Editing Radium (section)

==Chemistry==
Radium only exhibits the oxidation state of +2 in solution.{{sfn|Kirby|Salutsky|1964|page=4}} It forms the colorless Ra{{sup|2+}} [[cation]] in [[aqueous solution]], which is highly [[base (chemistry)|basic]] and does not form [[coordination complex|complexes]] readily.{{sfn|Kirby|Salutsky|1964|page=4}} Most radium compounds are therefore simple [[ionic bond|ionic]] compounds,{{sfn|Kirby|Salutsky|1964|page=4}} though participation from the [[Electron configuration|6s and 6p electrons]] (in addition to the valence 7s electrons) is expected due to [[relativistic quantum chemistry|relativistic effects]] and would enhance the [[covalent bond|covalent]] character of radium compounds such as [[Radium fluoride|RaF{{sub|2}}]] and Ra[[astatine|At]]{{sub|2}}.<ref name=Thayer>{{cite book |last1=Thayer |first1=John S. |chapter=Relativistic Effects and the Chemistry of the Heavier Main Group Elements |title=Relativistic Methods for Chemists |volume=10 |year=2010 |page=81 |doi=10.1007/978-1-4020-9975-5_2 |isbn=978-1-4020-9974-8 |series=Challenges and Advances in Computational Chemistry and Physics |publisher=Springer |location=Dordrecht }}</ref> For this reason, the [[standard electrode potential]] for the [[half-reaction]] Ra{{sup|2+}} (aq) + 2e{{sup|-}} → Ra (s) is −2.916&nbsp;[[volt|V]], even slightly lower than the value −2.92&nbsp;V for barium, whereas the values had previously smoothly increased down the group (Ca: −2.84&nbsp;V; Sr: −2.89&nbsp;V; Ba: −2.92&nbsp;V).{{sfn|Greenwood|Earnshaw|1997|page=111}} The values for barium and radium are almost exactly the same as those of the heavier alkali metals [[potassium]], [[rubidium]], and [[caesium]].{{sfn|Greenwood|Earnshaw|1997|page=111}}

===Compounds===
[[File:Ra-226 nitrate (10 mCi) - Photo by Dr Andrew R. Burgoyne - Oak Ridge National Laboratory.jpg|thumb|{{sup|226}}Ra nitrate (10 mCi) - Photo by Dr Andrew R. Burgoyne - Oak Ridge National Laboratory]]
Solid radium compounds are white as radium ions provide no specific coloring, but they gradually turn yellow and then dark over time due to self-[[radiolysis]] from radium's [[alpha decay]].{{sfn|Kirby|Salutsky|1964|page=4}} Insoluble radium compounds [[Coprecipitation|coprecipitate]] with all barium, most [[strontium]], and most [[lead]] compounds.{{sfn|Kirby|Salutsky|1964|page=8}}

[[Radium oxide]] (RaO) is poorly characterized, as the reaction of radium with air results in the formation of [[radium nitride]].<ref>{{Cite book |last=Tyler |first=Paul McIntosh |url=https://books.google.com/books?id=1KSfyGTUXpcC&pg=PA2 |title=Radium |date=1930 |publisher=U.S. Department of Commerce, Bureau of Mines |language=en}}</ref> [[Radium hydroxide]] (Ra(OH)<sub>2</sub>) is formed via the reaction of radium metal with water, and is the most readily soluble among the alkaline earth hydroxides and a stronger base than its barium congener, [[barium hydroxide]].{{sfn|Kirby|Salutsky|1964|pages=4-8}} It is also more soluble than [[actinium hydroxide]] and [[thorium hydroxide]]: these three adjacent hydroxides may be separated by precipitating them with [[ammonia]].{{sfn|Kirby|Salutsky|1964|pages=4-8}}

[[Radium chloride]] (RaCl<sub>2</sub>) is a colorless, [[Luminescence|luminescent]] compound. It becomes yellow after some time due to self-damage by the [[alpha radiation]] given off by radium when it decays. Small amounts of barium impurities give the compound a [[Rose (color)|rose color]].{{sfn|Kirby|Salutsky|1964|pages=4-8}} Its  It is soluble in water, though less so than [[barium chloride]], and its solubility decreases with increasing concentration of [[hydrochloric acid]]. Crystallization from aqueous solution gives the dihydrate RaCl<sub>2</sub>·2H<sub>2</sub>O, [[Isomorphism (crystallography)|isomorphous]] with its barium analog.{{sfn|Kirby|Salutsky|1964|pages=4-8}}

[[Radium bromide]] (RaBr<sub>2</sub>) is also a colorless, luminous compound.{{sfn|Kirby|Salutsky|1964|pages=4-8}} In water, it is more soluble than radium chloride. Like radium chloride, crystallization from aqueous solution gives the dihydrate RaBr<sub>2</sub>·2H<sub>2</sub>O, isomorphous with its barium analog. The ionizing radiation emitted by radium bromide excites [[nitrogen]] molecules in the air, making it glow. The [[alpha particle]]s emitted by radium quickly gain two electrons to become neutral [[helium]], which builds up inside and weakens radium bromide crystals. This effect sometimes causes the crystals to break or even explode.{{sfn|Kirby|Salutsky|1964|pages=4-8}}

[[Radium nitrate]] (Ra(NO<sub>3</sub>)<sub>2</sub>) is a white compound that can be made by dissolving [[radium carbonate]] in [[nitric acid]]. As the concentration of nitric acid increases, the solubility of radium nitrate decreases, an important property for the chemical purification of radium.{{sfn|Kirby|Salutsky|1964|pages=4-8}}

Radium forms much the same insoluble salts as its lighter congener barium: it forms the insoluble [[radium sulfate|sulfate]] (RaSO<sub>4</sub>, the most insoluble known sulfate), [[radium chromate|chromate]] (RaCrO<sub>4</sub>), [[radium carbonate|carbonate]] (RaCO<sub>3</sub>), [[radium iodate|iodate]] (Ra(IO<sub>3</sub>)<sub>2</sub>), [[radium tetrafluoroberyllate|tetrafluoroberyllate]] (RaBeF<sub>4</sub>), and nitrate (Ra(NO<sub>3</sub>)<sub>2</sub>). With the exception of the carbonate, all of these are less soluble in water than the corresponding barium salts, but they are all [[isostructural]] to their barium counterparts. Additionally, [[radium phosphate]], [[radium oxalate|oxalate]], and [[radium sulfite|sulfite]] are probably also insoluble, as they [[coprecipitation|coprecipitate]] with the corresponding insoluble barium salts.{{sfn|Kirby|Salutsky|1964|pages=8-9}} The great insolubility of radium sulfate (at 20&nbsp;°C, only 2.1&nbsp;[[milligram|mg]] will dissolve in 1&nbsp;[[kilogram|kg]] of water) means that it is one of the less biologically dangerous radium compounds.{{sfn|Kirby|Salutsky|1964|page=12}} The large ionic radius of Ra{{sup|2+}} (148&nbsp;pm) results in weak ability to form [[Coordination complex|coordination complexes]] and poor extraction of radium from aqueous solutions when not at high pH.{{sfn|Keller|Wolf|Shani|2011|pages=97–98}}