Question

Assertion: In the rock salt type structure, all the octahedral voids are occupied by $N{a^ + }$ ions.
Reason: Number of octahedral voids = number of $C{l^ - }$ ions in the packing.
A) Both Assertion and Reason are correct and Reason is the correct explanation of assertion.
B) Both Assertion and Reason are correct but Reason is not the correct explanation of assertion
C) Assertion is correct but Reason is not correct
D) Assertion is not correct but Reason is correct
E) Both Assertion and Reason are not correct.

Answer 1

Use the structural formation of sodium chloride where chloride ions forms the lattice and sodium ions are present in the voids. So as you know they form a FCC type of arrangement which have atoms at the eight corner and also on each face. When these atoms get arranged in such a pattern voids formed in which sodium takes its place.

Complete answer:
In the sodium chloride crystal, we have two ions among which anion bigger in size forms the lattice by taking sites at the eight corners and along with it, it also takes place at one each face. Now as chloride ions form the lattice with the help of radius ratio rule, cations start taking their sites in the void which forms. As in FCC there are four O.V. octahedral voids present and eight T.V tetrahedral voids. The number of tetrahedral voids are double than the number of O.V.
By the above question it is given that we are talking about sodium chloride crystal which is having FCC arrangement also called as ccp thus the cations sodium ions put themselves in the sites. So, the assertion is true that in rock salt like structure, all octahedral voids are occupied by $N{a^ + }$ ions but the reason is not true. We know that the number of octahedral voids is equal to the number of $N{a^ + }$ ions in the packing and not equal to the $C{l^ - }$ ions.
Hence we can say that option B. is correct.

Note: In FCC lattice, when anion arranges itself in the pattern it will form octahedral voids in some places. These voids are only four in number when we talk about unit cells. The cations which are smaller in size take place in these voids. We can also find out the formula of that chemical by the information of voids and atoms.