Question

Which of the following is not correctly matched with the given example?
[A] An element of the first transition series which has the highest second ionization enthalpy-Cu.
[B] An element of the first transition series with highest third ionisation enthalpy-Zn.
[C] An element of the first transition series with lowest enthalpy of atomisation-Zn.
[C] Last element of third transition series-Cd.

Answer 1

Hint: To answer such questions the first thing that we need to do is write down the electronic configuration of the element given. Presence of half-filled or full-filled d-orbitals will gain extra stability thus, ionisation enthalpy will increase.

Complete answer:
 Before answering the question, let us discuss the periodic properties and terms mentioned in the question.
Firstly, we have a transition series. 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.
Next, we have ionization enthalpy. As we can understand from the name itself, it is the amount of energy required for the ionisation of the atom i.e. removal of the loosely bound outermost electron from the atom. Among the transition metals, as we know they have d-orbitals and half-filled and full-filled d-orbitals have higher stability thus elements with such d-configuration will not want to give up their electrons easily.
First ionisation is the removal of the first electron from the atom. Second and third ionisation is the removal of second and third electrons from the atom.
Lastly, enthalpy of atomisation is the energy we need to provide for the conversion of a solid to its gaseous form.
Now we will discuss each of the given options and check if they are correct.

[A] An element of the first transition series which has the highest second ionization enthalpy-Cu.
Atomic number of copper is 29. Therefore we can write that the electronic configuration of copper is $\left[ Ar \right]3{{d}^{10}}4{{s}^{1}}$. After removal of 1 electron, it will become $C{{u}^{+}}$ and the electronic configuration will become $\left[ Ar \right]3{{d}^{10}}$. As we can see, it has a full-filled d-orbital already therefore it will not want to lose another electron thus it has a high second ionization enthalpy. Therefore, this option is correct.

[B] An element of the first transition series with highest third ionisation enthalpy-Zn.
Atomic number of zinc is 30. Thus, its electronic configuration is $\left[ Ar \right]3{{d}^{10}}4{{s}^{2}}$. As we can see from the configuration that after losing 2 electrons it will have a noble gas configuration. Therefore, on losing the third electron it will have a high ionisation enthalpy. Therefore, this option is also correct.
Next, we have option [C] An element of the first transition series with lowest enthalpy of atomisation-Zn.
We know that atomisation enthalpy depends on the number of unpaired electrons and as there is no unpaired electron in zinc, the metallic bonding is weak for zinc thus, it has low atomization enthalpy.
And lastly, we have [C] Last element of third transition series-Cd.
As we have discussed above that the transition series ends with the last element in the third transition series which is mercury, therefore this option is incorrect.
So, the correct answer is “Option C”.

Note: All the elements in the transition series are metals and they have d-orbitals thus they are also called transition metals and in inorganic chemistry we study then under d-block elements. Like all the other metals the transition metals are also malleable and ductile and they also conduct electricity. They are called ‘transition’ metals because they lie in between the metallic (alkali and alkaline earth metals) and the non-metallic (halogen and noble gases) elements.