10
$\begingroup$

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?

$\endgroup$
6
$\begingroup$

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.

$\endgroup$
  • 1
    $\begingroup$ Well, there are crystalline and amorphous solids... $\endgroup$ – Mithoron Aug 21 '15 at 11:43
  • $\begingroup$ 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. ;-) $\endgroup$ – Karl Aug 21 '15 at 14:17
  • $\begingroup$ 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. $\endgroup$ – Karl Aug 21 '15 at 19:28
2
$\begingroup$

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.)

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.