81. Thallium
Name: Thallium
Symbol: Tl
Atomic Number: 81
Atomic Mass: 204.3833 amu
Melting Point: 303.5 °C (576.65 °K, 578.3 °F)
Boiling Point: 1457.0 °C (1730.15 °K, 2654.6 °F)
Number of Protons/Electrons: 81
Number of Neutrons: 123
Classification: Other Metals
Crystal Structure: Hexagonal
Density(293 K): 11.85 g/cm3
Color: bluish
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.
Thallium (Greek thallos meaning "a green shoot or twig") was discovered by Sir William Crookes in 1861 in England while he was making spectroscopic determinations for tellurium on residues from a sulfuric acid plant. The name comes from Thallium's bright green spectral emission lines. In 1862 Crookes and Claude-Auguste Lamy isolated the metal independent of each other.
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