why does praseodymium possess electronic config. 4f36s2 instead of the expected one 4f25d16s2

why does praseodymium possess electronic config. 4f36s2 instead of the expected one 4f25d16s2


1 Answers

dolly bhatia
200 Points
6 years ago
4f electrons in free lanthanide atoms:
Outer electrons of the xenon core in lanthanide atoms are 5s and 5p. Bohr assumed that the 4f electrons in lanthanide atoms are inner electrons. If so, they must spend most of their time closer to the nucleus than do the outer 5s and 5p electrons of the core. Quantum mechanics can give us an idea of where these three types of electrons are to be found. One can draw radial distribution functions of the 4f, 5s and 5p electrons in praseodymium atom. They are obtained by self-consistent field calculations and show us probability of finding each kind of electron at different distances from nucleus.
Distributions suggest that, relative to the 5s and 5p electrons, the 4f electrons in the praseodymium atom are inner electrons because the 4f electron is most likely to be found at a distance which lies well inside highest peaks in the 5s and 5p distributions. The 4f electron spends most of its time closer to the nucleus than either a 5s or 5p electron.
The general electronic configuration of Lanthanides is [Xe] 4f1-145d16s2. Complete electronic configuration of Lanthanides can be given as 1s2 2s2 2p6 3s2 3d10 4s2 4p6 4d10 5s2 5p6. Lanthanoids have partially filled 4f-orbital. However, first member Lanthanum and last member Lutetium of lanthanide series have no partially filled 4f-orbital and should therefore be excluded from Lanthanoids.4f orbital is filled only after completing 5s, 5p and 6s orbitals. When f-orbital contains 0, 7 or 14 electrons, it is stable. Electronic configuration of Lanthanum is 2, 8, 18, 18, 9, 2. In Lanthanum, 4f subshell is vacant and 5d subshell contains one electron. Thus, in lanthanides, 4f orbitals are successively filled which means from Cerium (58) to Lutetium (71), additional electron must occupy vacant 5f orbital and 5d orbital must remain singly filled up. So, electronic configuration of Lanthanides should be 2, 8, 18, 19-32, 9, 2.
But, in observed electronic configuration, 5d1 electron gets shifted to 4f orbital and 5d orbital remains vacant because energies of 5d and 4f orbitals are closely similar.

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