Symbol	Se
Atomic number	34
Group	16 (Oxygen group)
Period	4
Block	p
Classification	Nonmetal

Appearance	Black, red, and gray (not pictured) allotropes
Color	Gray

Number of protons	34 p+
Number of neutrons	45 n0
Number of electrons	34 e-

From Wikipedia, the free encyclopediaSelenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between those of its periodic table column-adjacent chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature, or as pure ore compounds.

Phase at STP	Solid
Density	4.81 g/cm3
Atomic weight	78.9718 u

Melting point	494 K 220.85 °C 429.53 °F
Boiling point	958 K 684.85 °C 1264.73 °F
Heat of vaporization	26.32 kJ/mol

Electronegativity (Pauling Scale)	2.55
Electron affinity	194.959 kJ/mol
Oxidation states	−2, −1, +1, +2, +3, +4, +5, +6 (a strongly acidic oxide)
Ionization energies	941 kJ/mol 2045 kJ/mol 2973.7 kJ/mol 4144 kJ/mol 6590 kJ/mol 7880 kJ/mol 14990 kJ/mol

Electron configuration Shorthand configuration	[Ar] 3d10 4s2 4p4
Electron configuration Full configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4
Electron configuration chart	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4
Electrons per shell	2, 8, 18, 6
Valence electrons	6
Valency electrons	2,4,6
Bohr model	Figure: Shell diagram of Selenium (Se) atom.
Orbital Diagram	1s 2s 2p 3s 3p 3d 4s 4p

Discovery and first isolation

Jöns Jakob Berzelius, Johan Gottlieb Gahn (1817)

Discovery of selenium Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jacob Berzelius and Johan Gottlieb Gahn. Both chemists owned a chemistry plant near Gripsholm, Sweden, producing sulfuric acid by the lead chamber process. The pyrite from the Falun Mine created a red precipitate in the lead chambers which was presumed to be an arsenic compound, so the pyrite's use to make acid was discontinued. Berzelius and Gahn wanted to use the pyrite and they also observed that the red precipitate gave off a smell like horseradish when burned. This smell was not typical of arsenic, but a similar odor was known from tellurium compounds. Hence, Berzelius's first letter to Alexander Marcet stated that this was a tellurium compound. However, the lack of tellurium compounds in the Falun Mine minerals eventually led Berzelius to reanalyze the red precipitate, and in 1818 he wrote a second letter to Marcet describing a newly found element similar to sulfur and tellurium. Because of its similarity to tellurium, named for the Earth, Berzelius named the new element after the Moon.

CAS Number	7782-49-2
ChemSpider ID	4885617
EC number	231-957-4
PubChem CID Number	6326970