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Thallium (Tl)
Thallium (symbol Tl) is the chemical element with atomic number 81: a post-transition metal, solid at 0 °C, with an atomic mass of 204.38 u. Discovered in 1861 (William Crookes).
Thallium is a soft metal whose compounds are among the most toxic known: colorless, odorless and tasteless, they were long used as rat poison… and by poisoners. It also has helpful uses, in heart imaging and radiation detectors.
Key facts
- Atomic number
- 81
- Symbol
- Tl
- French name
- Thallium
- Atomic mass
- 204.38 u
- Molar mass
- 204.38 g/mol
- Family
- Post-transition metals
- Position
- period 6, group 13, p-block
- State at 0 °C
- Solid
- Melting point
- 303.9 °C
- Boiling point
- 1,472.9 °C
- Density
- 11.85 g/cm³
- Electronegativity (Pauling)
- 1.62
- First ionization energy
- 589 kJ/mol
- Atomic radius
- 190 pm
- Electron configuration
- [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹
- Abundance in Earth's crust
- 0.85 ppm (60th most abundant element)
- Indicative price
- 4,200 $/kg (order of magnitude)
- Radioactive
- no
- Discovery
- 1861, William Crookes
Origin of the name
From the Greek thallos, green shoot, the color of its spectral line.
What does thallium look like?
A blue-gray metal so soft it can be cut with a knife. It quickly gains a dull oxide coat in air.
Where is thallium found?
About 0.8 ppm of Earth's crust. It hides mainly in sulfide ores of zinc, lead and iron, and in some clays and coals.
Natural sources
- Lorándite
- Crookesite
- Hutchinsonite
- Sphalerite (traces)
How is thallium produced?
Recovered as a by-product from the dust and residues of roasting zinc and lead ores. World production is no more than about ten tonnes a year.
Top producers
- China
- Kazakhstan
- Russia
What is thallium used for?
- Thallium-201 for cardiac scintigraphy (imaging the heart muscle).
- Thallium-doped sodium iodide crystals for detecting gamma rays.
- High-index optical glass and infrared optics.
- High-temperature superconductors (research).
In everyday life
- A cardiac stress scintigraphy scan
Role in living things
No biological role. It mimics potassium and gets into cells, which explains both its toxicity and its value for imaging the heart muscle.
Hazards of thallium
Extremely toxic: less than a gram of a thallium salt can kill an adult. Telltale symptoms are nerve pain and hair loss a few weeks after exposure. The antidote is Prussian blue, which traps it in the gut.
Cosmic origin
- Dying stars: Made slowly in red giants (stars like the Sun at the end of their lives).
- Neutron stars: Created when two neutron stars collide — the “forge” of gold.
History of thallium
In 1861 the Englishman William Crookes noticed an unknown green line in the spectrum of sulfuric-acid residues. He named the element thallium, from the Greek thallos, “green shoot.” The Frenchman Claude-Auguste Lamy independently isolated the metal in 1862.
Did you know?
- In 1977, a London nurse reading Agatha Christie's The Pale Horse recognized the symptoms of thallium poisoning in a little girl, and saved her life.
- It was banned as a rat poison in many countries in the 1970s because of accidental poisonings.
- Its name evokes the spring-green color of its spectral line.
Isotopes of thallium
Natural thallium consists of two stable isotopes, 203 and 205. Thallium-201 has been used for decades to examine the heart.
- thallium-201: radioactive, half-life 3.04 days. Heart-muscle scintigraphy to spot areas with poor blood supply.
- thallium-203: 29.52% of natural thallium, stable
- thallium-204: radioactive, half-life 3.79 years. A beta source in industrial thickness gauges.
- thallium-205: 70.48% of natural thallium, stable
- thallium-208: radioactive, half-life 3.05 min. A link in the thorium decay chain, it emits the most energetic gamma rays of natural radioactivity.
Related elements
Same family (post-transition metals): Aluminium (Al), Gallium (Ga), Indium (In), Tin (Sn), Lead (Pb), Bismuth (Bi), Polonium (Po).
Same column (group 13): Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), Nihonium (Nh).