Question

Assertion: $NO$ is diamagnetic in solid or liquid state but paramagnetic in gaseous state.
Reason: $NO$ contains one unpaired electron.
A.Both assertion and reason are correct and reason is the correct explanation for assertion
B.Both assertion and reason are correct but reason is not the correct explanation for assertion
C.Assertion is correct but reason is incorrect
D.Assertion is incorrect but reason is correct

Answer 1

A compound is said to be paramagnetic if there are unpaired electrons present in the substance. In other words, a paramagnetic compound has a partially filled atomic orbital. To answer this question, we must recall the electronic configuration of nitrogen in nitric oxide.

Complete step by step solution:
$NO$ is a neutral oxide and is known as nitric oxide. We know that nitrogen contains five electrons in its valence shell, whereas oxygen contains six valence electrons.
If we draw the molecular orbital diagram for nitric oxide, we will observe that there is one unpaired electron present in its ${\pi ^*}2p$ orbital. Thus, we can say that it is paramagnetic in nature. However on solidifying, the molecules of nitric oxide carrying an unpaired electron, undergo dimerization in which the unpaired electrons are paired making the molecule diamagnetic.
Thus, we can say that the assertion is correct.
Also we have seen that there are eleven electrons in nitric oxide and thus have one unpaired electron. So we can also say that the reason is correct. But the reason does not give any information about the diamagnetic nature of the solid state of nitric oxide.
Hence, both the assertion and reason are correct statements but the reason is not the correct explanation for the assertion.

Hence, the correct answer is B.

Note:
Molecular orbital theory (MOT) is a method of depicting the electronic structures of molecules using quantum mechanics. According to this theory, electrons in a molecule are not assigned to individual chemical bonds between atoms like it was done in previous theories, but are treated to be moving under the influence of the atomic nuclei in the whole molecule.