Thallium is named after the Greek word thallos, meaning "green shoot". Thus, it was first identified from the colors of its flame, which includes a bright green light. Thallium was discovered in 1861 by William Crookes and Claude-Auguste Lamy. Although both chemists worked separately, they found the element in the same way. That is, as a residue while producing strong acids using the mineral pyrite. Later, it was discovered that thallium is found in large quantities in other minerals, including thallium alum. Pure thallium is toxic and must be handled with care when used. A thallium chloride compound is used in scans to study a patient's blood circulation. Thallium oxide also helps to strengthen glass for use in cameras and binoculars.

Summary of properties (Tl)

Atomic weight[204.382, 204.385
Discoverer (year)Crookes, William (1861)
Natural formmetallic solid (hexagonal)
Electron configuration[Xe] 414 5d10 6s2 6p1
Melting point (ºC)303
Boiling point (ºC)1457
Abundance in earth's crust (ppm)0.85
Isotope (abundance)203Tl (29.52), 205Tl (70.48)
Density g/cm311.85
Van der Waals radius (pm)196
Covalent radius (pm)144
Electronegativity (Pauling)1.62
Vaporization enthalpy (kJ/mol)162.10
Enthalpy of fusion (kJ/mol)4.14
Specific heat capacity (J/g·K) at 25ºC and 1 atm0.13
Thermal conductivity (W/cm·K) at 25ºC and 1 atm0.460
Oxidation state+3, +1
Electron affinity (eV)0.38
1st Ionization potential (eV)6.1082

Back to the Periodic Table of the Elements.