Atomic number 81

Thallium

Tl
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81 electrons in 6 shells: 2, 8, 18, 32, 18, 3.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Post-transition metal
Period: 6
Group: 13
Block: p
Standard atomic weight: 204.38(1) Da
Electron configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹
Phase at 25 °C: solid
Melting point: 304 °C
Boiling point: 1473 °C
Density: 11.8 g/cm³
Electronegativity (Pauling): 1.62
Discovered: 1861, William Crookes

Neighbours in the table

In 1861 William Crookes examined sludge left from making sulfuric acid, expecting the yellow lines of tellurium. His spectroscope showed instead a bright green line never seen before, Jefferson Lab records. Crookes and Lamy both isolated the metal in 1862, according to Los Alamos.

Thallium stacks 81 electrons in shells of 2, 8, 18, 32, 18 and 3, ending [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹. Its three outer electrons largely set its combining power, in Chem1’s account, and +3 is one of its two oxidation states in Jefferson Lab’s list. That is a bookkeeping charge, as if all bonds were ionic. The other, +1, appears in thallium sulfate, Tl₂SO₄, and thallium iodide, TlI.

The first ionisation energy, the energy to remove one outer electron, is 6.108 eV. Each group 13 element has a lower first ionisation energy than the element before it in its period, mercury for thallium, Chem1 notes. The dip is often attributed to the easier loss of a lone p electron. The metal is very soft and malleable and can be cut with a knife, Los Alamos says. Metals are malleable, in Chem1’s account, because their bonding has no preferred direction, each ion bathed in a fluid of electrons.

Where it is found

A kilogram of crust holds 0.85 milligrams of thallium in Jefferson Lab’s figure, or about 0.7 parts per million in the US Geological Survey’s estimate. Most is bound up in potassium minerals of clay, granite and soil, generally not commercially recoverable in the survey’s view. Its own minerals include crooksite, lorandite and hutchinsonite, and it occurs in pyrite and in manganese nodules on the ocean floor, according to Los Alamos.

Most recoverable thallium comes from copper, lead and zinc sulfide ores, via roasting flue dust, the survey says; Los Alamos adds pyrite roasted for sulfuric acid. World output in 2023 was estimated at about 10,000 kilograms, led by China, Kazakhstan and Russia; most producers withhold their figures. The United States has produced none since 1981 by the survey’s count, or since 1984 according to the US Agency for Toxic Substances and Disease Registry.

Natural thallium has two stable isotopes: thallium-205 at 70.48 per cent and thallium-203 at 29.52 per cent. Their slightly different properties let natural processes fractionate, or partly sort, them, according to IUPAC. Abundances and atomic weight therefore vary between materials, and 204.38 Da is the conventional value.

Why it matters

Thallium-201 lets physicians image the heart: ²⁰¹Tl scintigraphy, imaging the heart muscle during exercise tests, is used to detect coronary artery disease, in IUPAC’s description. It can start as thallium-203, struck by protons that knock out three neutrons to give lead-201, which then captures an electron; thallium-205 serves as an alternative target, IUPAC notes. The half-life of thallium-201 is 3.0421 days, entirely by electron capture, in the Atomic Mass Data Center’s table. Its use is declining as better alternatives spread, the US Geological Survey reports, though some cardiac stress tests still rely on it.

Crystals of sodium iodide doped with thallium, its activator, serve in gamma-radiation detectors and photoelectric cells, the survey says. Thallium sulfide changes its electrical conductivity under infrared light, and it is used in photocells, Los Alamos says; thallium bromide-iodide crystals have served as infrared optics. The survey still rates thallium ahead of its substitutes in detectors and infrared equipment, on performance and cost.

An alloy of mercury with 8.5 per cent thallium freezes at about −60 °C, some 20 degrees below mercury, Los Alamos reports. The survey lists thallium-mercury alloys in low-temperature thermometers and switches, and thallium-barium-calcium-copper oxides among high-temperature superconductors. In 2024 the metal fetched an estimated 9,500 dollars a kilogram.

Properties

Properties of thallium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	81
Standard atomic weight	204.38(1) Da	Da	IUPAC/CIAAW 2024
Melting point	304 °C	°C	Los Alamos: 304 °C
Boiling point	1473 °C	°C	Los Alamos: 1473 °C
Density	11.8 g/cm³	g/cm³
Electronegativity (Pauling)	1.62
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹
Phase at 25 °C	solid

The standard atomic weight of this element is an interval in nature, from 204.382 to 204.385 Da; the value shown is IUPAC's conventional abridged value.

Safety

Thallium metal is highly toxic: PubChem’s record classes it Danger, fatal if swallowed or inhaled, and able to damage organs through prolonged or repeated exposure. Chronic poisoning can cause nerve damage in the limbs, liver injury and hair loss, according to Haz-Map. The metal and its compounds are strictly controlled, the US Geological Survey notes, and PubChem’s Laboratory Chemical Safety Summary for thallium gives the full classification.

Questions

Why is thallium called thallium?

The name comes from the Greek thallos, a green shoot or twig, chosen for the bright green line in the element's spectrum, according to the note by IUPAC's atomic-weights commission in PubChem's element record. In Los Alamos's wording, the element was named after that green spectral line, which was how it was first recognised. William Crookes spotted the line with a spectroscope in 1861, and the metal itself was isolated in 1862.

Why was thallium used as rat poison?

Thallium sulfate, Tl₂SO₄, was widely spread as a rodenticide and ant killer. The salt has neither odour nor taste, Los Alamos notes. Household use in the United States was banned in 1975 by Los Alamos's date, or in 1974 according to Jefferson Lab. Most reported thallium poisonings follow swallowing, according to Haz-Map.

Why does thallium tarnish so quickly?

A freshly cut surface soon turns a bluish-grey much like lead, Los Alamos says, and a heavy oxide builds up in air. Pure thallium reacts fast with oxygen and water vapour, leaving a black powder, in Jefferson Lab's text in PubChem's record. Jefferson Lab gives that reaction as the reason pure thallium has no uses of its own, though the US Geological Survey lists thallium-mercury alloys in low-temperature thermometers and switches.

Is thallium dangerous to touch?

Yes. Skin contact with the metal is dangerous, Los Alamos warns, and Japan's 2021 classification of thallium and its compounds adds fatal in contact with skin, in PubChem's record. It enters the body through skin contact, by swallowing, or as inhaled dust or fumes, the US Geological Survey adds. The emergency-response guidance quoted in that record rates it highly toxic, possibly fatal if inhaled, swallowed or absorbed through skin. The full classification is in PubChem's Laboratory Chemical Safety Summary.