Tl
Thallium
Element 81 · Post-transition metal
Thallium (Tl)
Thallium is a soft, heavy, grey metal. It is extremely toxic and was once called “the poisoner’s poison”.
- Post-transition metal
- Period 6
- Group 13
- p-block
- solid at room temperature
Key properties
- Atomic mass
- 204.383
- u
- Electron configuration
- [Xe] 4f14 5d10 6s2 6p1
- Melting point
- 303.9 °C
- 577 K
- Boiling point
- 1,472.9 °C
- 1,746 K
- Density
- 11.8 g/cm³
- Electronegativity
- 1.62
- Pauling
- Atomic radius
- 196 pm
- van der Waals
- 1st ionisation energy
- 6.108 eV
Compare with every element:Electronegativity chartAtomic radius chartIonisation energy chartDensity chart of the elementsMelting point chartElectron configuration chart
Did you know?
Notable facts about thallium
- 01
Thallium sulfate was once sold as a rat and ant poison.
- 02
Thallium poisoning causes hair loss, which helped doctors diagnose it.
- 03
Thallium-201 is used in nuclear heart stress tests.
Applications
What is thallium used for?
- Cardiac imaging (Tl-201)
- Infrared optics
- Electronics research
- Low-melting glass
History
Discovery & name origin
Discovered by
Year
Where
England
Name origin
Greek thallos, “green shoot”, for its bright green spectral line.
Atomic structure
Atomic properties
Atomic structure
Atomic properties
- Atomic number
- 81
- Atomic mass
- 204.383 u
- Group
- 13
- Period
- 6
- Block
- p-block
- Electron configuration
- [Xe] 4f14 5d10 6s2 6p1
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1
- Electrons per shell
- 2, 8, 18, 32, 18, 3
- Electronegativity (Pauling)
- 1.62
- Electron affinity
- 0.2 eV
- Atomic radius (van der Waals)
- 196 pm
- Stable isotopes
- 2
Oxidation states
Highlighted: common oxidation states (+3, +1).
Successive ionisation energies
Energy to remove each electron in turn (kJ/mol). Big jumps show where a new, inner shell begins.
- 1st589.4
- 2nd1,971
- 3rd2,878
Physical
Physical properties & colour
Physical
Physical properties & colour
- State at 20 °C
- Solid
- Melting point
- 577 K · 303.9 °C · 578.9 °F
- Boiling point
- 1,746 K · 1,472.9 °C · 2,683.1 °F
- Density
- 11.8 g/cm³
- Molar heat capacity
- 26.32 J/(mol·K)
- Appearance
- Silvery white
- CPK model colour
- Copied
The CPK colour is the convention used for this element’s atoms in molecular models.
FAQ
Thallium questions
What is Tl on the periodic table?
Tl is the symbol for thallium, element 81 on the periodic table. It sits in group 13, period 6, in the post-transition metal category, and has an atomic mass of 204.383.
What is the atomic number of thallium?
Thallium has atomic number 81, so every thallium atom has 81 protons in its nucleus. Its symbol is Tl and its atomic mass is 204.383.
Who discovered thallium?
Thallium was discovered by William Crookes in 1861 (England). The name comes from Greek thallos, “green shoot”, for its bright green spectral line.
What is thallium used for?
Thallium is used for: cardiac imaging (Tl-201); infrared optics; electronics research; low-melting glass.
Is thallium a metal?
Yes. Thallium is a post-transition metal, one of the metallic elements.
What is the electron configuration of thallium?
The electron configuration of thallium is [Xe] 4f14 5d10 6s2 6p1. Its 81 electrons are arranged in shells as 2, 8, 18, 32, 18, 3.