Question

In which period of the periodic table, will an element, to be found differentiating electrons, be a 4d electron?

Answer 1

Hint: The correct answer is one of the periods whose elements are known as transition elements. The correct answer here belongs to the 4d- transition series. The period starts with yttrium and ends with cadmium.

Complete answer:
Before answering the question, let us understand what a differentiating electron is.
The last electron that we add while writing the electronic configuration of an atom is the differentiating electron. In words, we can also say that it is the electron that makes the current element different from the element lying next to it in the periodic table.
Let us take an example to understand it better. Let us consider gallium. We can write the electronic configuration of gallium as $\left[ Ar \right]3{{d}^{10}}4{{s}^{2}}4{{p}^{1}}$. In the periodic table, the element that lies just before gallium is zinc. We know that the atomic number of zinc is 30. Therefore, we can write the electronic configuration of zinc as $\left[ Ar \right]3{{d}^{10}}4{{s}^{2}}$.
As we can see that gallium and zinc have the same electronic configuration except the electron in the 4p- orbital. Therefore, the differentiating electron is 4p in this case.
In the question it is asked that for which period the differentiating electron will be 4d.
We know that the fifth period element will contain 4d- orbitals and the general electronic configuration of the fifth period elements is $\left[ Kr \right]4{{d}^{1-10}}5{{s}^{0-2}}$. Here, as we can see, 5s-orbital have a higher principal quantum number but still they are at a lower energy level than the 4d- orbital. Therefore, the electron will enter 5s-orbital first and then occupy 4d-orbital i.e. the last electron will enter in 4d and not 5s. Therefore, the differentiating electron will be in 4d.
Thus, in the fifth period of the periodic table, we will find an element which will have a differentiating 4d electron.
Therefore, the correct answer is the fifth period.

Note:
The elements of the fifth period are considered as transition metals. There are 4 transition metal series, 3d, 4d, 5d and 6d. They are called transition metals because they are placed between the alkali, alkali earth metals and the halogens which are non-metal.