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25 votes

Why is crystallography still used in some cases for small molecule structure determination?

Our group uses both methods a lot so here are some examples why you would use X-ray, in addition to NMR, in organic synthesis. Your compound isn't soluble enough: Colleagues produce very large ...
DSVA's user avatar
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22 votes
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How were x-ray diffraction patterns deciphered before computers?

I agree with @andselisk that this question is quite broad. I will focus on two specific questions asked The only equation I know for x-ray diffraction is Bragg's Law but is this the only equation ...
Karsten's user avatar
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20 votes
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What is the smallest molar volume?

Boron is a covalent solid with high melting point, like diamond (though not quite), and hence its crystals are hard to make. Unlike diamond crystals, they are not nice and probably wouldn't make a ...
Ivan Neretin's user avatar
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18 votes

Are there any crystals that are theoretically possible, but haven't yet been made?

When we are talking about the prediction of materials, it is generally about the structure and corresponding free energy, not the crystallization process. That being said there are a huge number of ...
Greg's user avatar
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16 votes

Why is crystallography still used in some cases for small molecule structure determination?

Coming from natural product chemistry, of course the structure elucidation by NMR is the most commonly used method, especially in isolation. NMR requires only a little substance dissolved in whichever ...
Jan's user avatar
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16 votes

How are the number of tetrahedral voids twice the number of octahedral voids in a CCP structure?

To be precise, if in a close packed structure (ccp or fcc) there are $n$ atoms or ions then the number of octahedral voids and tetrahedral voids will be $n$ and $2n$ respectively. For example, there ...
Mitchell's user avatar
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15 votes

Are there any crystals that are theoretically possible, but haven't yet been made?

Arguably, there are more such crystals than those which have been made. It is just that nobody bothered to write them all down, because hey, what's the point. A tiny minority of those theoretical ...
Ivan Neretin's user avatar
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14 votes

Why does this lysozyme powder not look crystalline?

The typical protein crystal does not withstand mechanical stress equally well than e.g., the ones of table salt, or refined sucrose. Hence the varying shapes of grains. Photos about nice protein ...
Buttonwood's user avatar
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11 votes
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Why are snowflakes shaped as they are?

Now, as I understand it, all processes proceed so as to maximize the "randomness" of its constituent particles. (Oversimplified version of the Second Law of Thermodynamics, yes, I know... just don't ...
getafix's user avatar
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11 votes
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Miller Indices: How to deal with some weird(-ish) cases?

IFF I remember correctly... Sometimes you have to "move" the plane, in order to see where the axis crosses. Take example 7, if you look at the example plane, the one actually shown, it is (0 ...
Stian's user avatar
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11 votes

If you accumulate 100 identical spheres how many will end up on the surface?

Experimental approach Here are 100 marbles in a polyethylene bag: If a marble touches the bag, it is counted as being on the surface. I did not count, and the bag was not perfectly round. A balloon ...
Karsten's user avatar
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10 votes
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How many crystal forms of HCl have been identified - where to find the unit cells/lattice constants?

From the first picture depicted, I would refrain from stating there are five HCl molecules per unit cell. One thing, while there is some choice in the definition of a unitcell (there are several ...
Buttonwood's user avatar
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10 votes

Calculate the third and fourth nearest neighbours in bcc

You can think of the body centered cubic lattice as two simple cubic lattice, one with points at coordinates $(ma,na,pa)$ where $m,n,p$ are integers, the other with points at $((m+(1/2))a,(n+(1/2))a,(...
Oscar Lanzi's user avatar
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10 votes
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Calculate the third and fourth nearest neighbours in bcc

With $\sqrt3\over2$ being that close to $1$, BCC packing is better not looked at in terms of coordination spheres. But if you insist... Say you are sitting in the center of a cell. Then: Your first ...
Ivan Neretin's user avatar
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10 votes
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What material properties are unpredictable (not reproducible) but can be measured consistently and at a low cost?

Impossible for a homogeneous material, however ... basically every nonhomogeneous material fits your description. Say the pattern of microphase separation in a copolymer. Or the arrangement of filler ...
Karl's user avatar
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9 votes
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Is the 'central Tin atom' in crystalline tin oxide really offset like this?

From http://www.crystallography.net/cod/1000062.html one can see that Sn sits in the middle, without any displacement, and mimics the classic Rutile-type structure: I checked original Wikimedia image,...
andselisk's user avatar
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Structure of crystalline boron trioxide

This statement is relevant for the initially [incorrectly] determined structure of $\ce{B2O3}$ by Berger [1, 2] who used powder x-ray diffraction data. You can still access the structure and visualize ...
andselisk's user avatar
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9 votes
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Given a CIF (crystallographic interchange format) file (and thus cartesian atomic coordinates), how can you find the bonds between atoms?

Your question does not detail out what you mean by how is it done. Indeed, your question may be answered twice: determine the distances between atoms, pairwise compare the sum of van der Waals radii;...
Buttonwood's user avatar
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8 votes

What does it mean to say "the 222 space group"?

This is a crystallographer’s shorthand for a certain type of space group. You may have heard of the 14 Bravais lattices, e.g. cubic, monoclinic, orthorhombic. These lattices all assume perfect sphere’...
Jan's user avatar
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8 votes

What is the smallest molar volume?

A mole of neutrons in a neutron star would take up about $10^{-20}$ m$^3$. And in a black hole, they would be even smaller.
Acccumulation's user avatar
8 votes

If you accumulate 100 identical spheres how many will end up on the surface?

Note that the classical idea of an atomic nucleus as a set of nucleon balls is very wrong, similarly as another classical idea of an atom as a planetary system of orbiting and spinning electron balls. ...
Poutnik's user avatar
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8 votes

Why does Samarium have such a large c-axis lattice constant? (>26 Å) Are there significantly larger lattice constants of the elements?

This is the answer to the question Are any significantly larger lattice constants seen in pure crystal forms of the elements? and provides arguably the largest single linear parameter of a unit cell ...
andselisk's user avatar
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8 votes

What does the 'S' stand for in base-centered cubic crystals and Bravais lattices?

S also has German origins: seitenflächenzentriert (side face centered). Reference: International Tables for Crystallography, Volume A, Space Group Symmetry, Fifth Edition, Springer 2005, pg. 748. One ...
AChem's user avatar
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7 votes

How to add hydrogens to a crystal structure?

Olex2 (free, available for Windows, MacOS, Linux) can also be used to add hydrogens to any structure with the similar to PyMOL's command hadd. Example: hemoglobin ...
andselisk's user avatar
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7 votes
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Use of geometry index for the determination of coordination environment

For your first question, the original paper (J. Chem. Soc., Dalton Trans. 1984, 1349–1356) that described the geometry index $\tau_5$ defined it as an "index of trigonality". For example, they write ...
Tyberius's user avatar
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Number of atoms per unit cell

The denominator signifies the number of cubes that are needed to completely encompass the whole point. For example, a corner point can be thought of as a center of 8 whole cubes, while a face centre ...
Ayushmaan's user avatar
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7 votes
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Do we really have technology with the resolution to distinguish between layers of single atoms of materials?

X-rays do not allow you to look at the material just like an microscope. You get the spacing information from the diffraction pattern of only crystalline materials and a lot of mathematics. On the ...
AChem's user avatar
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7 votes

Converting fractional coordinates into cartesian coordinates for crystallography

You have to use a matrix to convert. This is derived in most textbooks on crystallography, such as McKie & McKie 'Essentials of Crystallography'. The matrix is $$ M=\begin{bmatrix} a & 0 & ...
porphyrin's user avatar
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7 votes
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Whats the difference between unit cell and motif?

Conceptually, the unit cell is the larger object, the (if relating to 3D crystals), the parallelepiped containing the crystallographic motif. It is the unit cell which, by mere translation along $\...
Buttonwood's user avatar
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7 votes

Given a CIF (crystallographic interchange format) file (and thus cartesian atomic coordinates), how can you find the bonds between atoms?

You didn't ask about macromolecules, but I'll write about it anyway. Macromolecular CIF files (mmCIF) also don't contain bonds. This information is stored in separate dictionaries. In particular, the ...
marcin's user avatar
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