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Element Fact
Selenium is an essential trace element in the diets that acts as antioxidants.

Thallium, Tl

thallium sample

Atomic number: 81
Atomic mass: 204.3833(2)
Natural abundance: 0.7 ppm
203Tl (202.972329) - 29.524%
204Tl (204.974412) - 70.476%

History and Production

From Greek thallos, meaning a green shoot or twig. It was discovered spectroscopically by W. Crookes in 1861. The metal was subsequently isolated by Crookes and at the same time by C.A. Lamy in 1862. The name was given due to the characteristic bright green line in its flame spectrum. The metal can be recovered from the flue dusts produced during sulfide roasting for manufacturing of sulfuric acid. It is also obtained from the smelting of lead and zinc ores. There is no commercial use of the metal itself. However, the sulfide compound is used in photocells while thallium bromide-iodide crystals have been used as infrared optical materials. The oxide has also been used to make glasses with a high refractive index.

Physical Data

Thallium exhibits a metallic luster when freshly cut and soon develops a bluish-gray tinge. It is soft, malleable and can be cut with a knife. It has a natural abundance of about 0.7 ppm and tend to occur as sulfide minerals. The metal and its compounds are toxic and suspected to be carcinogenic.

Interatomic distance: 340.0 pm
Melting point: 304C
Boiling point: 1473C
Thermal conductivity/Wm-1K-1: 46.1 (27C)
Density/kgm-3: 11850 (20C), 11290 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 182.2 kJ/mol
Gibbs free energy of formation: 147.4 kJ/mol
Entropy: 181.0 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Xe] 4f14 5d10 6s2 6p1
Term symbol: 2P1/2
Electron affinity: 19.2971 kJ/mol   Electronegativity (Pauline): 1.80
Ionization energy (first, second, third): 589.352, 1971.00, 2878.16 kJ/mol

Chemical properties

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