Editing Sulfur (section)

===Electron transfer reactions===
[[Image:Lapis lazuli block.jpg|thumb|upright|[[Lapis lazuli]] owes its blue color to a [[trisulfur]] radical anion ({{chem|S|3|-}})]]

Sulfur polycations, {{chem2|S8(2+)}}, {{chem2|S4(2+)}} and {{chem2|S19(2+)}} are produced when sulfur is reacted with oxidizing agents in a strongly acidic solution.<ref>Shriver, Atkins. Inorganic Chemistry, Fifth Edition. W. H. Freeman and Company, New York, 2010; pp 416</ref> The colored solutions produced by dissolving sulfur in [[oleum]] were first reported as early as 1804 by C.&nbsp;F. Bucholz, but the cause of the color and the structure of the polycations involved was only determined in the late 1960s. {{chem2|S8(2+)}} is deep blue, {{chem2|S4(2+)}} is yellow and {{chem2|S19(2+)}} is red.<ref name="Greenwood-1997b" />

Reduction of sulfur gives various [[polysulfide]]s with the formula {{chem|S|''x''|2-}}, many of which have been obtained in crystalline form. Illustrative is the production of [[sodium tetrasulfide]]:

{{block indent|{{chem2|4 Na + S8 -> 2 Na2S4}}}}

Some of these dianions dissociate to give [[radical anion]]s. For instance, {{chem2|S3-|link=Trisulfur}} gives the blue color of the rock [[lapis lazuli]].

[[File:S@CNT.jpg|thumb|Two parallel sulfur chains grown inside a single-wall [[carbon nanotube]] (CNT, a). Zig-zag (b) and straight (c) S chains inside double-wall CNTs<ref>{{cite journal |doi=10.1038/ncomms3162 |pmid=23851903 |pmc=3717502 |title=Conducting linear chains of sulphur inside carbon nanotubes |journal=Nature Communications |volume=4 |pages=2162 |year=2013 |last1=Fujimori |first1=Toshihiko |last2=Morelos-Gómez |first2=Aarón |last3=Zhu |first3=Zhen |last4=Muramatsu |first4=Hiroyuki |last5=Futamura |first5=Ryusuke |last6=Urita |first6=Koki |last7=Terrones |first7=Mauricio |last8=Hayashi |first8=Takuya |last9=Endo |first9=Morinobu |last10=Young Hong |first10=Sang |last11=Chul Choi |first11=Young |last12=Tománek |first12=David |last13=Kaneko |first13=Katsumi |bibcode=2013NatCo...4.2162F}}</ref>]]
This reaction highlights a distinctive property of sulfur: its ability to [[catenation|catenate]] (bind to itself by formation of chains). [[Protonation]] of these polysulfide anions produces the [[polysulfane]]s, H<sub>2</sub>S<sub>''x''</sub>, where ''x'' = 2, 3, and 4.<ref>{{cite book |title=Handbook of Preparative Inorganic Chemistry |edition=2nd |editor-first=G. |editor-last=Brauer |publisher=Academic Press |year=1963 |location=New York |volume=1 |page=421}}</ref> Ultimately, reduction of sulfur produces sulfide salts:

{{block indent|16 Na + S<sub>8</sub> → 8 Na<sub>2</sub>S}}

The interconversion of these species is exploited in the [[sodium–sulfur battery]].