Assertion : `4d` and `5d` series elements have nearly same atomic radius.
Reason : Lanthanoid contraction.

Options

A. If both assetion and reason are true and the reason is the correct explanation of the assertion.
B. If both assertion and reason are ture but reason is not the correct explanation of the assertion.
C. If assertion is true but reason is false.
D. If assertion is false but reason is true.

Answer

A

Explanation

The atomic radii of the second (`4d` series ) and third, (`5d` series) transition series are almost the same.
This phenomenon is associated with te intervention of the `4f` orbitals which must be filled before the `5d` series of element begin. The filling of `4f` before `5d` orbital results in a regular decrease in atomic radii called Lanthanid contraction which essentially compensates fro the expected in atomic size with incresing atomic number. The net result of the lanthanoid contraction is the the second and the third `d` series exhibit similar radii (e.g., `Zr 160 "pm", Hf 159 "pm")`.