Periodic Table of Elements




Thallium was discovered by Sir William Crookes in England in 1861 spectroscopically, on selenium compounds. Claude-Auguste Lamy and Sir William Crookes isolated the metal independent of each other in 1862. The name comes from thallium's bright green spectral emission lines.

Name: Thallium
Symbol: Tl
Atomic number: 81
Atomic weight: 204.38
State: solid
Group, period, block: 13, 6, p
Color: silvery white
Classification: post-transition metal
Electron configuration: 4f14 5d10 6s2 6p1

Physical properties

Density: 11.85 g/cm-3
Melting point: 577 K,304 C,579 F
Boiling point: 1746 K,1473 C,2683 F

Atomic properties

Oxidation states: 3, 2, 1
Electronegativity: 1.62 (Pauling scale)
Ionization energies: 1st: 589.4 kJmol-1
Covalent radius: 1457 pm
Van der Waals radius: 196 pm
  • Tl
  • Electron Configuration


There are two naturally occurring isotopes of thallium exist, 203Tl, and 205Tl.