Assertion : The order of atomic radii of `Cu, Ag` and `Au` is `Cu lt Ag ~~ Au`.
Reason : The atomic radii of `4d` series elements are larger than those of `3d` series elements but generally the radii of `4d` and `5d` series elements are almost identical.

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

B

Explanation

The atomic radii, in general, increase down the group. The atomic radii of second series are larger than those of first transition series. In the atoms of the second transition series, the number of shells are more than those of the first transition series. As a result, the atoms of second transition series are larger than those of the elements of the first transition series. But the atomic radii of second and third trasition series are almost the same. This phenomenon is associated with theintervention of the `4f` orbitals which must be filled before the `5d` series of elements begin. The filling of `4f` before `5d` orbital results in a regular decrease in atomic radii called Lantanoid cantraction which essentially compensates for the expected increase in atomic size with increasing atomic number.