Crystal Chemistry help? Watch

thenextchemist
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Hi,

I'm learning about the different structures (ccp based and hcp based).
How do I know whether a molecule is ccp or hcp - what do I have to know in order to find this?

And also, e.g. the NaCl crystal structure
Na+ and Cl- ions have coordination numbers of 6 and therefore octahedral geometries.
And with CaF2, the coordination numbers are 8 and 4, and so have tetrahedral and cubic coordination.
How do I work the coordination numbers out and therefore, the geometry because all the crystal systems are different?
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MexicanKeith
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(Original post by Bloom77)
Hi,

I'm learning about the different structures (ccp based and hcp based).
How do I know whether a molecule is ccp or hcp - what do I have to know in order to find this?

And also, e.g. the NaCl crystal structure
Na+ and Cl- ions have coordination numbers of 6 and therefore octahedral geometries.
And with CaF2, the coordination numbers are 8 and 4, and so have tetrahedral and cubic coordination.
How do I work the coordination numbers out and therefore, the geometry because all the crystal systems are different?
In general you can't simply 'work out' the crystal structure.
With experience you can make educated guesses and if you study chemistry to degree level then you quickly memorise the structures of numerous solids that come up regularly.

Having said that, you can come up with some sensible predictors, for example radius ratio rules can be useful for highly ionic systems (ie they work reasonably for group 1 halides). And certain structure are favoured by more polarisable ions because they enable directional bonding (eg CdCl2 and CdI2 structures occur regularly when a polarising cation is combined with a polarisable anion).
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thenextchemist
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(Original post by MexicanKeith)
In general you can't simply 'work out' the crystal structure.
With experience you can make educated guesses and if you study chemistry to degree level then you quickly memorise the structures of numerous solids that come up regularly.

Having said that, you can come up with some sensible predictors, for example radius ratio rules can be useful for highly ionic systems (ie they work reasonably for group 1 halides). And certain structure are favoured by more polarisable ions because they enable directional bonding (eg CdCl2 and CdI2 structures occur regularly when a polarising cation is combined with a polarisable anion).
Thank you!

I understand the maximum radius ratio for octahedral coordination in cp systems is 0.414.
So, if I were to work out the radius ratio for a given molecule, and it were lower than 0.414, it is going to be an octahedral coordination in cp systems?

P.S. new to this topic
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MexicanKeith
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(Original post by Bloom77)
Thank you!

I understand the maximum radius ratio for octahedral coordination in cp systems is 0.414.
So, if I were to work out the radius ratio for a given molecule, and it were lower than 0.414, it is going to be an octahedral coordination in cp systems?

P.S. new to this topic
Sorry forgot to mention in my first reply, don't use the word molecule when you don't mean molecule!

So, for a 1:1 close packed system you can work out using simple trigonometry the limits in which cubic, octahedral, tetrahedral etc. geometry are possible.
so if you have similar size ions, then many ions can fit round one another.

Hence cation-anion ratio of more 0.732 can give a hint of cubic geometry.

lower ratios lead to lower coordination numbers, because the smaller cation cannot easily be cubically coordinated.

below the 0.414 ratio you mention you go from octahedral geometry (like in NaCl) to tetrahedral geometry (ie Wurtzite or Sphalerite structures)
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