Editing Protactinium (section)

==Properties==
Protactinium is an [[actinide]] positioned in the [[periodic table]] to the left of [[uranium]] and to the right of [[thorium]], and many of its physical properties are intermediate between its neighboring actinides. Protactinium is denser and more rigid than thorium, but is lighter than uranium; its melting point is lower than that of thorium, but higher than that of uranium. The thermal expansion, electrical, and thermal conductivities of these three elements are comparable and are typical of [[post-transition metal]]s. The estimated [[shear modulus]] of protactinium is similar to that of [[titanium]].<ref>Seitz, Frederick and Turnbull, David (1964) [https://books.google.com/books?id=F9V3a-0V3r8C&pg=PA289 Solid state physics: advances in research and applications], Academic Press, pp. 289–291, {{ISBN|0-12-607716-9}}.</ref> Protactinium is a metal with silvery-gray luster that is preserved for some time in air.<ref name="CRC">{{cite book |author = Hammond, C. R. |title = The Elements, in Handbook of Chemistry and Physics |edition = 81st |publisher = CRC press |isbn = 978-0-8493-0485-9 |date = 2004-06-29 |url-access = registration |url = https://archive.org/details/crchandbookofche81lide }}</ref><ref name="Myasoedov">{{cite book |last1=Myasoedov |first1=B. F. |last2=Kirby |first2=H. W.|last3=Tananaev |first3=I. G. |title=The Chemistry of the Actinide and Transactinide Elements |s2cid=93796247 |editor1-first=L. R. |editor1-last=Morss |editor2-first=N. M. |editor2-last=Edelstein |editor3-first=J. |editor3-last=Fuger |edition=3rd |date=2006 |publisher=Springer |location=Dordrecht, The Netherlands |chapter=Chapter 4: Protactinium |isbn=978-1-4020-3555-5|bibcode=2011tcot.book.....M |doi=10.1007/978-94-007-0211-0 }}</ref> Protactinium easily reacts with oxygen, water vapor, and acids, but not with alkalis.<ref name="Emsley" />

At room temperature, protactinium crystallizes in the body-centered [[Tetragonal crystal system|tetragonal]] structure, which can be regarded as distorted body-centered cubic lattice; this structure does not change upon compression up to 53 GPa. The structure changes to face-centered cubic (''fcc'') upon cooling from high temperature, at about 1200&nbsp;°C.<ref name="exp" /><ref>Young, David A. (1991) [https://books.google.com/books?id=F2HVYh6wLBcC&pg=PA222 Phase diagrams of the elements], University of California Press, p. 222, {{ISBN|0-520-07483-1}}.</ref> The thermal expansion coefficient of the tetragonal phase between room temperature and 700&nbsp;°C is 9.9{{e|-6}}/°C.<ref name="exp" />

Protactinium is [[paramagnetism|paramagnetic]] and no magnetic transitions are known for it at any temperature.<ref>Buschow, K. H. J. (2005) [https://books.google.com/books?id=N9mvytGEBtwC&pg=PA129 Concise encyclopedia of magnetic and superconducting materials], Elsevier, pp. 129–130, {{ISBN|0-08-044586-1}}.</ref> It becomes [[Superconductivity|superconductive]] at temperatures below 1.4&nbsp;K.<ref name="Emsley" /><ref name="super">{{cite journal| display-authors =4| author =Fowler, R. D. |title = Superconductivity of Protactinium| journal = [[Physical Review Letters]]| volume = 15 |page = 860|date = 1965| doi = 10.1103/PhysRevLett.15.860| bibcode=1965PhRvL..15..860F| issue =22| last2 =Matthias| first2 =B.| last3 =Asprey| first3 =L.| last4 =Hill| first4 =H.| last5 =Lindsay| first5 =J.| last6 =Olsen| first6 =C.| last7 =White| first7 =R.}}</ref> Protactinium tetrachloride is paramagnetic at room temperature, but becomes [[ferromagnetism|ferromagnetic]] when cooled to 182&nbsp;K.<ref>{{cite journal|last1=Hendricks|first1=M. E.|title=Magnetic Properties of Protactinium Tetrachloride|journal=[[Journal of Chemical Physics]]|volume=55|pages=2993–2997|date=1971|doi=10.1063/1.1676528|issue=6|bibcode = 1971JChPh..55.2993H }}</ref>

Protactinium exists in two major [[oxidation state]]s: +4 and +5, both in solids and solutions; and the +3 and +2 states, which have been observed in some solids. As the electron configuration of the neutral atom is [Rn]5f<sup>2</sup>6d<sup>1</sup>7s<sup>2</sup>, the +5 oxidation state corresponds to the low-energy (and thus favored) 5f<sup>0</sup> configuration. Both +4 and +5 states easily form [[hydroxide]]s in water, with the predominant ions being Pa(OH)<sup>3+</sup>, {{chem2|Pa(OH)2(2+)}}, {{chem2|Pa(OH)3(+)}}, and Pa(OH)<sub>4</sub>, all of which are colorless.<ref name="g1265">[[#Greenwood|Greenwood]], p. 1265</ref> Other known protactinium ions include {{chem2|PaCl2(2+)}}, {{chem2|PaSO4(2+)}}, PaF<sup>3+</sup>, {{chem2|PaF2(2+)}}, {{chem2|PaF6(-)}}, {{chem2|PaF7(2-)}}, and {{chem2|PaF8(3-)}}.<ref name="g1275">[[#Greenwood|Greenwood]], p. 1275</ref><ref name="trif" />