Question

(A) Antiferromagnetic substances possess zero magnetic moment.
(R) MnO is an antiferromagnetic substance.
A. Both (R) and (A) are true but the reason is the correct explanation of the assertion.
B. Both (R) and (A) are true but reason is not the correct explanation of assertion.
C. Assertion (A) is true but reason (R) is false.
D. Assertion (A) and Reason (R) both are false.
E. Assertion (A) is false but reason (R) is true.

Answer 1

Hint: Try to recall that in solid state, the metal ions of ferromagnetic substances are grouped together in small regions called domains and in anti ferromagnetic substances the alignment of domains is in a compensatory way. Now, by using this you can easily find the correct option from the given ones.

Complete step by step solution:
It is known to you that in solid state, the metal ions of ferromagnetic are grouped together into small regions called domains. Thus, each acts as a tiny magnet. In an unmagnetized piece of ferromagnetic substance, the domains are randomly oriented and their magnetic moments get cancelled.
If the alignment of magnetic moments of domains is in a compensatory way so as to give zero net magnetic moment because of cancellation of the individual magnetic moments then we get anti-ferromagnetism in the material. The common example is MnO.
Therefore, from above we can conclude that both assertion and reason are true and option A is the correct option to the given question.

Additional information:
When the magnetic moments of domains are aligned in parallel and antiparallel directions in unequal numbers resulting in net magnetic moment, we get ferrimagnetism.
When there is spontaneous alignment of magnetic moments of domains in the same direction, we get ferromagnetism. These have a strong magnetic effect. This ordering of domains persists even when the magnetic field is removed and the ferromagnetic substance becomes a permanent magnet.

Note: It should be remembered to you that all the magnetically ordered solids (ferromagnetic, anti-ferromagnetic and ferromagnetic) transform to paramagnetic state at higher temperature due to randomisation of their spins.
Also, you should remember that ferromagnetic substances are weakly attracted by magnetic field as compared to ferromagnetic substances.