Question

All the lanthanoid elements form stable compounds containing the +3 cation. Of the few other ionic forms known, Ce forms the stable + 4 series of ionic compounds and Eu the stable + 2 series. Account for these unusual ionic forms in terms of their electronic configurations.

Answer 1

   Lanthanides are metallic elements in the periodic table from “Lanthanum to lutetium”. Lanthanides are elements of f-block in which 4f electron shell are filled. The atomic numbers run from 51 to 71. All the Lanthanide elements contain the +3 cations and forms the stable compounds.

   The electronic configuration of Cerium is  ${ 4f }^{ 1 }{ 5d }^{ 1 }{ 6s }^{ 2 }$ and it is stable at +4  cation.  ${ Ce }^{ 3+ }$ can lose one 'f' electron to form  ${ Ce }^{ 4+ }$ with the fixed Xenon configuration.  

   The Europium configuration  is  ${ 4f }^{ 7 }{ 6s }^{ 2 }$ and it is stable at +2 cation.  ${ Eu }^{ 3+ }$ earn an electron for the formation of ${ Eu }^{ 2+ }$ with the ${ 4f }^{ 7 }{ 6s }^{ 2 }$ configuration and has the additional “stability of a partially-filled shell”.