Question

In rock salt structure, what percentage of the octahedral voids are occupied by the cations?
A) 50%
B) 100%
C) 25%
D) 33%

Answer 1

Hint: For determining the octahedral voids in rock salt structure, you should be familiar with its structure. A $NaCl$ structure is also called rock salt structure. In this, sodium ions which are cations, occupy all the octahedral voids (or holes) of the crystal lattice while chloride ions, which are anions, occupy fcc (face-centred cubic) sites.

Complete step by step answer:
Rock salt structure or $NaCl$ crystal belong to face-centred cubic crystal (also called \[fcc\]). The face centred cubic unit cell is the repeating unit of rock salt structure, i.e., the structure of NaCl is formed by repeating the fcc unit cell. It has an organised structure and has a 1:1 stoichiometry ratio of Na : Cl.
Arrangement of $N{a^ + }$ and $C{l^ - }$ ions: $N{a^ + }$ and $C{l^ - }$ ions are arranged in $NaCl$ crystal as a face-centred cubic lattice. In the face-centred cubic lattice, lattice points are located at the eight corners of the unit cell and at the centres of each face of the unit cell. Moreover, in $NaCl$ structure, all the anions ($C{l^ - }$) have a face-centred cubic arrangement and all the cations ($N{a^ + }$) are present at the octahedral voids. It can also be explained as a \[fcc\] array of anions with cations in the octahedral voids.
It is quite clear from the above explanation that all the octahedral voids are occupied by $N{a^ + }$ ions in the rock salt structure. Therefore, the percentage of the octahedral voids occupied by cations in the rock salt structure would be 100%. So, the correct answer is “Option B”.

Note: From the experimental analysis and structure elucidation, it was revealed that each sodium ion is shared equally by six chloride ions, and each chloride ion is shared equally by six sodium ions in $NaCl$ structure. Thus, the coordination number is 6 for both cations as well as anions in rock salt structure.