Question

Why the atomic radii of the second and third transition series are almost same?

Answer 1

In elements of second transition series, the number of shells increases compared to first transition series so, their atomic radii is greater than that of the first transition series. The atomic radii of the second and third transition series are almost same due to lanthanoid contraction.
Lanthanoid contraction is the imperfect shielding of one electron by another in the same set of orbitals.

The filling of 4f before 5d orbital results in a regular decrease in atomic radii because f orbitals have very poor shielding effect which leads to higher nuclear attraction on outermost electrons. Also the drastic increase in positive nuclear charge from 4d to 5d increase its attraction toward the electrons.