Atomic number:34

Group numbers:16

Period:4

Electronic configuration:[Ar] 3d¹⁰ 4s² 4p⁴

Formal oxidation number:-2 +4 +6

Electronegativities:2.55

Atomic radius / pm:116

Relative atomic mass:78.96 ± 0.03

 

Selenium was discovered by Jöns Jacob Berzelius (SE) in 1817. The origin of the name comes from the Greek word selene meaning moon. It is a soft metalloid similar to sulfur, unaffected by water and soluble in alkalis and nitric acid. The colour ranges from grey metallic to red glassy. Selenium burns in air and is toxic by inhalation or ingestion. Also it conducts electricity when struck by light. Selenium is obtained from lead, copper and nickel refining. Light causes it to conduct electricity more easily. It is used in photoelectric cells, TV cameras, xerography machines and as a semiconductor in solar batteries and rectifiers, also colours glass red.

Density / g dm^-3:4790 (grey, 293 K)

 3987 (m.p.)

Molar volume / cm³mol^-1:16.48 (grey, 293 K)

 19.80 (m.p.)

Electrical resistivity / μΩcm:12 (20 °C)

Thermal conductivity / W m^-1K^-1:2.04

Melting point / °C:221

Boiling point / °C:685

Heat of fusion / kJ mol^-1:5.1

Heat of vaporization / kJ mol^-1:90

Heat of atomization / kJ mol^-1:226.4

1st ionization energy / kJ mol^-1:940.97

2nd ionization energy / kJ mol^-1:2044.54

3rd ionization energy / kJ mol^-1:2973.74

in the atmosphere / ppm:-

in the Earth's crust / ppm:0.05

in the oceans / ppm:-