Solution:
SOLUTION
In lanthanides there is a regular decrease in the atomic radii as well as ionic radii of trivalent ions as the atomic number increases from
Ce to Lu. This decrease in size of atoms and ions is known as Lanthanide contraction.
Although the atomic radii do show some irreguairities but ionic radii decreases from La to Lu.
Thus the correct
Yb+3 < Pm+3 < Ce+3 < La+3
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