Symbol	Se
Atomnummer	34
Gruppe	16 (Chalkogener)
Periode	4
Blok	p
Klassifikation	Ikkemetal

Udseende	Black, red, and gray (not pictured) allotropes
Farve	Grå

Antal protoner	34 p+
Antal neutroner	45 n0
Antal elektroner	34 e-

Fra Wikipedia, den gratis encyklopædiSelen (forkortes Se) er et grundstof med atomnummer 34. Selen er et giftigt ikkemetal, der kemisk er i familie med svovl og tellur. Det findes i adskillige former, men en enkelt form er en stabil, metalagtig form, der leder strøm bedre i lys end i mørke, og som derfor bruges i fotodetektorer. Grundstoffet findes i svovlårer såsom pyrit.

Fase ved STP	Fast
Massefylde	4.81 g/cm3
Atommasse	78.9718 u

Smeltepunkt	494 K 220.85 °C 429.53 °F
Kogepunkt	958 K 684.85 °C 1264.73 °F
Fordampningsvarme	26.32 kJ/mol

Elektronegativitet (Pauling Scale)	2.55
Elektronaffinitet	194.959 kJ/mol
Oxidationstrin	−2, −1, +1, +2, +3, +4, +5, +6 (a strongly acidic oxide)
Ioniseringsenergier	941 kJ/mol 2045 kJ/mol 2973.7 kJ/mol 4144 kJ/mol 6590 kJ/mol 7880 kJ/mol 14990 kJ/mol

Elektronkonfiguration Kortfattet konfiguration	[Ar] 3d10 4s2 4p4
Elektronkonfiguration Fuld konfiguration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4
Elektronkonfigurationsdiagram	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4
Elektroner pr. Skal	2, 8, 18, 6
Valenselektroner	6
Valency-elektroner	2,4,6
Bohrs atommodel	Figur: Skaldiagram af Selen (Se) atom.
Orbital diagram	1s 2s 2p 3s 3p 3d 4s 4p

Opdagelse og første isolation

Jöns Jakob Berzelius, Johan Gottlieb Gahn (1817)

Opdagelse af selen 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