Editing Selenium (section)

==Characteristics==
===Physical properties===
[[File:Selenium trigonal.jpg|thumb|left|upright|Structure of hexagonal (gray) selenium]]
Selenium forms several [[allotrope]]s that interconvert with [[temperature]] changes, depending somewhat on the rate of temperature change. When prepared in chemical reactions, selenium is usually an [[Amorphous solid|amorphous]], brick-red powder. When rapidly melted, it forms the black, vitreous form, usually sold commercially as beads.<ref name="house2008">{{cite book |title=Inorganic chemistry |first=James E. |last=House |publisher=Academic Press |year=2008 |isbn=978-0-12-356786-4 |page=524}}</ref> The structure of black selenium is irregular and complex and consists of [[polymer]]ic rings with up to 1000&nbsp;atoms per ring. Black selenium is a brittle, lustrous solid that is slightly soluble in [[carbon disulfide|CS<sub>2</sub>]]. Upon heating, it softens at 50&nbsp;°C and converts to gray selenium at 180&nbsp;°C; the transformation temperature is reduced by presence of [[halogen]]s and [[amine]]s.<ref name="ge">{{Greenwood&Earnshaw |pages=751–752}}</ref>

The red α, β, and γ forms are produced from solutions of black selenium by varying the evaporation rate of the solvent (usually CS<sub>2</sub>). They all have a relatively low, [[monoclinic]] crystal symmetry ([[space group]] 14) and contain nearly identical puckered '''cyclooctaselenium''' (Se<sub>8</sub>) rings as in [[sulfur]].<ref>{{cite journal |last1=Olav Foss and Vitalijus Janickis |title=Crystal structure of γ-monoclinic selenium |journal=Journal of the Chemical Society, Dalton Transactions |date=1980 |issue=4 |pages=624–627 |doi=10.1039/DT9800000624}}</ref> The eight atoms of a ring are not equivalent (i.e. they are not mapped one onto another by any symmetry operation), and in fact in the γ-monoclinic form, half the rings are in one configuration (and its mirror image) and half in another.<ref>{{cite web |title=β–Se (Al) Structure: A_mP32_14_8e |url=https://aflowlib.org/prototype-encyclopedia/A_mP32_14_8e.html |website=Encyclopedia of Crystallographic Prototypes |access-date=2024-01-10 |archive-date=2024-02-08 |archive-url=https://web.archive.org/web/20240208074603/https://aflowlib.org/prototype-encyclopedia/A_mP32_14_8e.html |url-status=dead }}</ref><ref>{{cite web |title=<!-- was β –Se (Al) .. using what I saw at target: --> Se (A<sub>k</sub>) Structure: A_mP64_14_16e |url=https://aflowlib.org/prototype-encyclopedia/A_mP64_14_16e.html |website=Encyclopedia of Crystallographic Prototypes |access-date=2024-01-10 |archive-date=2024-02-08 |archive-url=https://web.archive.org/web/20240208073655/https://aflowlib.org/prototype-encyclopedia/A_mP64_14_16e.html |url-status=dead }}</ref> The packing is most dense in the α form. In the Se<sub>8</sub> rings, the Se–Se distance varies depending on where the pair of atoms is in the ring, but the average is 233.5&nbsp;pm, and the Se–Se–Se angle is on average 105.7°. Other selenium allotropes may contain Se<sub>6</sub> or Se<sub>7</sub> rings.<ref name="ge" />

The most stable and dense form of selenium is gray and has a [[chiral]] [[hexagon]]al crystal lattice (space group 152 or 154 depending on the chirality)<ref>{{cite web |title=γ–Se (A8) Structure: A_hP3_152_a |url=https://aflowlib.org/prototype-encyclopedia/A_hP3_152_a.html |website=Encyclopedia of Crystallographic Prototypes |access-date=2023-12-03 |archive-date=2023-12-03 |archive-url=https://web.archive.org/web/20231203011831/https://aflowlib.org/prototype-encyclopedia/A_hP3_152_a.html |url-status=dead }}</ref> consisting of helical polymeric chains, where the Se–Se distance is 237.3&nbsp;pm and Se–Se–Se angle is 103.1°. The minimum distance between chains is 343.6&nbsp;pm. Gray selenium is formed by mild heating of other allotropes, by slow cooling of molten selenium, or by condensing selenium vapor just below the melting point. Whereas other selenium forms are [[Insulator (electricity)|insulators]], gray selenium is a [[semiconductor]] showing appreciable [[photoconductivity]]. Unlike the other allotropes, it is insoluble in CS<sub>2</sub>.<ref name="ge" /> It resists oxidation by air and is not attacked by nonoxidizing [[acid]]s. With strong reducing agents, it forms polyselenides. Selenium does not exhibit the changes in viscosity that sulfur undergoes when gradually heated.<ref name="house2008" /><ref>{{YouTube|nDEfR2Nw50s|Video of selenium heating}}</ref>

===Isotopes===
{{Main|Isotopes of selenium}}

Selenium has seven naturally occurring [[isotope]]s. Five of these, <sup>74</sup>Se, <sup>76</sup>Se, <sup>77</sup>Se, <sup>78</sup>Se, <sup>80</sup>Se, are stable, with <sup>80</sup>Se being the most abundant (49.6% natural abundance). Also naturally occurring is the long-lived [[primordial radionuclide]] <sup>82</sup>Se, with a [[half-life]] of 8.76×10<sup>19</sup> years.{{NUBASE2020|ref}} The non-primordial radioisotope [[Selenium-79|<sup>79</sup>Se]] also occurs in minute quantities in [[uranium]] ores as a product of [[nuclear fission]]. Selenium also has numerous unstable [[synthetic isotope]]s ranging from <sup>64</sup>Se to <sup>95</sup>Se; the most stable are <sup>75</sup>Se with a half-life of 119.78&nbsp;days and <sup>72</sup>Se with a half-life of 8.4&nbsp;days.{{NUBASE2016|ref}} Isotopes lighter than the stable isotopes primarily undergo [[beta plus decay]] to [[isotopes of arsenic]], and isotopes heavier than the stable isotopes undergo [[beta minus decay]] to [[isotopes of bromine]], with some minor [[neutron emission]] branches in the heaviest known isotopes.

{| class="wikitable"
|+Selenium isotopes of greatest stability
! Isotope
! Nature
! Origin
! Half-life
|-
|<sup>74</sup>Se
|Primordial
|
|Stable
|-
|<sup>76</sup>Se
|Primordial
|
|Stable
|-
|<sup>77</sup>Se
|Primordial
|Fission product
|Stable
|-
|<sup>78</sup>Se
|Primordial
|Fission product
|Stable
|-
|<sup>79</sup>Se
|Trace
|Fission product
|{{val|1=327000}} yr<ref>{{cite web |url=https://www.ptb.de/en/org/6/nachrichten6/2010/60710_en.htm |title=The half-life of <sup>79</sup>Se <!-- title=Physikalisch-Technische Bundesanstalt (PTB) --> |date=2010-09-23 |publisher=Physikalisch-Technische Bundesanstalt |access-date=2012-05-29 |df=dmy-all |archive-url=https://web.archive.org/web/20110927042432/http://www.ptb.de/en/org/6/nachrichten6/2010/60710_en.htm |archive-date=2011-09-27}}</ref><ref>{{cite journal |last2=Bühnemann |first2=Rolf |last3=Hollas |first3=Simon |last4=Kivel |first4=Niko |last5=Kossert |first5=Karsten |last6=Van Winckel |first6=Stefaan |last7=Gostomski |first7=Christoph Lierse v. |date=2010 |title=Preparation of radiochemically pure <sup>79</sup>Se and highly precise determination of its half-life |journal=Applied Radiation and Isotopes |volume=68 |issue=12 |pages=2339–2351 |doi=10.1016/j.apradiso.2010.05.006 |pmid=20627600 |last1=Jörg |first1=Gerhard |bibcode=2010AppRI..68.2339J |display-authors=3}}</ref>
|-
|<sup>80</sup>Se
|Primordial
|Fission product
|Stable
|-
|<sup>82</sup>Se
|Primordial
|Fission product*
|8.76{{e|19}} yr{{NUBASE2020|ref}}{{efn|For all practical purposes, <sup>82</sup>Se is stable.}}
|}