(i) In the 1st, 2nd and 3rd transition series, the 3d, 4d, and 5d orbitals are filled with electrons respectively. We know that elements in the same group generally have similar electronic configurations.

In the first transition series, two elements generally show unusual electronic configurations. They are:

Cu (29) = 3d¹⁰4s¹

Cr (24) = 3d⁵4s¹

Similarly, the second transition series also include some exceptions. These are:

Ag(47) = 4d¹⁰5s¹

Pd(46) = 4d¹⁰ 5s⁰

Rh(45) = 4d⁸5s¹

Ru (44) = 4d⁷5s¹

Tc (43) = 4d⁶5s¹

Mo (42) = 4d⁵5s¹

The third transition series include some exceptions as well. These are:

Au (79 ) = 5d¹⁰ 6s¹

Pt (78) = 5d⁹ 6s¹

W (74) = 5d⁴ 6s²

As a result of these exceptions, it happens many times that the electronic configurations of the elements present in the same group are not similar.

(ii) In each of the three transition series the number of oxidation states shown by the elements is the maximum in the middle of the series and the minimum at the extreme ends. However, +2 and +3 oxidation states are quite stable for all elements present in the first transition series. All metals present in the first transition series form stable compounds in the +2 and +3 oxidation states. The stability of the +2 and +3 oxidation states decreases in the second and the third transition series, wherein higher oxidation states are more important.

Some stable complexes are :

 [Fe_II(CN)₆​]⁴⁻, [Co_III(NH₃​)₆​]³⁺, [Ti(H₂​O)₆​]³⁺

The issue that lies here is that no such coordination complexes are known for the second and third transition series such as Mo, W, Rh, In. They form such coordination complexes in which their oxidation states are high. For example: WCl₆, ReF₇, RuO₄, etc.