Editing Hafnium (section)

==Characteristics==

===Physical characteristics===
[[File:Hafnium bits.jpg|thumb|left|Pieces of hafnium]]
<section begin=properties />
Hafnium is a shiny, silvery, [[ductility|ductile]] [[metal]] that is [[corrosion]]-resistant and chemically similar to zirconium<ref name="ASTM" /> in that they have the same number of [[valence electron]]s and are in the same group. Also, their [[relativistic quantum chemistry|relativistic effects]] are similar: The expected expansion of atomic radii from period 5 to 6 is almost exactly canceled out by the [[lanthanide contraction]]. Hafnium changes from its alpha form, a hexagonal close-packed lattice, to its beta form, a body-centered cubic lattice, at {{convert|2388|K}}.<ref>{{cite journal |last1=O'Hara |first1=Andrew |last2=Demkov |first2=Alexander A. |title=Oxygen and nitrogen diffusion in α-hafnium from first principles |journal=[[Applied Physics Letters]] |date=2014 |volume=104 |issue=21 |page=211909 |doi=10.1063/1.4880657 |bibcode=2014ApPhL.104u1909O }}</ref>  The physical properties of hafnium metal samples are markedly affected by zirconium impurities, especially the nuclear properties, as these two elements are among the most difficult to separate because of their chemical similarity.<ref name="ASTM" /><section end=properties />

A notable physical difference between these metals is their [[density]], with zirconium having about one-half the density of hafnium. The most notable [[nuclear physics|nuclear]] properties of hafnium are its high [[thermal neutron|thermal]] [[neutron capture cross section]] and that the nuclei of several different hafnium isotopes readily absorb two or more [[neutron]]s apiece.<ref name="ASTM" /> In contrast with this, zirconium is practically transparent to thermal neutrons, and it is commonly used for the metal components of nuclear reactors—especially the cladding of their [[nuclear fuel rod]]s.

===Chemical characteristics===
{{See also|Hafnium#Chemical compounds|label1=&sect;&nbsp;Chemical compounds}}
[[File:Hafnium(IV) oxide.jpg|thumb|left|[[Hafnium(IV) oxide|Hafnium dioxide]] (HfO<sub>2</sub>)]]
Hafnium reacts in air to form a [[Passivation (chemistry)|protective film]] that inhibits further [[corrosion]].  Despite this, the metal is attacked by hydrofluoric acid and concentrated sulfuric acid, and can be oxidized with [[halogen]]s or burnt in air.  Like its sister metal zirconium, finely divided hafnium can ignite spontaneously in air. The metal is resistant to concentrated [[alkali]]s.

As a consequence of [[lanthanide contraction]], the chemistry of hafnium and zirconium is so similar that the two cannot be separated based on differing chemical reactions. The melting and boiling points of the compounds and the [[solubility]] in solvents are the major differences in the chemistry of these twin elements.<ref name="Holl">{{cite book|publisher = [[Walter de Gruyter]]|date = 1985|edition = 91–100|pages=1056–1057|isbn = 978-3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|first = Arnold F.|last = Holleman |author2 = Wiberg, Egon |author3=Wiberg, Nils |language = de|doi=10.1515/9783110206845|author-link2=Egon Wiberg}}</ref>

===Isotopes===
{{Main|Isotopes of hafnium}}
At least 40 isotopes of hafnium have been observed, ranging in [[mass number]] from 153 to 192.<ref name="EC">{{cite web|title = Periodic Table of Elements: Hf – Hafnium|url = http://environmentalchemistry.com/yogi/periodic/Hf-pg2.html#Nuclides|publisher = J.K. Barbalace Inc.|access-date=2021-11-12|first=Kenneth L.|last=Barbalace|website=environmentalchemistry.com}}</ref><ref name="Audi">{{NUBASE 2016}}</ref><ref name=PRC108>{{cite journal |first1=K. |last1=Haak |first2=O. B. |last2=Tarasov |first3=P. |last3=Chowdhury |display-authors=et al. |title=Production and discovery of neutron-rich isotopes by fragmentation of <sup>198</sup>Pt |date=2023 |journal=Physical Review C |volume=108 |number=34608 |page=034608 |doi=10.1103/PhysRevC.108.034608|bibcode=2023PhRvC.108c4608H |s2cid=261649436 }}</ref> The five stable isotopes have mass numbers ranging from 176 to 180 inclusive. The radioactive isotopes' [[half-life|half-lives]] range from 400&nbsp;[[SI prefix|ms]] for <sup>153</sup>Hf<ref name="Audi" /> to {{val|7.0|e=16}} years for the most stable one, the [[primordial radionuclide|primordial]] <sup>174</sup>Hf.<ref name="EC" /><ref name=174Hf2020>{{cite journal |title=Search for α decay of naturally occurring Hf-nuclides using a Cs<sub>2</sub>HfCl<sub>6</sub> scintillator |year=2020 |first1=V. |last1=Caracciolo |first2=S. |last2=Nagorny |first3=P. |last3=Belli |first4=R. |last4=Bernabei |first5=F. |last5=Cappella |first6=R. |last6=Cerulli |first7=A. |last7=Incicchitti |first8=M. |last8=Laubenstein |first9=V |last9=Merlo |first10=S. |last10=Nisi |first11=P. |last11=Wang |display-authors=3 |journal=Nuclear Physics A |volume=1002 |number=121941 |page=121941 |doi=10.1016/j.nuclphysa.2020.121941|arxiv=2005.01373 |bibcode=2020NuPhA100221941C |s2cid=218487451 }}</ref>

The [[extinct radionuclide]] <sup>182</sup>Hf has a half-life of {{val|8.9|0.1|u=million years}}, and is an [[Hafnium–tungsten dating|important tracker isotope]] for the formation of [[planetary core]]s.<ref name=":0">{{cite journal | vauthors = Kleine T, Walker RJ | title = Tungsten Isotopes in Planets | journal = Annual Review of Earth and Planetary Sciences | volume = 45 | issue = 1 | pages = 389–417 | date = August 2017 | pmid = 30842690 | pmc = 6398955 | doi = 10.1146/annurev-earth-063016-020037 | bibcode = 2017AREPS..45..389K }}</ref> The [[nuclear isomer]] <sup>178m2</sup>Hf was at the [[Hafnium controversy|center of a controversy]] for several years regarding its potential use as a weapon.

===Occurrence===
[[File:Zircão.jpeg|thumb|left|Zircon crystal (2×2 cm) from [[Tocantins]], [[Brazil]]]]

Hafnium is estimated to make up about between 3.0 and 4.8 [[Parts per million|ppm]] of the [[Earth]]'s upper [[crust (geology)|crust]] by mass.<ref>{{Cite book |last1=Haygarth |first1=John C. |url=https://onlinelibrary.wiley.com/doi/10.1002/9781118788417.ch1 |title=Zirconium and Hafnium |last2=Graham |first2=Ronald A. |date=2013-09-30 |publisher=John Wiley & Sons, Inc. |isbn=978-1-118-78841-7 |editor-last=Mishra |editor-first=Brajendra |location=Hoboken, NJ, USA |pages=1–71 |language=en |doi=10.1002/9781118788417.ch1}}</ref>{{rp|5}} <ref name=CRC>ABUNDANCE OF ELEMENTS IN THE EARTH’S CRUST AND IN THE SEA, ''CRC Handbook of Chemistry and Physics,'' 97th edition (2016–2017), p. 14-17</ref> It does not exist as a free element on Earth, but is found combined in [[solid solution]] with zirconium in natural [[zirconium]] compounds such as [[zircon]], ZrSiO<sub>4</sub>, which usually has about 1–4% of the Zr replaced by Hf. Rarely, the Hf/Zr ratio increases during crystallization to give the isostructural mineral [[hafnon]] {{chem2|(Hf,Zr)SiO4}}, with atomic Hf > Zr.<ref>{{cite book|title = The Rock-Forming Minerals: Orthosilicates|first1 = William Alexander|last1 = Deer |author-link1=William Alexander Deer|last2 = Howie|first2= Robert Andrew |author-link2=Robert A. Howie|last3=Zussmann|first3=Jack|isbn=978-0-582-46526-8|date = 1982|publisher = [[Longman|Longman Group Limited]]|pages=418–442|volume=1A|url=https://books.google.com/books?id=Yi0SAQAAMAAJ&q=9780582465268}}</ref> An obsolete name for a variety of zircon containing unusually high Hf content is ''alvite''.<ref>{{cite journal|title = The Mineralogy of Hafnium|first = O. Ivan|last = Lee|journal = [[Chemical Reviews]]|date = 1928|volume = 5|issue=1|pages=17–37|doi = 10.1021/cr60017a002|url=https://archive.org/details/in.ernet.dli.2015.27353/page/n23/mode/2up}}</ref>

A major source of zircon (and hence hafnium) ores is [[heavy mineral sands ore deposits]], [[pegmatite]]s, particularly in [[Brazil]] and [[Malawi]], and [[carbonatite]] intrusions, particularly the Crown Polymetallic Deposit at [[Mount Weld]], [[Western Australia]]. A potential source of hafnium is [[Trachyte|trachyte tuffs]] containing rare zircon-hafnium silicates [[eudialyte]] or [[armstrongite]], at [[Dubbo]] in [[New South Wales]], Australia.<ref>{{cite web|url = http://www.alkane.com.au/projects/nsw/dubbo/DZP%20Summary%20June07.pdf|title = The Dubbo Zirconia Project |last=Chalmers|first=Ian|date = June 2007|publisher = Alkane Resources Limited|access-date = 2008-09-10|archive-url = https://web.archive.org/web/20080228054038/http://www.alkane.com.au/projects/nsw/dubbo/DZP%20Summary%20June07.pdf|archive-date = 2008-02-28}}</ref>