Editing Lanthanide (section)

===Effect of 4f orbitals===
Viewing the lanthanides from left to right in the periodic table, the seven [[Atomic orbital#Orbitals_table|4f atomic orbital]]s become progressively more filled (see above and {{section link|Periodic table|Electron configuration table}}). The electronic configuration of most neutral gas-phase lanthanide atoms is [Xe]6s<sup>2</sup>4f<sup>''n''</sup>, where ''n'' is 56 less than the atomic number ''Z''. Exceptions are La, Ce, Gd, and Lu, which have 4f<sup>''n''−1</sup>5d<sup>1</sup> (though even then 4f<sup>''n''</sup> is a low-lying excited state for La, Ce, and Gd; for Lu, the 4f shell is already full, and the fifteenth electron has no choice but to enter 5d). With the exception of lutetium, the 4f orbitals are chemically active in all lanthanides and produce profound differences between lanthanide chemistry and [[transition metal]] chemistry. The 4f orbitals penetrate the [Xe] core and are isolated, and thus they do not participate much in bonding. This explains why crystal field effects are small and why they do not form π bonds.<ref name=CottonSA2006/> As there are seven 4f orbitals, the number of unpaired electrons can be as high as 7, which gives rise to the large [[magnetochemistry|magnetic moments]] observed for lanthanide compounds. 

Measuring the magnetic moment can be used to investigate the 4f electron configuration, and this is a useful tool in providing an insight into the chemical bonding.<ref name="BochkarevFedushkin1997">{{cite journal|last1=Bochkarev|first1=Mikhail N.|last2=Fedushkin|first2=Igor L.|last3=Fagin|first3=Anatoly A.|last4=Petrovskaya|first4=Tatyana V.|last5=Ziller|first5=Joseph W.|last6=Broomhall-Dillard|first6=Randy N. R.|last7=Evans|first7=William J.|title=Synthesis and Structure of the First Molecular Thulium(II) Complex: [TmI<sub>2</sub>(MeOCH<sub>2</sub>CH<sub>2</sub>OMe)<sub>3</sub>]|journal=Angewandte Chemie International Edition in English|volume=36|issue=12|year=1997|pages=133–135|doi=10.1002/anie.199701331}}</ref> The [[lanthanide contraction]], i.e. the reduction in size of the Ln<sup>3+</sup> ion from La<sup>3+</sup> (103 pm) to Lu<sup>3+</sup> (86.1&nbsp;pm), is often explained by the poor shielding of the 5s and 5p electrons by the 4f electrons.<ref name=CottonSA2006/>

[[File:Rareearthoxides.jpg|thumb|Lanthanide oxides: clockwise from top center: praseodymium, cerium, lanthanum, neodymium, samarium and gadolinium.]]
The chemistry of the lanthanides is dominated by the +3 oxidation state, and in Ln<sup>III</sup> compounds the 6s electrons and (usually) one 4f electron are lost and the ions have the configuration [Xe]4f<sup>(''n''−1)</sup>.<ref>{{cite web|author=Winter, Mark |url=http://www.webelements.com/lanthanum/atoms.html|title=Lanthanum ionisation energies|publisher=WebElements Ltd, UK|access-date=2 September 2010}}</ref> All the lanthanide elements exhibit the [[oxidation state]] +3. In addition, Ce<sup>3+</sup> can lose its single f electron to form Ce<sup>4+</sup> with the stable electronic configuration of xenon. Also, Eu<sup>3+</sup> can gain an electron to form Eu<sup>2+</sup> with the f<sup>7</sup> configuration that has the extra stability of a half-filled shell. Other than Ce(IV) and Eu(II), none of the lanthanides are stable in oxidation states other than +3 in aqueous solution. 

In terms of reduction potentials, the Ln<sup>0/3+</sup> couples are nearly the same for all lanthanides, ranging from −1.99 (for Eu) to −2.35 V (for Pr). Thus these metals are highly reducing, with reducing power similar to alkaline earth metals such as Mg (−2.36 V).<ref name = "Greenwood&Earnshaw"/>