(i) In the$1^{st}{2}^{nd }and 3^{rd}$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.

$Cr\left(24\right)=3d^{5}4s^1$
$Cu\left(29\right)=3d^{10}4s^1$

In the first transition series, two elements show unusual electronic configurations:

Similarly, there are exceptions in the second transition series. These are:

$Mo\left(42\right)=4d^{5}5s^1$
$Tc\left(43\right)=4d^{6}5s^1$
$Ru\left(44\right)=4d^{7}5s^1$
$Rh\left(46\right)=4d^{8}5s^1$
$Pd\left(46\right)=4d^{10}5s^0$
$Ag\left(47\right)=4d^{10}5s^1$

There are some exceptions in the third transition series as well. These are:

$W\left(74\right)=5d^{4}6s^2$
$Pt\left(78\right)=5d^{9}6s^1$
$Au\left(79\right)=5d^{10}6s^1$

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 ${\left[F\stackrel{n}{e} {\left(Cn\right)}_6\right]}^{4-},{\left[C\stackrel{}{o{(NH}_{}}\right]}^{}$₃O)6lll3+,Ti(H₂O)63+ 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: WCl${}_6$, ReF${}_7$,

RuO${}_4$, 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 4f electrons in the third transition series.

Certain elements in the second transition series have higher first ionisation enthalpies than

elements corresponding to the same vertical column in the first transition series.

There are also elements in the 2nd transition series whose first ionisation enthalpies are

lower than those of the elements corresponding to the same vertical column in the 1st

transition series.

(iv) Atomic size generally decreases from left to right across a period. Now, among the

three transition series, atomic sizes of the elements in the second transition series are

greater than those of the elements corresponding to the same vertical column in the first

transition series. However, the atomic sizes of the elements in the third transition series

are virtually the same as those of the corresponding members in the second transition

series. This is due to lanthanoid contraction.