Statement I: 4d and 5d series elements having closer atomic radii

Statement II: In d-block elements 6d series is incomplete

Statement I and Statement II are true and statement II is not correct explanation of statement I

The answer is 2. Statement I and Statement II are true but statement II is not correct explanation of statement I.

Statement I is true because the atomic radii of the 4d and 5d transition metals are very similar. This is due to the lanthanide contraction, which causes the atomic radii of the lanthanide elements to decrease as the atomic number increases. The lanthanide contraction affects the 4d and 5d transition metals because the 4f orbitals are filled after the 5d orbitals.

Statement II is also true, but it is not the correct explanation for statement I. The 6d series is incomplete because the lanthanide contraction makes it difficult for the 6d orbitals to be filled. However, this does not affect the atomic radii of the 4d and 5d transition metals.

The correct explanation for statement I is that the atomic radii of the 4d and 5d transition metals are similar because the 4f orbitals have a poor shielding effect. This means that the 4f electrons do not effectively shield the 5d electrons from the nuclear charge. As a result, the 5d electrons are more strongly attracted to the nucleus, which causes the atomic radii of the 4d and 5d transition metals to be similar.

Therefore, both statements are true, but statement II is not the correct explanation for statement I.