Question
What do you understand by lanthanoid contraction?
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Answer
Atomic number increases in lanthanoids, there is an overall decrease in atomic radius and ionic radius from La to Lu (lanthanum to lutetium), this is called lanthanoid contraction.
This decrease in the values of atomic radii is not regular as is the regular decrease in $M ^{+3}$ ions. This contraction is also similar to the normal transition ranges. The reason for this is the increasing nuclear charge and shielding effect.
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