Editing Europium (section)

===Chemical properties===
Europium is the most reactive rare-earth element. It rapidly oxidizes in air, so that bulk oxidation of a centimeter-sized sample occurs within several days.<ref>{{cite web|url=http://www.elementsales.com/re_exp/index.htm|author=Hamric, David |date=November 2007 |title = Rare-Earth Metal Long Term Air Exposure Test|work=elementsales.com|access-date=2009-08-08}}</ref> Its reactivity with water is comparable to that of [[calcium]], and the reaction is
:2 Eu + 6 H<sub>2</sub>O → 2 Eu(OH)<sub>3</sub> + 3 H<sub>2</sub>

Because of the high reactivity, samples of solid europium rarely have the shiny appearance of the fresh metal, even when coated with a protective layer of mineral oil. Europium ignites in air at 150 to 180&nbsp;°C to form [[europium(III) oxide]]:<ref>{{cite book |doi=10.1016/B978-0-12-813840-3.00002-8 |chapter=Potential of europium and samarium β -diketonates as red light emitters in organic light-emitting diodes |title=Lanthanide-Based Multifunctional Materials |date=2018 |last1=Ugale |first1=Akhilesh |last2=Kalyani |first2=Thejo N. |last3=Dhoble |first3=Sanjay J. |pages=59–97 |isbn=978-0-12-813840-3 }}</ref>
:4 Eu + 3 O<sub>2</sub> → 2 Eu<sub>2</sub>O<sub>3</sub>

Europium dissolves readily in dilute [[sulfuric acid]] to form pale pink<ref>{{Greenwood&Earnshaw2nd|page=1243}}</ref> solutions of [Eu(H<sub>2</sub>O)<sub>9</sub>]<sup>3+</sup>:
:2 Eu + 3 H<sub>2</sub>SO<sub>4</sub> + 18 H<sub>2</sub>O → 2 [Eu(H<sub>2</sub>O)<sub>9</sub>]<sup>3+</sup> + 3 {{chem|SO|4|2−}} + 3 H<sub>2</sub>

====Eu(II) vs. Eu(III)====
Although usually trivalent, europium readily forms divalent compounds. This behavior is unusual for most lanthanides, which almost exclusively form compounds with an oxidation state of +3. The +2 state has an [[electron configuration]] 4''f''<sup>7</sup> because the half-filled ''f''-shell provides more stability. In terms of size and [[coordination number]], europium(II) and [[barium]](II) are similar. The [[sulfate]]s of both [[Barium sulfate|barium]] and [[Europium(II) sulfate|europium(II)]] are also highly insoluble in water.<ref>{{cite book|doi = 10.1002/9780470132333.ch19|chapter = Europium(II) Salts|title = Inorganic Syntheses|date = 1946|last1 = Cooley|first1 = Robert A.|last2 = Yost|first2 = Don M.|last3 = Stone|first3 = Hosmer W.|isbn = 978-0-470-13233-3|volume = 2|pages = 69–73}}</ref> Divalent europium is a mild reducing agent, oxidizing in air to form Eu(III) compounds. In anaerobic, and particularly geothermal conditions, the divalent form is sufficiently stable that it tends to be incorporated into minerals of calcium and the other alkaline earths. This ion-exchange process is the basis of the "negative [[europium anomaly]]", the low europium content in many lanthanide minerals such as [[monazite]], relative to the [[chondrite|chondritic]] abundance. [[Bastnäsite]] tends to show less of a negative europium anomaly than does monazite, and hence is the major source of europium today. The development of easy methods to separate divalent europium from the other (trivalent) lanthanides made europium accessible even when present in low concentration, as it usually is.<ref>{{Ullmann|volume=31|page=199|last1=McGill|first1=Ian|contribution=Rare Earth Elements|doi=10.1002/14356007.a22_607}}.</ref>