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Why do $\ce{NaHCO3}$ and $\ce{KHCO3}$ have different crystal structures?

We had this in a test today and this got me stumped...

My questions are:

  • Do $\ce{NaHCO3}$ and $\ce{KHCO3}$ possess crystal structures?
  • If they do, why should they have different crystal structures, because they are in the same group and must possess almost similar physical properties.
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  • $\begingroup$ NaCl and CsCl involve congeners too, but I don't see them having the same structure. $\endgroup$ Jul 10, 2021 at 12:02

2 Answers 2

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The reason why the crystal structure of $\ce{NaHCO3}$ is face centered and the crystal structure of $\ce{KHCO3}$ is body centered is due to the fact that the cation/anion ratio is different, meaning that certain types of hole are now too small for the cation to fit into them and as a result it must go into a different hole.

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NaHCO3 isn’t face centered, nor is KHCO3 body centered. They do have crystal structures though, both being primitive monoclinic (in which case that the edges of elementary cells have different lengths, and there is one odd angle), but their key features are very different. In KHCO3, the anions form dimers, while in NaHCO3 you find infinitely long chains of anions, being held together by hydrogen bonds. It‘s a really good question why that is this way, actually nobody knows why. Main driver might be that Na+ cations prefer octahedric coordination spheres of O that might be easier to achieve when you have infinite HCO3 chains, while the significantly larger K wants coordination spheres with seven to eight O. But that is not sufficient as explanation, because there might be a way to stack infinite HCO3 chains in a way that K finds a suitable place to fit in.

But the differences between the sodium and potassium compounds are not the weirdest thing to find when looking at hydrogen carbonate structures. For example, check out Ammonium Bicrbonate. Ammonium and Potassium compounds often crystallize in a very similar way because their size is comparable. However, that rule breaks for the bicarbonate structures: There, the NH4HCO3 is much more related to the NaHCO3 structure, and you find chains instead of dimers. It’s mind boggling.

So you’re obviously asking the right questions. You might want to consider getting deeper into the art of crystal structure determination ;)

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