Europium breaks the ranks. It is the most reactive rare-earth element and, in contrast to most other lanthanides, it also forms stable divalent compounds due to the stable half-filled f electron shell for Eu(II). Many of its applications are related to the f electrons which transitions lead to sharp lines in the luminescence spectrum, which can, for example, be used in colour TV screens or in anti-counterfeiting.  For three decades, Europium has been used in bio probes in immunoassays to detect the presence of a compound or biomolecule. Also, its discovery is connected to the lines in the emission spectrum. In 1901 Demarçay reported the isolation of a new element, for which he proposed the name Europium and which previously was observed as an anomalous red line in the emission spectrum of samarium.  Curiously, its stable divalent state leads to a geochemical phenomenon that is known as "europium anomaly" which describes that it is often found to be enriched or depleted in minerals compared to other rare earth elements. This can happen for example, when Eu(II) replaces Ca(II) in some minerals. 
 Suyver, F.; Meijerink, A. Europium Beveiligt de Euro. Chemisch2Weekblad 2002, 98 (4), 12–13.
 Bünzli, J.-C. Europium in the Limelight. Nature Chem 2010, 2 (8), 696–696.
 Goldschmidt, V. M. Geochemistry; Oxford Univ. Press: London, 1958.
 Axt, M.; Alifantes, J.; Costa, V. E. U. Use of Chiral Lanthanide Shift Reagents in the Elucidation of NMR Signals from Enantiomeric Mixtures of Polycyclic Compounds †. J. Chem. Soc., Perkin Trans. 2 1999, No. 12, 2783–2788.
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