ElementsWhat is thallium?
Thallium is a soft, grey metal. It and its compounds are very poisonous.
What is thallium used for?
Special glass and electronics, and a radioactive form used in heart scans.
Where the name comes from
From the Greek word thallos, a green shoot, for the green line in its spectrum.
Key facts
- Symbol
- Tl
- Atomic number
- 81
- Atomic mass
- 204.383 u
- Family
- Post-transition metal
- Period
- 6
- Group
- 13
- Block
- p
- State at room temperature
- Solid
- Melting point
- 303.9 °C (577 K, 579 °F)
- Boiling point
- 1,473 °C (1,746 K, 2,683 °F)
- Density
- 11.8 g/cm³
- Electrons per shell
- 2, 8, 18, 32, 18, 3
- Electron configuration
- [Xe]6s2 4f14 5d10 6p1
- Common oxidation states
- +3, +1
- Electronegativity (Pauling)
- 1.62
- First ionisation energy
- 6.108 eV
- Electron affinity
- 0.2 eV
- Atomic radius (van der Waals)
- 196 pm
- Discovered
- 1861
Place in the periodic table
Thallium is element number 81. It sits in period 6 (row 6) of the periodic table, in group 13 (column 13), and belongs to the p-block. Just before it comes mercury (80), and just after it comes lead (82). In the same column, indium is above it and nihonium is below it. Elements in one column tend to behave alike.
Family: Post-transition metals.
Physical properties
At room temperature thallium is a solid. It melts at 303.9 °C and boils at 1,473 °C, so it stays liquid over a range of 1,169 degrees. Its density is 11.8 grams per cubic centimetre, so a one-litre block would weigh about 11.8 kilograms. By density it ranks 20th among the 96 elements with a measured value.
Inside the atom
An atom of thallium has 81 protons in its nucleus and, when neutral, 81 electrons around it. The electrons are arranged in 6 shells: 2, 8, 18, 32, 18, 3. The outermost shell holds 3 electrons. Written in full, the electron configuration is [Xe]6s2 4f14 5d10 6p1.
Its electronegativity on the Pauling scale is 1.62. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 6.108 electronvolts (about 589 kilojoules per mole). One mole of thallium weighs 204.383 grams, so a single gram contains about 2.95 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains thallium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.