Editing Rare-earth element (section)

==Classification==
Before the time that [[ion exchange]] methods and [[elution]] were available, the separation of the rare earths was primarily achieved by repeated [[precipitation (chemistry)|precipitation]] or [[crystallization]]. In those days, the first separation was into two main groups, the cerium earths (lanthanum, cerium, praseodymium, neodymium, and samarium) and the yttrium earths (scandium, yttrium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium). 
Europium, gadolinium, and terbium were either considered as a separate group of rare-earth elements (the terbium group), or europium was included in the cerium group, and gadolinium and terbium were included in the yttrium group. In the latter case, the f-block elements are split into half: the first half (La–Eu) form the cerium group, and the second half (Gd–Yb) together with group 3 (Sc, Y, Lu) form the yttrium group.

The reason for this division arose from the difference in [[solubility]] of rare-earth double sulfates with sodium and potassium. The sodium double sulfates of the cerium group are poorly soluble, those of the terbium group slightly, and those of the yttrium group are very soluble.<ref>B. Smith Hopkins: "Chemistry of the rarer elements", D. C. Heath & Company, 1923.</ref> Sometimes, the yttrium group was further split into the erbium group (dysprosium, holmium, erbium, and thulium) and the ytterbium group (ytterbium and lutetium), but today the main grouping is between the cerium and the yttrium groups.<ref>{{Ullmann |volume=31 |page=184 |last1=McGill |first1=Ian |contribution=Rare Earth Elements |doi=10.1002/14356007.a22_607}}</ref> Today, the rare-earth elements are classified as light or heavy rare-earth elements, rather than in cerium and yttrium groups.

===Light versus heavy classification===
The classification of rare-earth elements is inconsistent between authors.<ref name=Zepf-2013>{{cite book |title=Rare earth elements: a new approach to the nexus of supply, demand and use: exemplified along the use of neodymium in permanent magnets |last=Zepf |first=Volker |date=2013 |publisher=Springer |isbn=978-3-642-35458-8 |location=Berlin; London |language=en}}</ref> The most common distinction between rare-earth elements is made by [[atomic number]]s. Those with low atomic numbers are referred to as light rare-earth elements (LREE), those with high atomic numbers are the heavy rare-earth elements (HREE), and those that fall in between are typically referred to as the middle rare-earth elements (MREE).<ref name="Rollinson">{{cite book |last=Rollinson |first=Hugh R. |title=Using geochemical data: evaluation, presentation, interpretation |date=1993 |publisher=Longman Scientific & Technical |isbn=978-0-582-06701-1 |location=Harlow, Essex, England |oclc=27937350}}</ref> Commonly, rare-earth elements with atomic numbers 57 to 61 (lanthanum to promethium) are classified as light and those with atomic numbers 62 and greater are classified as heavy rare-earth elements.<ref name=Brownlow>{{cite book |title=Geochemistry |last=Brownlow |first=Arthur H |date=1996 |publisher=Prentice Hall |oclc=33044175 |isbn=978-0-13-398272-5 |location=Upper Saddle River, N.J.}}</ref> 

Increasing atomic numbers between light and heavy rare-earth elements and decreasing [[atomic radius|atomic radii]] throughout the series causes chemical variations.<ref name=Brownlow/> Europium is exempt of this classification as it has two valence states: Eu{{sup|2+}} and Eu{{sup|3+}}.<ref name=Brownlow/> Yttrium is grouped as a heavy rare-earth element due to chemical similarities.<ref name=gsl/> The break between the two groups is sometimes put elsewhere, such as between elements 63 (europium) and 64 (gadolinium).<ref>{{cite web |title=Seltene Erden – Daten & Fakten |url=https://www.oeko.de/oekodoc/1110/2011-001-de.pdf |publisher=Öko-Institut e.V. |date=Jan 2011}}</ref> The actual metallic densities of these two groups overlap, with the "light" group having densities from 6.145 (lanthanum) to 7.26 (promethium) or 7.52 (samarium) g/cc, and the "heavy" group from 6.965 (ytterbium) to 9.32 (thulium), as well as including yttrium at 4.47. Europium has a density of 5.24.