(a). Why second asnd third transition series elements show similar size?
(b). Why electronic configuration of lanthanoids not known with certainty?
(c). The electronic configuration of actinide elements are not known with certainty. Explain.
(d). Why there is similarties (horizontal and vertical) in successive members of the transition series?

Explanation

(a). In the third transition series after lanthanum, there is lanthanoid contraction. Due to this constraction, the size of any atom of the third transition series is almost the same as that of the elements lying just above in the second transition series. This leads to similarity in their properties.
(b). In the lanthanoids 4f and 5d subshells are very close in energy. The outermost 6s orbital remains filled with 2 electrons `(6s^2)`. The electron can easily jump from 4f to 5d or vice versa. Hence, their electronic configurations are not known with certainly.
(c). In actinides, 5f and 6d subshells are close in energy. Hence, they show a large number of oxidation states. The electrons can jump from 5f to 6d or vice versa. Hence, their electronic configuration are not known with certainty.
(d). this is vecause of the fact that along a horizontal row, electrons enter an incomplete inner shell in building up an atom, while the outer level remains almost unchanged. In vertical columns, similarities are due to similar electronic configuration even in the d-subshells.