# Why is there a difference in crystal shapes of sodium chloride (NaCl) and potassium chloride (KCl)?

According to Sodium chloride and Potassium chloride:

Sodium chloride (NaCl) crystallizes, among various other shapes, into octahedron and tetragonal pyramid shapes which are absent in potassium chloride (KCl). I am a quite confused about this because both KCl and NaCl have a crystal structure. What is the explanation?

And what are some books where I can find the shapes that salt crystals form?

• Crystal shapes are a complicated story. Apr 21, 2022 at 19:39
• There are some papers on that topic. The crystal habit/shape or in broader terms morphology depend a lot on concentration, temperature, impurities, etc. According to some papers that I found an octahedral shape can be achieved for KCl as well. So you might have to search for 'KCl octahedral morphology' and follow the cited papers. But it's a complicated story. Apr 22, 2022 at 7:58
• I feel the urge to reiterate once again that it is a really, really complicated story. There is no simple rule. Apr 22, 2022 at 19:39
• Maybe you want to remove the last question as it is too broad. Apr 23, 2022 at 6:16
• Crystal habits from solutions containing additives differ because the interactions with these additives differ. Here is one example for sodium chloride: pubs.acs.org/doi/10.1021/acs.cgd.7b01170
– Karsten
Apr 23, 2022 at 10:24

To compare between $$\ce{KCl}$$ and $$\ce{NaCl}$$ structure, you need to know what crystallographic planes are and their "distance between the atoms ratio".

$$\ce{KCl}$$ adopts a simple cubic system in which each atom lies at the corner of the cube.

The first set of planes ABFE, CDHG, ADHE, BCGF, ABCD, EFGH are alike. They are called (100) planes. Let the distance between the atoms be $$\mathrm{d_1}$$.

Parallel planes ADGF and ABGH are called (110) planes and are inclined at 45° to the cubic faces. Distance between atoms $$\mathrm{d_2 = \frac{d_1}{\sqrt{2}}}$$

Third set of planes like AFH are (111) planes. Distance $$\mathrm{d_3 = \frac{d_1}{\sqrt{3}}}$$.

Thus, for simple cubic system, $$\mathrm{d_1 : d_2 : d_3 = 1 : \frac{1}{\sqrt{2}} : \frac{1}{\sqrt{3}}}$$.

This ratio is in agreement with practical values. However, this calculation doesn't answer if the $$\ce{K}$$ and $$\ce{Cl}$$ atoms are in alternative fashion. This can be answered when compared to $$\ce{NaCl}$$.

The values obtained for the planes of $$\ce{NaCl}$$ are almost similar to that of $$\ce{KCl}$$ except a weak reflection at the (111) planes at about 5°. This makes the ratio, $$\mathrm{d_1 : d_2 : d_3 = 1 : \frac{1}{\sqrt{2}} : \frac{2}{\sqrt{3}}}$$ which agrees that $$\ce{NaCl}$$ adopts a face-centred cubic system.

This difference can be explained by the fact that the number of electrons (atomic number) determines the amount by which an atom scatters X-rays. Because the atomic number of potassium and chlorine are not that apart, $$\ce{KCl}$$ had adopted the simple cubic lattice but in case of $$\ce{NaCl}$$, difference between the atomic number between sodium and chlorine is more, hence they adopted the face-centred cubic lattice. But, we can say that both $$\ce{NaCl}$$ and $$\ce{KCl}$$ have same structure but with different arrangement.

Pictures taken from Modern Physics by Kiruthiga Sivaprasath, S. Chand Publishing, 2008

• I thought NaCl and KCl crystallized in the same spacegroup, with anions fcc and cations in "holes" with octahedral coordination. For KCl, your picture does not show a complete unit cell but just an eighth of it (KCl is not simple cubic).
– Karsten
Apr 23, 2022 at 10:28
• At STP NaCl and KCl do crystallize in the same structure, see e.g. en.wikipedia.org/wiki/Category:Rock_salt_crystal_structure . It is the CsCl structure that can be viewed as interpenetrating primitive lattices. I really don't understand what "both NaCl and KCl have same structure but with different arrangement" is trying to say. Apr 23, 2022 at 13:34