Editing Thorium (section)

===Reactivity===
Thorium is a highly [[reactivity (chemistry)|reactive]] and electropositive metal. With a [[standard reduction potential]] of −1.90&nbsp;V for the {{chem2|Th(4+)}}/Th couple, it is somewhat more electropositive than zirconium or aluminium.{{sfn|Stoll|2005|p=6}} Finely divided thorium metal can exhibit [[pyrophoricity]], spontaneously igniting in air.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} When heated in air, thorium [[swarf|turnings]] ignite and burn with a brilliant white light to produce the dioxide. In bulk, the reaction of pure thorium with air is slow, although corrosion may occur after several months; most thorium samples are contaminated with varying degrees of the dioxide, which greatly accelerates corrosion.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} Such samples slowly tarnish, becoming grey and finally black at the surface.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}

At [[standard temperature and pressure]], thorium is slowly attacked by water, but does not readily dissolve in most common acids, with the exception of [[hydrochloric acid]], where it dissolves leaving a black insoluble residue of ThO(OH,Cl)H.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}<ref name="CRC">{{cite book| last= Hammond| first= C. R.| title= The Elements, in Handbook of Chemistry and Physics| edition= 81st| publisher= [[CRC Press]]| isbn= 978-0-8493-0485-9| date= 2004| url-access= registration| url= https://archive.org/details/crchandbookofche81lide}}</ref> It dissolves in concentrated [[nitric acid]] containing a small quantity of catalytic [[fluoride]] or [[fluorosilicate]] ions;{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}<ref name="ekhyde">{{cite book|url= http://www.radiochemistry.org/periodictable/pdf_books/pdf/rc000034.pdf|author= Hyde, E. K.|title= The radiochemistry of thorium|publisher= [[National Academy of Sciences]]|date= 1960|access-date= 29 September 2017|archive-date= 5 March 2021|archive-url= https://web.archive.org/web/20210305130415/http://www.radiochemistry.org/periodictable/pdf_books/pdf/rc000034.pdf}}</ref> if these are not present, [[passivation (chemistry)|passivation]] by the nitrate can occur, as with uranium and plutonium.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}{{sfn|Greenwood|Earnshaw|1997|p=1264}}<ref>{{cite journal |last1=Moore |first1=Robert Lee |last2=Goodall |first2=C. A. |first3=J. L. |last3=Hepworth |first4=R. A. |last4=Watts |date=May 1957 |title=Nitric Acid Dissolution of Thorium. Kinetics of Fluoride-Catalyzed Reaction |journal=Industrial & Engineering Chemistry |volume=49 |issue=5 |pages=885–887 |doi=10.1021/ie50569a035}}</ref>

[[File:Kristallstruktur Uran(IV)-fluorid.png|thumb|alt=Crystal structure of thorium tetrafluoride|Crystal structure of thorium tetrafluoride<br/>{{chem2|Th(4+)}}: <span style="color:silver; background:silver;">__</span>&nbsp;&nbsp;/&nbsp;&nbsp;{{chem2|F−}}: <span style="color:#9c0; background:#9c0;">__</span>]]