Question

Four successive members of the first series transition metals are listed below for which one of them the standard potential $\left( {{E}_{{{{M}^{2+}}}/{M}\;}} \right)$ value has a positive sign?
[A] Ni (Z = 28)
[B] Cu (Z = 29)
[C] Fe (Z = 26)
[D] Co (Z = 27)

Answer 1

Hint: To solve this write down the electronic configuration of the elements mentioned and also their electronic configuration in +2 states. Do not forget that electrons will leave from d-orbitals and not the outermost s-orbital. Also, in the electrochemical series this element is placed below hydrogen.

Complete answer:
Here, we have to find out the standard reduction potential is positive for which of the following transition metals. For that, firstly let us write down their reduction reaction-
     \[{{M}^{2+}}+2{{e}^{-}}\to M\]
Now, let us write down the electronic configuration of the metals along with the electronic configuration of their +2 states.
Firstly we have nickel. Ni = $\left[ Ar \right]3{{d}^{8}}4{{s}^{2}}$ and $N{{i}^{2+}}=\left[ Ar \right]3{{d}^{6}}4{{s}^{2}}$
Then we have copper. Cu = $\left[ Ar \right]3{{d}^{10}}4{{s}^{1}}$ and $C{{u}^{2+}}=\left[ Ar \right]3{{d}^{9}}$
Next we have iron. Fe = $\left[ Ar \right]3{{d}^{6}}4{{s}^{2}}$ and $F{{e}^{2+}}=\left[ Ar \right]3{{d}^{4}}4{{s}^{2}}$
And lastly we have cobalt. Co = $\left[ Ar \right]3{{d}^{7}}4{{s}^{2}}$ and $C{{o}^{2+}}=\left[ Ar \right]3{{d}^{5}}4{{s}^{2}}$
We can see from the above electronic configuration that in case of copper, it has a fulfilled d-orbital but in +2 oxidation state it loses that stability. Half-filled or full-filled d-orbitals have higher energy due to higher value of exchange energy for them.
Therefore, when copper is in +2 state it will convert readily to copper metal or we can say that copper +2 ion will not be formed readily.
Thus, it shows a positive reduction potential value unlike the other elements mentioned.

Therefore, the correct answer is option [B] Cu (Z = 29).

Note:
The elements lying from group 3 to group 12 in the periodic table are called the transition elements. They are placed in the period 4, 5 and 6 and they have 3d, 4d and 5d-orbitals and are called first, second and third transition series respectively. The transition series starts from scandium which has atomic number 21 and ends with the last metal in the third transition series which is mercury. However, there are other newly found transition metals too.