Editing Chalcogen (section)

===Allotropes===
{{see also|Allotropes of oxygen|Allotropes of sulfur}}
[[File:Phase diagram of sulfur (1975).png|thumb|upright=1.2|Phase diagram of sulfur showing the relative stabilities of several allotropes<ref>{{cite web|url=https://www.osti.gov/biblio/4010212 |title=Phase Diagrams of the Elements|author=Young, David A. |date=September 11, 1975|publisher=Lawrence Livermore Laboratory |doi=10.2172/4010212 |osti = 4010212}}</ref>]]
[[File:Chalkogene.jpg|thumb|left|The four stable chalcogens at [[Standard temperature and pressure|STP]]]]
[[File:Phase diagram of solid oxygen.svg|thumb|[[Phase diagram]] for [[solid oxygen]]]]

Oxygen's most common [[allotrope]] is diatomic oxygen, or O<sub>2</sub>, a reactive paramagnetic molecule that is ubiquitous to [[aerobic organism]]s and has a blue color in its [[liquid oxygen|liquid state]]. Another allotrope is O<sub>3</sub>, or [[ozone]], which is three oxygen atoms bonded together in a bent formation. There is also an allotrope called [[tetraoxygen]], or O<sub>4</sub>,<ref>{{cite journal|title = The ε Phase of Solid Oxygen: Evidence of an O4 Molecule Lattice|year = 1999|bibcode = 1999PhRvL..83.4093G|last1 = Gorelli|first1 = Federico A.|last2 = Ulivi|first2 = Lorenzo|last3 = Santoro|first3 = Mario|last4 = Bini|first4 = Roberto|volume = 83|page = 4093|journal = Physical Review Letters|doi = 10.1103/PhysRevLett.83.4093|issue = 20}}</ref> and six allotropes of [[solid oxygen]] including "red oxygen", which has the formula O<sub>8</sub>.<ref>{{cite journal|title = Observation of an O8 molecular lattice in the ε phase of solid oxygen|doi=10.1038/nature05174|journal =Nature|volume =443|issue =7108|pages =201–4|pmid =16971946|year = 2006|last1 = Lundegaard|first1 = Lars F.|last2 = Weck|first2 = Gunnar|last3 = McMahon|first3 = Malcolm I.|last4 = Desgreniers|first4 = Serge|last5 = Loubeyre|first5 = Paul|bibcode = 2006Natur.443..201L|s2cid=4384225}}</ref>

Sulfur has over 20 known allotropes, which is more than any other element except [[allotropes of carbon|carbon]].<ref name="Greenwood">{{Greenwood&Earnshaw|pages = 645–662}}</ref> The most common allotropes are in the form of eight-atom rings, but other molecular allotropes that contain as few as two atoms or as many as 20 are known. Other notable sulfur allotropes include [[rhombic crystal system|rhombic]] sulfur and [[monoclinic]] sulfur. Rhombic sulfur is the more stable of the two allotropes. Monoclinic sulfur takes the form of long needles and is formed when liquid sulfur is cooled to slightly below its melting point. The atoms in liquid sulfur are generally in the form of long chains, but above 190&nbsp;°C, the chains begin to break down. If liquid sulfur above 190&nbsp;°C is [[freezing|frozen]] very rapidly, the resulting sulfur is amorphous or "plastic" sulfur. Gaseous sulfur is a mixture of diatomic sulfur (S<sub>2</sub>) and 8-atom rings.<ref>{{cite web|last = McClure|first = Mark R.|url = http://www.uncp.edu/home/mcclurem/ptable/sulfur/s.htm|title = sulfur|access-date = November 25, 2013|archive-url = https://web.archive.org/web/20140312220122/http://www2.uncp.edu/home/mcclurem/ptable/sulfur/s.htm|archive-date = March 12, 2014|url-status = dead|df = mdy-all}}</ref>

Selenium has at least eight distinct allotropes.<ref>{{Greenwood&Earnshaw2nd|page=751}}</ref> The gray allotrope, commonly referred to as the "metallic" allotrope, despite not being a metal, is stable and has a hexagonal [[crystal structure]]. The gray allotrope of selenium is soft, with a [[Mohs hardness]] of 2, and brittle. Four other allotropes of selenium are [[metastable]]. These include two [[monoclinic]] red allotropes and two [[amorphous]] allotropes, one of which is red and one of which is black.<ref>{{Cite web|vauthors = Butterman WC, ((Brown RD Jr)) |url =http://pubs.usgs.gov/of/2003/of03-018/of03-018.pdf |title = Selenium. Mineral Commodity Profiles|year = 2004 |publisher = Department of the Interior |url-status=live |archive-url=https://web.archive.org/web/20121003211018/http://pubs.usgs.gov/of/2003/of03-018/of03-018.pdf |archive-date=October 3, 2012 |access-date=November 25, 2013}}</ref> The red allotrope converts to the black allotrope in the presence of heat. The gray allotrope of selenium is made from [[spiral]]s on selenium atoms, while one of the red allotropes is made of stacks of selenium rings (Se<sub>8</sub>).<ref name="ReferenceB"/>{{dubious|date=September 2014}}

Tellurium is not known to have any allotropes,<ref>{{cite web |last = Emsley |first = John |url = http://www.rsc.org/periodic-table/element/52/tellurium |title = Tellurium |year = 2011 |access-date=November 25, 2013 |publisher=Royal Society of Chemistry}}</ref> although its typical form is hexagonal. Polonium has two allotropes, which are known as α-polonium and β-polonium.<ref>{{cite web|last = Emsley|first = John|url =http://www.rsc.org/periodic-table/element/84/polonium|title = Polonium|year = 2011 |access-date=November 25, 2013 |publisher=Royal Society of Chemistry}}</ref> α-polonium has a cubic crystal structure and converts to the rhombohedral β-polonium at 36&nbsp;°C.<ref name="ReferenceB"/>

The chalcogens have varying crystal structures. Oxygen's crystal structure is [[monoclinic crystal system|monoclinic]], sulfur's is [[orthorhombic crystal system|orthorhombic]], selenium and tellurium have the [[hexagonal crystal system|hexagonal]] crystal structure, while polonium has a [[cubic crystal system|cubic crystal structure]].<ref name="Jackson2002" /><ref name="The Elements">{{cite book|last = Gray|first = Theodore|title = The Elements|year = 2011|publisher = Black Bay and Leventhal publishers}}</ref>
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