Editing Uranium (section)

==Compounds==
{{Main|Uranium compounds}}
[[File:Uranium reactions.svg|thumb|upright=1.5|right|Reactions of uranium metal]]

===Oxidation states and oxides===

====Oxides====
{{See also|Uranium oxide}}
{{multiple image
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| image1 = U3O8lattice.jpg
| alt1 = Ball and stick model of layered crystal structure containing two types of atoms.
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| image2 = UO2lattice.jpg
| alt2 = Ball and stick model of cubic-like crystal structure containing two types of atoms.
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| footer = [[Triuranium octoxide]] (left) and [[uranium dioxide]] (right) are the two most common uranium oxides.
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Calcined uranium yellowcake, as produced in many large mills, contains a distribution of uranium oxidation species in various forms ranging from most oxidized to least oxidized. Particles with short residence times in a calciner will generally be less oxidized than those with long retention times or particles recovered in the stack scrubber. Uranium content is usually referenced to {{chem|U|3|O|8}}, which dates to the days of the [[Manhattan Project]] when {{chem|U|3|O|8}} was used as an analytical chemistry reporting standard.<ref>{{Cite book |last1=Kloprogge |first1=J. Theo |last2=Ponce |first2=Concepcion P. |last3=Loomis |first3=Tom A. |title=The periodic table : nature's building blocks : an introduction to the naturally occurring elements, their origins and their uses |date=2021 |publisher=Elsevier |isbn=978-0-12-821538-8 |location=Amsterdam |pages=861–862 |oclc=1223058470}}</ref>

[[Phase (matter)|Phase relationships]] in the uranium-oxygen system are complex. The most important oxidation states of uranium are uranium(IV) and uranium(VI), and their two corresponding [[oxide]]s are, respectively, [[uranium dioxide]] ({{chem|UO|2}}) and [[uranium trioxide]] ({{chem|UO|3}}).{{sfn|Seaborg|1968|p=779}} Other [[uranium oxide]]s such as [[uranium monoxide]] (UO), [[diuranium pentoxide]] ({{chem|U|2|O|5}}), and [[uranium peroxide]] ({{chem|UO|4|·2H|2|O}}) also exist.

The most common forms of uranium oxide are [[triuranium octoxide]] ({{chem|U|3|O|8}}) and {{chem|UO|2}}.<ref name="ANL-Chem">{{cite web |title=Chemical Forms of Uranium |publisher=Argonne National Laboratory |url=http://web.ead.anl.gov/uranium/guide/ucompound/forms/index.cfm |access-date=18 February 2007 |archive-url=https://web.archive.org/web/20060922180607/http://web.ead.anl.gov/uranium/guide/ucompound/forms/index.cfm |archive-date=22 September 2006 |url-status=dead}}</ref> Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions. Triuranium octoxide is (depending on conditions) the most stable compound of uranium and is the form most commonly found in nature. Uranium dioxide is the form in which uranium is most commonly used as a nuclear reactor fuel.<ref name="ANL-Chem"/> At ambient temperatures, {{chem|UO|2}} will gradually convert to {{chem|U|3|O|8}}. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.<ref name="ANL-Chem"/>

====Aqueous chemistry====
[[File:U Oxstufen.jpg|thumb|left|upright=0.85|Uranium in its oxidation states III, IV, V, VI]]
Salts of many [[oxidation state]]s of uranium are water-[[solubility|soluble]] and may be studied in [[aqueous solution]]s. The most common ionic forms are {{chem|U|3+}} (brown-red), {{chem|U|4+}} (green), {{chem|UO|2|+}} (unstable), and [[uranyl|{{chem|UO|2|2+}}]] (yellow), for U(III), U(IV), U(V), and U(VI), respectively.{{sfn|Seaborg|1968|p=778}} A few [[solid]] and semi-metallic compounds such as UO and [[Uranium monosulfide|US]] exist for the formal oxidation state uranium(II), but no simple ions are known to exist in solution for that state. Ions of {{chem|U|3+}} liberate [[hydrogen]] from [[water]] and are therefore considered to be highly unstable. The {{chem|UO|2|2+}} ion represents the uranium(VI) state and is known to form compounds such as [[uranyl carbonate]], [[uranyl chloride]] and [[uranyl sulfate]]. {{chem|UO|2|2+}} also forms [[complex (chemistry)|complexes]] with various [[organic compound|organic]] [[chelation|chelating]] agents, the most commonly encountered of which is [[uranyl acetate]].{{sfn|Seaborg|1968|p=778}}

Unlike the uranyl salts of uranium and [[polyatomic ion]] uranium-oxide cationic forms, the [[uranate]]s, salts containing a polyatomic uranium-oxide anion, are generally not water-soluble.

====Carbonates====
The interactions of carbonate anions with uranium(VI) cause the [[Pourbaix diagram]] to change greatly when the medium is changed from water to a carbonate containing solution. While the vast majority of carbonates are insoluble in water (students are often taught that all carbonates other than those of alkali metals are insoluble in water), uranium carbonates are often soluble in water. This is because a U(VI) cation is able to bind two terminal oxides and three or more carbonates to form anionic complexes.

{|class="wikitable" style="text-align:center; float:center"
|+[[Pourbaix diagram]]s<ref name="medusa">Puigdomenech, Ignasi (2004) [https://www.kth.se/che/medusa/chemeq-1.369367 ''Hydra/Medusa Chemical Equilibrium Database and Plotting Software'']. [[KTH Royal Institute of Technology]]</ref>
|-
|width=50% |[[File:Uranium pourdaix diagram in water.png|center|190x180px|alt=A graph of potential vs. pH showing stability regions of various uranium compounds]]
|width=50% |[[File:Uranium pourdiax diagram in carbonate media.png|center|190x180px|alt=A graph of potential vs. pH showing stability regions of various uranium compounds]]
|-
|Uranium in a non-complexing aqueous medium<br/>(e.g. [[perchloric acid]]/sodium hydroxide).<ref name="medusa" />
|Uranium in carbonate solution
|-
|[[File:Uranium fraction diagram with no carbonate.png|center|250x180px|alt=A graph of potential vs. pH showing stability regions of various uranium compounds]]
|[[File:Uranium fraction diagram with carbonate present.png|center|250x180px|alt=A graph of potential vs. pH showing stability regions of various uranium compounds]]
|-
|Relative concentrations of the different chemical forms of uranium in a non-complexing aqueous medium<br/>(e.g. [[perchloric acid]]/sodium hydroxide).<ref name="medusa" />
|Relative concentrations of the different chemical forms of uranium in an aqueous carbonate solution.<ref name="medusa" />
|}

====Effects of pH====
The uranium fraction diagrams in the presence of carbonate illustrate this further: when the pH of a uranium(VI) solution increases, the uranium is converted to a hydrated uranium oxide hydroxide and at high pHs it becomes an anionic hydroxide complex.

When carbonate is added, uranium is converted to a series of carbonate complexes if the pH is increased. One effect of these reactions is increased solubility of uranium in the pH range 6 to 8, a fact that has a direct bearing on the long term stability of spent uranium dioxide nuclear fuels.

===Hydrides, carbides and nitrides===
Uranium metal heated to {{convert|250|to|300|C|F}} reacts with [[hydrogen]] to form [[uranium hydride]]. Even higher temperatures will reversibly remove the hydrogen. This property makes uranium hydrides convenient starting materials to create reactive uranium powder along with various uranium [[carbide]], [[nitride]], and [[halide]] compounds.{{sfn|Seaborg|1968|p=782}} Two crystal modifications of uranium hydride exist: an α form that is obtained at low temperatures and a β form that is created when the formation temperature is above 250&nbsp;°C.{{sfn|Seaborg|1968|p=782}}

[[Uranium carbide]]s and [[uranium nitride]]s are both relatively [[Chemically inert|inert]] [[semimetal]]lic compounds that are minimally soluble in [[acid]]s, react with water, and can ignite in [[air]] to form {{chem|U|3|O|8}}.{{sfn|Seaborg|1968|p=782}} Carbides of uranium include uranium monocarbide (U[[carbon|C]]), uranium dicarbide ({{chem|UC|2}}), and diuranium tricarbide ({{chem|U|2|C|3}}). Both UC and {{chem|UC|2}} are formed by adding carbon to molten uranium or by exposing the metal to [[carbon monoxide]] at high temperatures. Stable below 1800&nbsp;°C, {{chem|U|2|C|3}} is prepared by subjecting a heated mixture of UC and {{chem|UC|2}} to mechanical stress.{{sfn|Seaborg|1968|p=780}} Uranium nitrides obtained by direct exposure of the metal to [[nitrogen]] include uranium mononitride (UN), uranium dinitride ({{chem|UN|2}}), and diuranium trinitride ({{chem|U|2|N|3}}).{{sfn|Seaborg|1968|p=780}}

===Halides===
[[File:Uranium hexafluoride crystals sealed in an ampoule.jpg|thumb|[[Uranium hexafluoride]] is the feedstock used to separate uranium-235 from natural uranium.|alt=Snow-like substance in a sealed glass ampoule.]]
<!--[[File:Uranium-hexafluoride-2D-V2.svg|thumb|upright|[[Uranium hexafluoride]] is the feedstock used to separate uranium-235 from natural uranium.|alt=Skeletal diagram of a chemical compound having a uranium atom in its center bonded to 6 fluorine atoms.]]-->
All uranium fluorides are created using [[uranium tetrafluoride]] ({{chem|UF|4}}); {{chem|UF|4}} itself is prepared by hydrofluorination of uranium dioxide.{{sfn|Seaborg|1968|p=782}} Reduction of {{chem|UF|4}} with hydrogen at 1000&nbsp;°C produces [[uranium trifluoride]] ({{chem|UF|3}}). Under the right conditions of temperature and pressure, the reaction of solid {{chem|UF|4}} with gaseous [[uranium hexafluoride]] ({{chem|UF|6}}) can form the intermediate fluorides of {{chem|U|2|F|9}}, {{chem|U|4|F|17}}, and [[Uranium pentafluoride|{{chem|UF|5}}]].{{sfn|Seaborg|1968|p=782}}

At room temperatures, {{chem|UF|6}} has a high [[vapor pressure]], making it useful in the [[gaseous diffusion]] process to separate the rare uranium-235 from the common uranium-238 isotope. This compound can be prepared from uranium dioxide and uranium hydride by the following process:{{sfn|Seaborg|1968|p=782}}

:{{chem|UO|2}} + 4 HF → {{chem|UF|4}} + 2 {{chem|H|2|O}} (500&nbsp;°C, endothermic)
:{{chem|UF|4}} + {{chem|F|2}} → {{chem|UF|6}} (350&nbsp;°C, endothermic)

The resulting {{chem|UF|6}}, a white solid, is highly [[chemical reaction|reactive]] (by fluorination), easily [[sublimation (chemistry)|sublimes]] (emitting a vapor that behaves as a nearly [[ideal gas]]), and is the most volatile compound of uranium known to exist.{{sfn|Seaborg|1968|p=782}}

One method of preparing [[uranium tetrachloride]] ({{chem|UCl|4}}) is to directly combine [[chlorine]] with either uranium metal or uranium hydride. The reduction of {{chem|UCl|4}} by hydrogen produces [[uranium trichloride]] ({{chem|UCl|3}}) while the higher chlorides of uranium are prepared by reaction with additional chlorine.{{sfn|Seaborg|1968|p=782}} All uranium chlorides react with water and air.

[[Bromide]]s and [[iodide]]s of uranium are formed by direct reaction of, respectively, [[bromine]] and [[iodine]] with uranium or by adding {{chem|UH|3}} to those element's acids.{{sfn|Seaborg|1968|p=782}} Known examples include: [[Uranium(III) bromide|{{chem|UBr|3}}]], [[Uranium(IV) bromide|{{chem|UBr|4}}]], [[Uranium(III) iodide|{{chem|UI|3}}]], and [[Uranium(IV) iodide|{{chem|UI|4}}]]. {{chem|UI|5}} has never been prepared. Uranium oxyhalides are water-soluble and include [[Uranyl fluoride|{{chem|UO|2|F|2}}]], {{chem|UOCl|2}}, [[Uranyl chloride|{{chem|UO|2|Cl|2}}]], and [[Uranyl bromide|{{chem|UO|2|Br|2}}]]. Stability of the oxyhalides decrease as the [[atomic weight]] of the component halide increases.{{sfn|Seaborg|1968|p=782}}