(a). Why second asnd third transition series elements show similar size?
(b). Why electronic configuration of lanthanoids not known with certainty?

Explanation

### Solution:

**(a) Why do the second and third transition series elements show similar size?**

1. **Definition of Transition Series**: Transition metals are elements that have partially filled d-orbitals. The second transition series includes elements from Zr (Z=40) to Ag (Z=47), while the third transition series includes elements from Hf (Z=72) to Au (Z=79).

2. **Lanthanide Contraction**: The similarity in size between the second and third transition series elements can be attributed to the phenomenon known as "lanthanide contraction." This occurs due to the poor shielding effect of the f-electrons in the lanthanides.

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