Atomic number: 81

Symbol: Tl

Category: transition metal 

Density at 25°C: solid: density: 11.85 g·cm−3

Well after the excitement of mercury thallium is almost back to the usual elements, but it is very soft indeed and can be cut easily with a knife. Thallium compounds are used in optics and were used in poisons. So thallium tends to be pretty toxic.

Thallium in a sealed ampoule in inert gas. image: adapted from wikipedia

Use, natural applications or common combinations: 

  • poisons
  • mercury-thallium alloy for cold thermometers that won't freeze until -60°C
  • optical photoresistors

Basic characteristics:

  • Standard atomic weight: [204.382; 204.385]
  • Melting point:  304 °C
  • Boiling point: 1473 °C
  • Radioactivity: all naturally stable. Some isotopes have very long half lives.