone production.
Medical applications
Stable iodine compounds and selected radioisotopes support imaging, antisepsis and thyroid medicine.
Specialty chemistry
Iodine reagents and compounds are used in pharmaceuticals, catalysts and organic synthesis.
Optical/electronic materials
Iodine-containing materials appear in specialized polarizers, lamps and other technical applications.
Iodine isotopes and natural abundance
¹²⁷I
Stable · essentially all natural iodineReference teaching nucleus: 53 protons and 74 neutrons.
¹²⁹I
Long-lived radionuclideImportant in environmental and nuclear-science tracing; it is not a stable natural isotope.
¹³¹I
Short-lived medical radionuclideWidely known in thyroid diagnostics/therapy and nuclear monitoring; use requires professional controls.
Five-question Iodine check
Atomic number?
Ordinary elemental form?
Typical iodide charge?
Vapor color?
Stable natural isotope?
Iodine questions students commonly ask
Each answer starts with the direct fact, then explains the chemistry, evidence or material context so the result is understandable rather than merely memorized.
What is iodine’s atomic number?
Short answer: 53.
Atomic number is defined by proton count, so 53 protons are what make an atom iodine. A neutral iodine atom also has 53 electrons, while isotopes can have different neutron counts without changing the element.
Key point: Atomic number = proton count.
Is iodine a metal?
Short answer: No. Iodine is a halogen nonmetal.
This guide classifies Iodine as a halogen. Its periodic position is Period 5, p-block, Group 17. Iodine is the Period 5 halogen in Group 17. Its seven outer electrons explain the strong tendency to gain one electron in iodide chemistry, while heavier-halogen polarizability permits positive oxidation states in compounds.
Key point: Periodic-table classification describes recurring atomic/chemical behavior; it does not make every element in a column physically identical.
What color is iodine?
Short answer: Solid iodine is bluish-black/dark lustrous; the vapor is violet.
The ordinary elemental-material description used here is: Bluish-black lustrous molecular solid at 20 °C. Solid iodine is dark and lustrous; iodine vapor is violet.
Key point: Elemental appearance can differ from the colors of its ions, compounds, oxide films or host materials.
How many valence electrons does iodine have?
Short answer: Seven in the neutral atom.
The neutral-atom ground-state reference used on this page is [Kr] 4d¹⁰ 5s² 5p⁵. This is an isolated-atom reference: bonding and ion formation can change which outer electrons are present or chemically active. The listed common oxidation-state context is −1, +1, +3, +5, +7, which helps connect the atomic configuration to ordinary chemistry without treating electron counting as a single universal rule.
Key point: Electron configuration is a ground-state atomic reference, not a literal picture of every compound.
Is iodide the same as elemental iodine?
Short answer: No. Iodide is I⁻; elemental iodine is mainly I₂ molecules.
The word “iodine” is often used casually for very different chemical forms. Elemental iodine is I₂; iodide is I⁻ in salts and biology. Keeping those forms separate prevents many health and chemistry misconceptions. Nutrition Iodized salt and other food systems supply iodine in safe chemical forms needed for thyroid hormone production.
Key point: The pure element, its ions, compounds and alloys are different materials and should not be treated as interchangeable.
Why is iodine important in nutrition?
Short answer: The body needs iodine to make thyroid hormones, normally supplied in ionic/compound forms rather than elemental iodine.
Nutrition as iodide Iodine is an essential nutrient, but biology uses iodide/iodinated molecules rather than chunks of elemental I₂. Nutrition Iodized salt and other food systems supply iodine in safe chemical forms needed for thyroid hormone production.
Key point: The mechanism matters: connect the observed behavior to electron structure, bonding, phase or the specific material form rather than memorizing the result alone.
Scientific sources for Iodine
- Royal Society of Chemistry - Iodine
- NIST - Atomic Data for Iodine
- USGS - Mineral Commodity Summaries 2026
Questions to ask next about Iodine
A good element lesson should lead to the next useful question, not end after a list of facts.
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