Thallium is a soft, toxic metal with specialized uses in electronic and optical materials. Its oxide can give glass a high refractive index, while thallium has also been used in photoelectric cells. Commercial recovery often occurs alongside copper, zinc, or lead refining.
FROM ELEMENT TO APPLICATION
AN ELEMENT IN CONTEXT
How can incoming light become an electrical signal?
Thallium sulfide in photoelectric devices
FOLLOW THIS RELATIONSHIP
Start with a compound
The detector material is thallium sulfide. Keep its chemical identity visible while distinguishing the semiconductor from pure thallium metal and the surrounding circuit.
Explore the model (optional)
TlFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
Optional: check your understanding
USE THE EXPLANATION
Which comparison best isolates the response to illumination?
Atomic identity, reference properties and structure models
INTERACTIVE 3D81 visible sites
An electron censusDrag to rotate · scroll or pinch to zoom
[Xe] 4f14 5d10 6s2 6p1. Grouping electrons by principal shell connects the neutral-atom configuration to periodic-table patterns.
What this representation means
A three-dimensional counting diagram. Dots are count markers, not measured electron positions; spherical placement and spacing are arbitrary. There are no electron trajectories here. Orbitals describe quantum states.
William Crookes found thallium while examining residues from sulfuric acid manufacture. A striking green line announced something that ordinary chemical inspection had missed. Claude Auguste Lamy independently investigated the element and obtained substantial metal the following year. Their overlapping work illustrates a recurring discovery pattern: spectroscopy can reveal an unfamiliar atom before isolation establishes the properties of the material containing it.
1861
Spectral identification
Crookes reports a new element from its green emission line.
1862
Metal isolated
Lamy obtains thallium metal and investigates its properties.
Thallium occurs dispersed through , including iron pyrites, and in worked for lead, zinc and copper. Industrial recovery therefore follows the processing of other metals more often than dedicated thallium mining. Its presence in an ore does not mean that visible pieces of native thallium occur there.
Thallium belongs to group 13, yet its chemistry often favors the +1 rather than the +3 state familiar from aluminum. The outer s participate less readily in bonding in this heavy atom. This difference makes neighboring positions on the periodic table a starting point, rather than a complete prediction.
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
577 K
at standard pressure unless noted
Boiling point in K
1746 K
101.325 kPa
Density in g/cm³
11.85 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
1.62 Pauling
Pauling scale
First ionization energy in kJ/mol
589.4 kJ/mol
gas-phase neutral atom, ground state
Covalent radius in pm
145 pm
single-bond covalent radius
Common oxidation states
+1, +3
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
0.85 mg/kg
upper continental crust, mass fraction
Has stable isotopes
Yesderived
Discovery year in CE
1861 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity across period 6
Tl 1.62 Pauling · Thallium
Electronegativity across period 6. Exact values.
Cs
0.79
Ba
0.89
La
1.1
Ce
1.12
Pr
1.13
Nd
1.14
Sm
1.17
Eu
1.2
Gd
1.2
Tb
1.1
Dy
1.22
Ho
1.23
Er
1.24
Tm
1.25
Yb
1.1
Lu
1.27
Hf
1.3
Ta
1.5
W
2.36
Re
1.9
Os
2.2
Ir
2.2
Pt
2.28
Au
2.54
Hg
2
Tl
1.62
Pb
2.33
Bi
2.02
Po
2
At
2.2
Electronegativity down group 13
Tl 1.62 Pauling · Thallium
Electronegativity down group 13. Exact values.
B
2.04
Al
1.61
Ga
1.81
In
1.78
Tl
1.62
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.
