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  4. Assertion: In transition elements radii of 5d series are virtually the same as those of the corresponding members of 4d series. Reason: The filling of 4f orbitals before 5d orbitals results in regular decrease in atomic radii.

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Q. Assertion : In transition elements radii of $5d$ series are virtually the same as those of the corresponding members of $4d$ series.
Reason : The filling of $4f$ orbitals before $5d$ orbitals results in regular decrease in atomic radii.

The d-and f-Block Elements

Solution:

It is the phenomenon occurring in the $3^{ rd }$ series of transition element. As they have electron filling up the 4 f shell which is filled with the $5 d$ series. The $4 f$ shell has very weak sheillding effect and is highly attracted towards the nucleus due to high nuclear charge so the filling of $4 f$ orbitals before $5 d$ orbital resulting in their atomic radius which is very similar to the elements of the $2^{\text {nd }}$ series i.e, Zr $=160$ atomic radius and Hf $=159$ atomic radius.