Lanthanides| lanthanum electron configuration Lanthanum and the f-block elements||URDU/HINDl#bsc#bs
•Ba56 [Xe] 6s2-orbital is completely filled
•La57 [Xe] 4f0 5d1 6s2
After 57La filling of 5d-orbital is discontinued 58Ce, [Xe] 4f2 5d0 6s2
•
Reason
As the nuclear charge increases by one unit from 57La to 58Ce, 4f orbitals which were higher in energy up to 57La, fall slightly below the 5d level.
•4f orbitals therefore begin to fill and are completely filled up to 71Lu before the filling of 5d-orbital is resumed.
•71Lu electronic configuration 4f14 5d1 6s2.
Order of filling of orbitals is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p etc etc this is in accordance with n+l rule, i.e filling of electrons in orbitals is in increasing order of n+l value , n is principal quantum number and l is azimuthal quantum number, if n+l value is same then orbital of lower n value filled first.
Most stable configurations are when orbitals are full filled , exactly half filled and empty and elements preferred to attain most stable configurations
According to Observed configuration, the single 5d electron shifts into the 4f-subshell in all the cases except in Gd (64) and Lu (71)
because it is favourable energetically to move the single 5d-electron into 4f-orbital in
most of the elements and give more appropriate electronic configuration which is widely accepted.
•The filling of 4f orbital is not regular, e.g. 64Gd, [Xe] 4f7 5d1 6s2
•The additional electron in Gd does not enter 4f level , but instead it go to 5d-level. This is because 4f and 5d orbitals in Gd are about the same energy level and Gd atom has a tendency to retain the configuration with half filled 4f orbitals which are relatively more stable.
•General V.S configuration can be represented as 4f0,2-14 5d0 or 1 6s2.
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