Question

Assertion:
A special line will be seen for the $2{p_x} - 2{p_y}$ transition
Reason: Energy is released in the form of a wave of light when the electron drops from the $2{p_x}$ to $2{p_y}$ level.
A.Both assertion and reason are correct and the reason is the correct explanation for the assertion.
B.Both assertion and reason are correct but the reason is not the correct explanation for the assertion.
C.Assertion is correct but the reason is incorrect.
D.Both assertion and reason are incorrect.

Answer 1

Hint:
Line is a spectrum indicative of release in energy. Release in energy can only happen if the orbitals are at different energy levels. Different subshells in the same orbital have degenerate energy levels.

Complete step by step answer:
Let us understand what it means for a special line to be visible.
When we study the line spectra of any molecule, we study it with respect to the electron transitions that take place. Now these transitions could be with respect to the shells, sub-shells or orbitals.
What happens is that, a sample of the material is bombarded with a fast moving particle which is usually a photon. When this photo strikes the sample with a certain energy this energy gets transferred to the sample atoms. This energy will now excite the outermost valence electron, which will get excited to a higher energy state.
Now, this state that the electron is in now is called the excited state.
There is a difference in the energy level of the excited electron before and after it was bombarded with a photon.
This difference in energy is responsible for the line spectrum .
Now, what happens here is that when the electron is in the excited state , it has a lot of energy in it but it is unstable and hence it wants to come back to its original state. To do that, it will have to lose the excess energy.
This excess energy is given out in the form of light. This is the light that we see in the line spectrum.
From the above discussion we have understood that, there needs to be an energy difference if a line has to be obtained.
In the assertion given to us, $2{p_x}$ and $2{p_y}$ belong to the same orbital. Hence, they have the same energy level. Which means there will be no energy difference between these two levels.
Hence, no possible transition in these two orbitals can bring about a line.
Hence option D is correct.

Note:This principle is fundamental to many techniques which are being used everyday, such as UV-VISIBLE spectroscopy. Even our ability to perceive the world in all of its colours is due to simple excitation and de excitation of electrons.