Question

What is the structure of potassium chloride?
A.Monoclinic
B.Orthorhombic
C.Rock salt crystal structure
D.None of the above

Answer 1

Hint: KCl has a structure similar to that of NaCl. Potassium chloride is an ionic compound having a bond between an alkali metal (K) and a halogen (Cl). The ratio between the cation and anion is 1:1. There are 4 atoms in each unit cell and it belongs to a cubic crystal system.
Complete step by step answer:
Most of the alkali metal halides have a structure similar to that of NaCl i.e., rock salt crystal structure.
This structure is likely to be formed if the cation is smaller than the anion (a cation/anion radius ratio of 0.414 to 0.732)
The cations and anions form a separate face-centered cubic lattice, with the two lattices interpenetrating to form a 3D checkerboard pattern.
Each cation (${K^ + }$) is coordinated to 6 $C{l^ - }$ anions at the vertices of an octahedron and similarly each $C{l^ - }$ anion is coordinated to 6 cations at the vertices of an octahedron. Hence, the coordination number is 6.
The unit cell of KCl consists of ${K^ + }$ and $C{l^ - }$ ions. It has a face-centered cubic close-packed structure.
$C{l^ - }$ ions having the larger size occupies the face-centers and corners of the unit cell while ${K^ + }$ occupies the octahedral voids
The total number of chloride ions per unit cell = sum of chloride ions at the face centers and corners = \[\left( {6 \times \dfrac{1}{2}} \right) + \left( {8 \times \dfrac{1}{8}} \right) = 4\] chloride ions
The total number of potassium ions per unit cell= sum of potassium ions at the 12 edge-centers and one in the center of the cube = $\left( {12 \times \dfrac{1}{4}} \right) + 1 = 4$ potassium ions
Each ion in this lattice has six of the other kind of ion as its nearest neighbours, and twelve of the same kind of ions as its second nearest neighbours. The structure of KCl is shown below:

 

The green spheres represent the chloride ions while the grey spheres represent the potassium ions in the figure.

Hence, the correct answer is (C) rock salt crystal structure.

Note: All alkali metal chlorides except CsCl have rock salt crystal structure. It has Cs ions at the body center and chloride ions at the corners.
The position of the octahedral voids in a face-centered cubic unit cell is at the edge centers and at the body center. Hence, the potassium atoms are said to occupy the edge centers and body center in this structure.