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.
Atomic number: 81
Atomic weight: 204.38
Group, period, block: 13, 6, p
Color: silvery white
Classification: post-transition metal
Electron configuration: 4f14 5d10 6s2 6p1
Density: 11.85 g/cm-3
Melting point: 577 K,304 °C,579 °F
Boiling point: 1746 K,1473 °C,2683 °F
Oxidation states: 3, 2, 1
Electronegativity: 1.62 (Pauling scale)
Ionization energies: 1st: 589.4 kJ·mol-1
Covalent radius: 145±7 pm
Van der Waals radius: 196 pm
There are two naturally occurring isotopes of thallium exist, 203Tl, and 205Tl.