Electronic configuration of the 14 elements Ce58 to Lu71 ( called 4f block elecments or Lanthanides)

All these elements follow La57 in the periodic table and hence called Lanthanides. The conf. of La57 is [Xe] 4f0 5d1 6s2 in which 5d sub-shell is singly filled (5d1 configuration) and 4f is vacant (4f0 conf). This conf shows that as we proceed from Ce58 to Lu71 (14 lanthanides) the additional electron should enter the vacant 4f sub-shell and 5d sub-shell should remain singly filled ( 5d1 conf) in all the 14 lanthanides, as it is in La57. Thus the expected conf of the atoms of lanthanides should be in which 5d sub-shell is singly filled and 4f sub-shell is progressively filled up with electron as we move from Ce58 to Lu71 in the series. We can say that the expected configuration of the atoms of lanthanides should be [Xe] 4f1-14 5d1 6s2.

But since the energies of 5d and 4f orbitals are closely similar in all the lanthanides, excepting Gd64 and Lu71, 5d1 electron gets shifted to 4f orbitals and hence 5d remains vacant. The presence of one electron in 5d inGd64 & Lu71 gives half-filled & completely filled respectively 4f orbitals which are more stable.

Valence shell conf of the atoms of 14 lanthanides can be written as 4f2-14 5d0-1 6s2.

Following steps can be followed for writing conf.

1. find out the difference between the atomic number of the given element and that of the noble gas present in the previous period. For ex, the difference between Pr59 & Xe54=59-54=5. Let this difference be denoted as x. This difference denotes the number of electrons present in the valence shell of the given element.

2.Now find out the sub-shells which are to be filled up with x electrons.

3.Then fill up these electrons in sub shells.

Thus the expected valence shell conf of Pr59, 6th period = 6s2 5d1 4f2, the actual is 6s2 5d0 4f3