(i) In the 1st, 2nd and 3rd transition series, the 3d, 4d and 5d orbitals are respectively filled. We know that elements in the same vertical column generally have similar electronic configurations.
In the first transition series, two elements show unusual electronic configurations:
Cr (24) = 3d54s1
Cu (29) = 3d104s1
Similarly, there are exceptions in the second transition series. These are:
Mo (42) = 4d55s1
Tc (43) = 4d65s1
Ru (44) = 4d75s1
Rh (45) = 4d85s1
Pd (46) = 4d105s0
Ag (47) = 4d105s1
There are some exceptions in the third transition series as well. These are:
W (74) = 5d46s2
Pt (78) = 5d96s1
Au (79) = 5d106s1
As a result of these exceptions, it happens many times that the electronic configurations of the elements present in the same group are dissimilar.
(ii) In each of the three transition series the number of oxidation states shown by the elements is the maximum in the middle 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.
For example [Fe(Cn)6]4-, [Co(NH3)6]3+, [Ti(H2O)6]3+ are stable complexes, but no such complexes are known for the second and third transition series such as Mo, W, Rh, In. They form complexes in which their oxidation states are high.
For example: WCl6, ReF7, RuO4, etc.
(iii) In each of the three transition series, the first ionisation enthalpy increases from left to right. However, there are some exceptions. The first ionisation enthalpies of the third transition series are higher than those of the first and second transition series. This occurs due to the poor shielding effect of 4felectrons in the third transition series.