Periodic Table


Atomic number: 81
Atomic weight: 204.3833
Symbol: Tl
Group number: 13
Electronic configuration: [Xe].4f14.5d10.6s2.6p1


When freshly exposed to air, thallium exhibits a metallic lustre, but soon develops a bluish-grey tinge, resembling lead in appearance. A heavy oxide builds up on thallium if left in air, and in the presence of water the hydroxide is formed. The metal is very soft and malleable. It can be cut with a knife.

The element and its compounds are toxic and should be handled carefully. Thallium may cause cancer.

General information

Discoveror: Sir William Crookes
Date discovered: 1861
Discovered at: England
Meaning of name: From the Greek word "thallos" meaning "green twig" or green shoot

Physical data

Standard state: solid at 298 K
Colour: silvery white
Density of solid at ambient temperature/kg m-3: 11850
Molar volume/cm3: 17.22

Radii /pm

Atomic: 156
Covalent (single bond):
Pauling radius for the ion [Tl]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f


Both values are quoted on the Pauling scale.

Pauling: 1.8
Allred Rochow: 1.44

Crystal Structure

structure: hcp (hexagonal close-packed)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 577
boiling point: 1746

Enthalpies /kJ mol-1

fusion: 4.2
vaporization: 164

single bond enthalpies:
Tl-F Tl-Cl Tl-Br Tl-I Tl-Tl
439 364 326 280 60.7

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
203Tl202.972320 (5)29.524 (14)
205Tl204.974401 (5)70.476 (14)

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Tl.html