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Question
Explain the cause of lanthanoid contraction?
Solution
Cause of the lanthanoid contraction:
1) As the atomic number of the members of lanthanoids series increases, the positive charge on nucleus increases by +1 unit and one more electron enters in the same 4f subshell.
2)There is inadequate shielding of one electron by another electron in the same 4f subshell.
3) The shielding of 4f electrons is less effective than the shielding of 5d electron as 4f orbital is more diffused in shape than 5d orbital
4) Thus, greater effective nuclear charge is experienced with the increase in the
atomic number. This results in slight pull of the valence shell towards the nucleus.
5)Thus, the atomic and ionic radii decrease slightly with increase in the atomic number, thereby causing lanthanoid contraction
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