to fill from lanthanum, and hence the expected electronic configuration of Lanthanum(La) is [Xe] f d s but the actual electronic configuration of Lanthanum is Figure . position of inner transition elements XII U4-D-Block-Jerald XII U4-D-Block-Jerald - - - - [Xe] f d s and it belongs to d block. Filling of 4f orbital starts from Cerium (Ce) and its electronic configuration is [Xe] f d s . As we move from Cerium to other elements the additional electrons are progressively filled in 4f orbitals as shown in the table.
Table : electronic configuration of Lanthanum and Lanthanoids Name of the element Atomic number Symbol Electronic configuration Lanthanum La [Xe] f d s Cerium Ce [Xe] f d s Praseodymium Pr [Xe] f d s Neodymium Nd [Xe] f d s Promethium Pm [Xe] f d s Samarium Sm [Xe] f d s Europium Eu [Xe] f d s Gadolinium Gd [Xe] f d s Terbium Tb [Xe] f d s Dysprosium Dy [Xe] f d s Holmium Ho [Xe] f d s Erbium Er [Xe] f d s Thulium Tm [Xe] f d s Ytterbium Yb [Xe] f d s Lutetium Lu [Xe] f d s In Gadolinium (Gd) and Lutetium (Lu) the 4f orbitals, are half-filled and completely filled, and one electron enters 5d orbitals. Hence the general electronic configuration of 4f series of elements can be written as [Xe] f d s Oxidation state of lanthanoids: The common oxidation state of lanthanoids is + . In addition to that some of the lanthanoids also show either + or + oxidation states. Gd + and Lu + ions have extra stability, it is due to the fact that they have exactly half filled and completely filled f-orbitals respectively.
Their electronic configurations are