Editing Lanthanum (section)

===Chemical===
As expected from [[periodic trend]]s, lanthanum has the largest [[atomic radius]] of the lanthanides. Hence, it is the most reactive among them, tarnishing quite rapidly in air, turning completely dark after several hours and can readily burn to form [[lanthanum(III) oxide]], {{chem|La|2|O|3}}, which is almost as [[Basic (chemistry)|basic]] as [[calcium oxide]].<ref>{{harvp|Greenwood|Earnshaw|1984|pp=1105–1107}}</ref> A centimeter-sized sample of lanthanum will corrode completely in a year as its oxide [[spallation|spalls]] off like iron [[rust]], instead of forming a protective oxide coating like [[aluminium]], scandium, yttrium, and lutetium.<ref>{{Cite web |title=Rare-Earth Metal Long Term Air Exposure Test |url=http://www.elementsales.com/re_exp/index.htm |access-date=2009-08-08}}</ref> Lanthanum reacts with the [[halogen]]s at room temperature to form the trihalides, and upon warming will form [[binary compound]]s with the nonmetals nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic.<ref name=Greenwood1106/><ref name=patnaik/> Lanthanum reacts slowly with water to form [[lanthanum hydroxide|lanthanum(III) hydroxide]], {{chem|La|(OH)|3}}.<ref name=webelements>{{Cite web |title=Chemical reactions of lanthanum |url=https://www.webelements.com/lanthanum/chemistry.html |access-date=2009-06-06 |publisher=Webelements}}</ref> In dilute [[sulfuric acid]], lanthanum readily forms the aquated tripositive ion {{chem|[La|(H|2|O)|9|]|3+}}: This is colorless in aqueous solution since {{chem|La|3+}} has no d or f electrons.<ref name=webelements/> Lanthanum is the strongest and [[HSAB theory|hardest]] base among the [[rare earth element]]s, which is again expected from its being the largest of them.<ref name=Greenwood1434>{{harvp|Greenwood|Earnshaw|1984|p=1434}}</ref>

Some lanthanum(II) compounds are also known, but they are much less stable.<ref name=patnaik/><!-- Although mentioned in the section on physical properties, someone looking for chemical characteristics easily misses it their --> Therefore, in officially naming compounds of lanthanum its oxidation number always is to be mentioned.