Iodine is a dark crystalline solid that can produce a purple vapor. Its compounds have roles in photography, display materials, and biological chemistry. Iodide occurs in seawater and concentrated brines, connecting a striking elemental appearance to dissolved ions and the industrial separation of natural mixtures.
FROM ELEMENT TO APPLICATION
AN ELEMENT IN CONTEXT
What must change for silver iodide to participate in a photographic record?
Silver iodide compound
FOLLOW THIS RELATIONSHIP
Use a light-sensitive compound
Silver iodide combines silver and iodine in a material that interacts with illumination. Its photographic role belongs to this compound’s chemical response.
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IFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
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Why does violet iodine vapor not explain the photographic image’s pattern?
Atomic identity, reference properties and structure models
INTERACTIVE 3D2 visible sites
I₂ · two atoms, one moleculeDrag to rotate · scroll or pinch to zoom
An elemental molecule has more than one atom but only one element. A single connector identifies the molecular pair; bond order and electron density are not encoded.
What this representation means
Original idealized molecular geometry. Display sizes are illustrative; this is not a measured coordinate dataset.
Bernard Courtois encountered iodine in 1811 while processing seaweed-derived material for saltpetre manufacture. Unusual violet fumes and resulting prompted investigation. Other chemists examined the substance, and in 1813 Joseph Louis Gay-Lussac and Humphry Davy helped establish its elemental identity. The story shows how industrial work can reveal unexpected chemistry, while keeping the initial observation distinct from the later public case for a new element.
1811
Unusual vapor observed
Courtois encountered iodine during seaweed-ash processing.
1813
Elemental identity established
Further investigation and public reports supported identification of iodine.
Iodine is recovered from underground brines and iodine-bearing deposits; seaweed was historically important. In these sources, it is generally present in chemical combinations rather than as visible violet vapor. Resource occurrence and the appearance of purified elemental iodine therefore describe different stages of the same material story.
Iodine is a halogen that forms molecular I2 as an elemental substance. Its dark solid and violet vapor differ visibly from the ionic forms found in salts. Iodide and iodate compounds must therefore be named explicitly: an iodine-containing material does not necessarily share the appearance or reactivity of elemental iodine.
Each section links its supporting references. Descriptions and diagrams were authored for this atlas. Source checks completed. Source checks dated 2026-09-06.
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Family labels and the group-3 layout are teaching conventions. A table position does not establish an unknown bulk property. Electronic configurations refer to neutral atoms; ions and material environments require their own account.
Read each reference property together with its units, conditions and the reference information below.
Not yet recorded: electron affinity, second ionization energy, ground-state term symbol, van der waals radius, thermal conductivity, specific heat capacity, electrical resistivity, crystal structure, lattice parameter a, space group, abundance in seawater, abundance in the universe, annual production, supply-risk listing, hazard classification, biological role, cas registry number, pubchem cid, wikidata item. These families are declared and awaiting an ingestion run against the source each names; they are listed here rather than shown as empty rows.
Physical
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Phase at 20 °C
solid
293.15 K, 101.325 kPa
Melting point in K
386.85 K
at standard pressure unless noted
Boiling point in K
457.4 K
101.325 kPa
Density in g/cm³
4.93 g/cm³
solids and liquids at 293 K; gases at 273 K and 101.325 kPa
Atomic and electronic
Intended reference: Pauling electronegativity scale · Widely tabulated; original in The Nature of the Chemical Bond
Electronegativity in Pauling
2.66 Pauling
Pauling scale
First ionization energy in kJ/mol
1008 kJ/mol
gas-phase neutral atom, ground state
Covalent radius in pm
139 pm
single-bond covalent radius
Common oxidation states
-1, +1, +5, +7
states met in ordinary compounds; not exhaustive
Occurrence and identity
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Abundance in the crust in mg/kg
0.45 mg/kg
upper continental crust, mass fraction
Has stable isotopes
Yesderived
Discovery year in CE
1811 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity across period 5
I 2.66 Pauling · Iodine
Electronegativity across period 5. Exact values.
Rb
0.82
Sr
0.95
Y
1.22
Zr
1.33
Nb
1.6
Mo
2.16
Tc
1.9
Ru
2.2
Rh
2.28
Pd
2.2
Ag
1.93
Cd
1.69
In
1.78
Sn
1.96
Sb
2.05
Te
2.1
I
2.66
Xe
2.6
Electronegativity down group 17
I 2.66 Pauling · Iodine
Electronegativity down group 17. Exact values.
F
3.98
Cl
3.16
Br
2.96
I
2.66
At
2.2
The trend uses the same reference-property values as the periodic table. Elements with no recorded value are left out of the line rather than drawn at zero.
