Thallium
[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹ · 2 · 8 · 18 · 32 · 18 · 3
A soft heavy metal whose salts have no taste or smell and cause a poisoning whose symptoms were mistaken for disease for centuries. Thallium earned the nickname poisoner's poison and is banned from domestic use almost everywhere.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 81
- Atomic mass
- 204.38 а.е.м.
- Electron configuration
- [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹
- Electrons per shell
- 2 · 8 · 18 · 32 · 18 · 3
- Block
- p-block
- Group
- Boron group
- Period
- 6
- Electronegativity (Pauling)
- 1.62
- Electronegativity (Allen)
- 1.79
- Atomic radius
- 156 pm
- Covalent radius
- 145 pm
- Van der Waals radius
- 196 pm
- Ionisation energy 1
- 589.4 kJ/mol
- Ionisation energy 2
- 1 971 kJ/mol
- Ionisation energy 3
- 2 878 kJ/mol
- Electron affinity
- 36.4 kJ/mol
- Common oxidation states
- +1, +3
- Oxidation states
- -5, -2, -1, +1, +2, +3
Physical properties
- State at 25 °C
- solid
- Density
- 11.85 g/cm³
- Melting point
- 577 K · 303.9 °C
- Boiling point
- 1 746 K · 1 472.9 °C
- Speed of sound
- 818 m/s
Thermal properties
- Heat of fusion
- 4.14 kJ/mol
- Heat of vaporisation
- 165 kJ/mol
- Specific heat capacity
- 0.129 J/(g·K)
- Thermal conductivity
- 46.1 W/(m·K)
Mechanical properties
- Young's modulus
- 8 GPa
- Shear modulus
- 2.8 GPa
- Bulk modulus
- 43 GPa
- Poisson ratio
- 0.45
- Mohs hardness
- 1.2
- Brinell hardness
- 26.4 MPa
Electrical and magnetic properties
- Electrical resistivity
- 180 nΩ·m
- Magnetic ordering
- diamagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- 2.38 K
Crystal structure
- Crystal structure
- hexagonal close-packed (hcp)
- Lattice constants
- a = 345.66 pm · c = 552.48 pm
Abundance
- In the crust
- 0.85 mg/kg
- In the ocean
- 1·10⁻⁵ mg/L
- In the universe
- 0.0005 mg/kg
- In the human body
- no data
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 203Tl | 29.52 % | stable | — |
| 205Tl | 70.48 % | stable | — |
Thallium-201, half-life 73 hours, is a mainstay of nuclear cardiology. Thallium-208 is a member of the thorium series and emits hard gamma rays.
Discovery
- Year of discovery
- 1861
- Discovered by
- William Crookes
- Where
- England
- Origin of the name
- from Greek thallos (green shoot), after its bright green spectral line
History
William Crookes discovered thallium in 1861 from a bright green spectral line in a residue from sulfuric acid manufacture. The name is Greek for green shoot.
Where it occurs
0.85 mg/kg of the crust. It has essentially no ores of its own, being dispersed through sulfides and won as a by-product.
How it is produced
From the dusts and residues of lead-zinc and copper production. World output is about 10 tonnes a year.
Role in living things
No biological role. The thallium ion is close in size to potassium and enters every cell in its place, hijacking potassium pumps — which explains its systemic toxicity.
Safety
A lethal dose is about one gram. The characteristic sign is hair loss after two weeks, which is what allowed retrospective diagnosis. Thallium rat poison has been banned since the 1970s.
Uses
- Thallium-201 in cardiac scintigraphy for assessing myocardial perfusion
- Infrared optics: thallium halides transmit in the far infrared
- Thallium-based high-temperature superconductors
- Doping glass for high-refractive-index optics
Curiosities
- Agatha Christie described thallium poisoning so accurately in The Pale Horse that the book helped doctors recognise real cases.
- Thallium was the poisoner's favourite precisely because it has no taste or smell.
- The name means green shoot, after the colour of its spectral line.
- Thallium poisoning is treated with Prussian blue, which binds the metal in the gut.