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I wonder why crystals of Naphthalene are needle-shaped after sublimation. Googling the fact, I could only confirm that it is true but failed to find the reason. Could someone please shed some light as to why this happens?

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2 Answers 2

Every solid material has a crystal structure. Sublimation tends to produce good crystals because the solid can grow slowly. In bulk form naphthalene crystals are broken and compressed and so bulk solid naphthalene looks quite different.

The exact same situation applies to snowflakes. They are flat hexagonal structures when seen individually. Crunch them into a snowball and they lose that pretty structure.

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Well, there are crystalline and amorphous solids... – Mithoron Aug 21 at 11:43
But you don't get amorphous solids out of a one-component gas phase. Or lets say it will be very hard even get it into the gas phase. ;-) – Karl Aug 21 at 14:17
Btw., crystals grown from the melt or solution likely form a totally different habitus that if grown from the gas phase. There's a good chance even a different phase will form. – Karl Aug 21 at 19:28

It's the symmetry!

(Sorry, that's the default answer to all questions on crystallography.)

You get needles if your crystallite grows faster in one direction than in any perpendicular direction. Which is the fact here, because aromatic rings love to stack, but don't have much energy to gain out of the van der Waals' interactions between the H-atoms sticking out in the plane.

If a crystallite grows slower in one direction than perpendicular to it, you will get platelet-like crystals.

And if the elementary cell of the crystal has a cubic symmetry, it will likely grow equally fast in all directions, and the crystals will be cubes, like $\ce{NaCl}$. (OK, this last statement is rather weak. It's easy to predict that growth rates are very different like in naphthalene, but that they're actually identical is dure. Sodium chloride is just a nice example where this is the case. And be careful about melts. They're never isotropic near a crystal surface, so anything can happen.)

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Citing former US Presidents is prohibited here, or what? – Karl Aug 21 at 18:12
What do you mean with this statement? – Martin - マーチン Aug 21 at 20:28
I was making a reference to William Jefferson Blythe III in the first line of my post. – Karl Aug 21 at 20:59
You can always rollback edits if you disagree. – Geoff Hutchison Aug 21 at 21:00
Please keep in mind that most of us probably don't know that when reading the particular statement. You also gave no attribution and when it stands like this, it can and will be easily misunderstood. Apart from that it is of course not prohibited to cite former US presidents here. – Martin - マーチン Aug 21 at 21:18

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