| Tecken | Se |
| Atomnummer | 34 |
| Grupp | 16 (Syregruppen) |
| Period | 4 |
| Block | p |
| Ämnesklass | Icke-metall |
| Utseende | Black, red, and gray (not pictured) allotropes |
| Färg | Grå |
| Antal protoner | 34 p+ |
| Antal neutroner | 45 n0 |
| Antal elektroner | 34 e- |
| Fas vid STP | Fast |
| Densitet | 4.81 g/cm3 |
| Relativ atommassa | 78.9718 u |
| Smältpunkt | 494 K 220.85 °C 429.53 °F |
| Kokpunkt | 958 K 684.85 °C 1264.73 °F |
| Ångbildningsvärme | 26.32 kJ/mol |
| Elektronegativitet (Paulingskalan) | 2.55 |
| Elektronaffinitet | 194.959 kJ/mol |
| Oxidationstal | −2, −1, +1, +2, +3, +4, +5, +6 (a strongly acidic oxide) |
| Jonisationspotential |
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| Upptäckt och första isolation | Jöns Jakob Berzelius, Johan Gottlieb Gahn (1817) |
Upptäckten av 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. | |