Editing Californium (section)

== Characteristics ==

=== Physical properties ===
Californium is a silvery-white [[actinide]] metal{{sfn|Jakubke|1994|p=166}} with a [[melting point]] of {{convert|900|±|30|C|-1}} and an estimated [[boiling point]] of {{convert|1743|K|-1}}.{{sfn|Haire|2006|pp=1522–1523}} The pure metal is malleable and is easily cut with a knife. Californium metal starts to vaporize above {{convert|300|C|-1}} when exposed to a vacuum.{{sfn|Haire|2006|p=1526}} Below {{convert|51|K|C F|0|abbr=on}}<!-- KEEP KELVIN --> californium metal is either [[ferromagnetism|ferromagnetic]] or [[ferrimagnetism|ferrimagnetic]] (it acts like a magnet), between 48 and 66&nbsp;K it is [[antiferromagnetism|antiferromagnetic]] (an intermediate state), and above {{convert|160|K|C F}}<!-- KEEP IN KELVIN (−110&nbsp;°C) --> it is [[paramagnetic]] (external magnetic fields can make it magnetic).{{sfn|Haire|2006|p=1525}} It forms [[alloy]]s with [[lanthanide]] metals but little is known about the resulting materials.{{sfn|Haire|2006|p=1526}}

The element has two [[Crystal structure|crystalline forms]] at [[Standard atmosphere (unit)|standard atmospheric pressure]]: a double-[[Close-packing of equal spheres|hexagonal close-packed]] form dubbed alpha (α) and a [[Cubic crystal system|face-centered cubic]] form designated beta (β).{{efn|A double hexagonal close-packed (dhcp) [[unit cell]] consists of two hexagonal close-packed structures that share a common hexagonal plane, giving dhcp an ABACABAC sequence.{{sfn|Szwacki|2010|p=80}} }} The α form exists below 600–800&nbsp;°C with a density of 15.10&nbsp;g/cm<sup>3</sup> and the β form exists above 600–800&nbsp;°C with a density of 8.74&nbsp;g/cm{{sup|3}}.{{sfn|O'Neil|2006|p=276}} At 48&nbsp;[[pascal (unit)|GPa]] of pressure the β form changes into an [[orthorhombic crystal system]] due to delocalization of the atom's [[Electron shell|5f electrons]], which frees them to bond.{{sfn|Haire|2006|p=1522}}{{efn|The three lower-mass transplutonium elements—[[americium]], [[curium]], and [[berkelium]]—require much less pressure to delocalize their 5f electrons.{{sfn|Haire|2006|p=1522}} }}

The [[bulk modulus]] of a material is a measure of its resistance to uniform pressure. Californium's bulk modulus is {{val|50|5|u=GPa}}, which is similar to trivalent lanthanide metals but smaller than more familiar metals, such as aluminium (70&nbsp;GPa).{{sfn|Haire|2006|p=1522}}

=== Chemical properties and compounds ===
{{Further|Californium compounds}}
{| class="wikitable"
|+ Representative californium compounds{{sfn|Jakubke|1994|p=166}}{{efn|Other +3 oxidation states include the sulfide and [[metallocene]].{{sfn|Cotton|Wilkinson|Murillo|Bochmann|1999|p=1163}}}}
! state !! compound !! formula !! color !!
|-
| +2 || [[californium(II) bromide]] || CfBr{{sub|2}} || yellow || style="background:#FFDA5C;" |
|-
| +2 || [[californium(II) iodide]] || CfI{{sub|2}} || dark violet || style="background:#702A97;" |
|-
| +3 || [[californium(III) oxide]] || Cf{{sub|2}}O{{sub|3}} || yellow-green || style="background:#B4E532;" |
|-
| +3 || [[californium(III) fluoride]] || CfF{{sub|3}} || bright green || style="background:#59FF00;" |
|-
| +3 || [[californium(III) chloride]] || CfCl{{sub|3}} || emerald green || style="background:#26B645;" |
|-
| +3 || [[californium(III) bromide]] || CfBr{{sub|3}} || yellowish green || style="background:#81C340;" |
|-
| +3 || [[californium(III) iodide]] || CfI{{sub|3}} || lemon yellow || style="background:#FFE826;" |
|-
| +3
|[[californium(III) polyborate]]
|Cf[B{{sub|6}}O{{sub|8}}(OH){{sub|5}}]
|pale green || style="background:#96CD91;" |
|-
| +4 || [[californium(IV) oxide]] || CfO{{sub|2}} || black brown || style="background:#3D3016;" |
|-
| +4 || [[californium(IV) fluoride]] || CfF{{sub|4}} || green || style="background:#2B9E22;" |
|}
Californium exhibits oxidation states of 4, 3, or 2. It typically forms eight or nine bonds to surrounding atoms or ions. Its chemical properties are predicted to be similar to other primarily 3+ valence actinide elements{{sfn|Seaborg|2004}} and the element [[dysprosium]], which is the lanthanide above californium in the periodic table.{{sfn|CRC|2006|p=4.8}} Compounds in the +4 oxidation state are strong [[oxidizing agent]]s and those in the +2 state are strong [[reducing agent]]s.{{sfn|Jakubke|1994|p=166}}

The element slowly tarnishes in air at room temperature, with the rate increasing when moisture is added.{{sfn|O'Neil|2006|p=276}} Californium reacts when heated with [[hydrogen]], [[nitrogen]], or a [[chalcogen]] (oxygen family element); reactions with dry hydrogen and aqueous [[mineral acid]]s are rapid.{{sfn|O'Neil|2006|p=276}} <!-- EXPLAIN Only californium-249 is suitable for chemical study.<ref name="Emeleus">{{cite book|last=Emeleus|first=H. J.|title=Advances in Inorganic Chemistry|page=33|year=1987|publisher=Academic Press | isbn = 978-0-12-023631-2 }}</ref> /EXPLAIN -->

Californium is only [[aqueous solution|water-soluble]] as the californium(III) [[cation]]. Attempts to [[redox|reduce or oxidize]] the +3 ion in solution have failed.{{sfn|CRC|2006|p=4.8}} The element forms a water-soluble [[chloride]], [[nitrate]], [[perchlorate]], and [[sulfate]] and is precipitated as a [[fluoride]], [[oxalate]], or [[hydroxide]].{{sfn|Seaborg|2004}} Californium is the heaviest actinide to exhibit covalent properties, as is observed in the californium borate.<ref>{{Cite journal|title = Unusual structure, bonding and properties in a californium borate|journal = Nature Chemistry|date = May 1, 2014|issn = 1755-4330|pages = 387–392|volume = 6|issue = 5|doi = 10.1038/nchem.1896|pmid = 24755589|language = en|first1 = Matthew J.|last1 = Polinski|first2 = Edward B. Garner|last2 = Iii|first3 = Rémi|last3 = Maurice|first4 = Nora|last4 = Planas|first5 = Jared T.|last5 = Stritzinger|first6 = T. Gannon|last6 = Parker|first7 = Justin N.|last7 = Cross|first8 = Thomas D.|last8 = Green|first9 = Evgeny V.|last9 = Alekseev|url = http://hal.in2p3.fr/in2p3-00966875|bibcode = 2014NatCh...6..387P|citeseerx = 10.1.1.646.749| s2cid=104331283 }}</ref>

=== Isotopes ===
{{main|Isotopes of californium}}
Twenty [[isotope]]s of californium are known ([[mass number]] ranging from 237 to 256<ref name="NNDC2008" />); the most stable are {{sup|251}}Cf with [[half-life]] 898 years, {{sup|249}}Cf with half-life 351 years, {{sup|250}}Cf at 13.08 years, and {{sup|252}}Cf at 2.645 years.<ref name="NNDC2008" /> All other isotopes have half-life shorter than a year, and most of these have half-lives less than 20 minutes.<ref name="NNDC2008" />

{{sup|249}}Cf is formed by [[beta decay]] of berkelium-249, and most other californium isotopes are made by subjecting berkelium to intense neutron radiation in a [[nuclear reactor]].{{sfn|CRC|2006|p=4.8}} Though californium-251 has the longest half-life, its production yield is only 10% due to its tendency to collect neutrons (high [[neutron capture]]) and its tendency to interact with other particles (high [[neutron cross section]]).{{sfn|Haire|2006|p=1504}}

{{sup|252}}Cf is a very strong [[neutron]] emitter, which makes it extremely [[radioactive]] and harmful.<ref>{{cite journal|author = Hicks, D. A. |title = Multiplicity of Neutrons from the Spontaneous Fission of Californium-252|journal = Physical Review|date = 1955|volume = 97|issue = 2|pages = 564–565|doi = 10.1103/PhysRev.97.564|last2 = Ise|first2 = John|last3 = Pyle|first3 = Robert V.|bibcode = 1955PhRv...97..564H |url = http://www.escholarship.org/uc/item/6031k6m2}}</ref><ref>{{cite journal|author = Hicks, D. A. |title = Spontaneous-Fission Neutrons of Californium-252 and Curium-244|journal = Physical Review |date = 1955|volume = 98|issue = 5|pages = 1521–1523|doi = 10.1103/PhysRev.98.1521|last2 = Ise|first2 = John|last3 = Pyle|first3 = Robert V.|bibcode = 1955PhRv...98.1521H }}</ref><ref>{{cite journal|author =Hjalmar, E.|author2 =Slätis, H.|author3 =Thompson, S.G. |title = Energy Spectrum of Neutrons from Spontaneous Fission of Californium-252| journal = Physical Review| date = 1955| volume = 100|issue =5|pages = 1542–1543| doi = 10.1103/PhysRev.100.1542|bibcode = 1955PhRv..100.1542H }}</ref> {{sup|252}}Cf, 96.9% of the time, [[alpha decay]]s to [[curium]]-248; the other 3.1% of decays are [[spontaneous fission]].<ref name="NNDC2008" /> One [[microgram]] (μg) of {{sup|252}}Cf emits 2.3&nbsp;million neutrons per second, an average of 3.7 neutrons per spontaneous fission.<ref name="osti">{{cite journal|author = Martin, R. C.|author2 = Knauer, J. B.|author3 = Balo, P. A.| title = Production, Distribution, and Applications of Californium-252 Neutron Sources| date = 1999|url = http://www.osti.gov/bridge/purl.cover.jsp?purl=/15053-AE6cnN/native/ |doi = 10.1016/S0969-8043(00)00214-1|journal = Applied Radiation and Isotopes |volume = 53|issue = 4–5|pages = 785–92|pmid = 11003521 }}</ref> Most other isotopes of californium, alpha decay to curium ([[atomic number]] 96).<ref name="NNDC2008" />