Question

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 a regular decrease in atomic radii.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

Lanthanoid Contraction :-

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