Thallium
Post-transition metal · Period 6 · Group 13 · p-block
The green-lined metal that poses as potassium inside the body
- Atomic mass
- 204.38 u
- State at 25 °C
- Solid
- Electronegativity
- 1.62
- Melting point
- 303.9 °C
- Boiling point
- 1,472.9 °C
- Configuration
- [Xe] 4f14 5d10 6s2 6p1
Nucleus drawn at reduced count for clarity. Drag to rotate.
Thallium, explained
Video lesson in production
Bhavya’s explainer for this element will play here.
Overview
Thallium is a soft, silvery-grey post-transition metal in group 13, below aluminium, gallium and indium. It is soft enough to cut with a knife and tarnishes within minutes in air, so samples are stored under oil. Its outer configuration is 6s2 6p1, and the two 6s electrons are held unusually tightly because of relativistic effects in such a heavy atom. This 'inert pair effect' means thallium prefers to lose only its single 6p electron, forming Tl+ rather than the Tl3+ ion that its lighter group-mates favour.
The Tl+ ion has almost the same size and charge as K+, and that coincidence explains most of thallium's notoriety. Cells take up Tl+ through potassium channels and pumps, where it disrupts enzymes and binds to sulfur groups in proteins, causing nerve damage and hair loss. The same mimicry is put to medical use: thallium-201 travels to heart muscle the way potassium does, revealing poorly supplied regions in cardiac scans. Thallium also forms many Tl(I) salts that resemble those of the alkali metals and silver.
Discovery
William Crookes spotted thallium in 1861 while examining selenium-rich residues from a sulfuric acid plant with a flame spectroscope: a bright green line appeared that matched no known element. He named it from the Greek thallos, meaning a green shoot or twig. In 1862 Claude-Auguste Lamy in France independently isolated enough of the metal to cast a small ingot, and the two men disputed priority for years.
- Where it’s found
- Thallium occurs at well under one part per million in the crust, mostly in sulfide ores, and is recovered as a byproduct from flue dust when zinc, lead and copper ores are roasted.
- What it’s used for
- Cardiac stress imaging
- Radiation detector crystals
- High-refractive-index optical glass
- Infrared optics
- Superconductor research
- In living things
- Thallium has no biological role and is one of the most toxic metals; the Tl+ ion disrupts potassium-dependent processes and damages nerves.
- Safety
- Highly toxic by ingestion, inhalation and skin contact, with cumulative effects such as hair loss and neuropathy. Handle only with gloves and fume extraction, and never near food.
How it compares
Where Thallium sits among all 118 elements on the trends that shape the table.
- Electronegativity1.62Pauling
#49 of 100 elements with data
- Ionization energy589kJ/mol
#72 of 104 elements with data
- Atomic radius190pm
#9 of 95 elements with data
- Electron affinity36.4kJ/mol
#62 of 109 elements with data
- Density11.8g/cm³
#34 of 114 elements with data
- Melting point577K
#79 of 110 elements with data
- Density
- 11.8 g/cm³
- Atomic radius
- 190 pm
- Covalent radius
- 144 pm
- Molar heat
- 26.32 J/(mol·K)
- Appearance
- silvery white
- Discovered by
- William Crookes
Inside the atom
81 electrons arranged in 6 shells: 2 · 8 · 18 · 32 · 18 · 3.
Orbital diagram
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1 · shorthand [Xe] 4f14 5d10 6s2 6p1
- 1s2
- 2s2
- 2p6
- 3s2
- 3p6
- 4s2
- 3d10
- 4p6
- 5s2
- 4d10
- 5p6
- 6s2
- 4f14
- 5d10
- 6p1
Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Thallium bonds.
Ionization energies
Signature reactions
The chemistry Thallium is known for.
Redox · Single displacement
Dissolving in dilute sulfuric acid
Thallium dissolves to give hydrogen bubbles and a colourless solution of thallium(I) sulfate. The product is the compound once used as rat poison, and the +1 oxidation state shows the inert pair effect at work.
Conditions: dilute acid, room temperature
Redox · Oxidation
Tarnishing in air
A freshly cut, bright surface turns dull grey within minutes as a film of thallium(I) oxide forms. This is why thallium is kept under oil or in sealed ampoules.
Conditions: exposure to air
Precipitation
Precipitating thallium(I) iodide
Adding iodide to a Tl+ solution gives a bright yellow precipitate, much as it does with silver ions. Low-solubility thallium(I) halides were long used to detect and separate thallium in analysis.
Conditions: aqueous solution, room temperature
Where you’ll meet it
Lorándite
Lorándite, a red thallium arsenic sulfide, is one of the few minerals in which thallium is a major component.
Photo: Wikimedia Commons
William Crookes
William Crookes discovered thallium in 1861 from a single green line in a flame spectrum.
Photo: Wikimedia Commons
Rodenticide
Thallium sulfate was widely used in rat poison baits until bans in the 1970s.
Photo: Wikimedia Commons
Myocardial perfusion imaging
Thallium-201 behaves like potassium and maps blood flow to heart muscle in cardiac stress scans.
Photo: Wikimedia Commons
Research
No research on Thallium yet
When Bhavya publishes a project that studies Thallium, it will be linked here.