[tortoise]

Why do Iodine crystals sublime?
 
And here is the answer:

They sublime because though there are covalent bonds within the molecule holding two iodine atoms together, only van der Waal's forces exist between the iodine molecules. Van der Waal's forces are very weak interactions between molecules of a substance hence the iodine molecules can escape from the solid structure easily.  

Is it right?

[Donaldson Tan]

why don't iodine melt, then vapourise?

[lemonoman]

It MIGHT be the same reason that dry Ice (solid carbon dioxide) doesn't melt either - it goes right from the solid to gas.  More resources may be available about that than about Iodine, but I don't know.  It's a good question.

[jdurg]

You are correct.  It's because of the relatively weak Van der Waals forces holding the I2 molecules together.  Iodine has a very high molecular mass (over 200 grams per mole) so the high mass of the molecule causes it to remain as a solid at room temperature.  However, the overall forces holding it together are very weak so if it gets enough energy to break free from itself, it forms a vapor right away instead of a liquid.  Bromine undergoes a similar process, but it's overall molecular mass isn't high enough to make it a solid.  Instead, it's a highly volatile liquid.

[tortoise]

Why don't iodine melt, then vapourise?

Intermolecular forces between iodine molecules are not strong enough to keep them together in a liquid state, after they gain enough kinetic energy to break free from the solid structure. In water, the intermolecular forces present are hydrogen bonds.These result rom slightly positively charged hydrogen atoms attracting slightly negatively charged oxygen atoms. Van der Waal's forces only result from small charges created by instantaneous and induced dipole forces.

[peacefulltortoise]

So does Iodine' melting and boiling point exist? I still saw these in book, why?

[Borek]

You have to look at phase diagram - all depends on the pressure and temperature acting together.

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/13.html

Scroll down to 13-18. Note that the "axes on a phase diagram are generally not drawn to scale".

[jdurg]

So does Iodine' melting and boiling point exist? I still saw these in book, why?

Yes, but in order for Iodine to melt it has to be under pressure.  You can actually see liquid iodine at standard pressures if you have the right container.  If you take a good amount of iodine crystals and place them in a test tube, then proceed to heat the test tube from the bottom, you'll see liquid iodine at the bottom of the tube.  This is because the massive weight of the iodine vapor that has formed actually creates a higher pressure right at the surface of the iodine.  This higher local pressure causes the iodine to liquify at a seemingly "normal" pressure.

[Bakhtiyar]

Iodine is solid at room remperature 293K (20°C) . The vapor pressure of iodine increases as temperature rises. Sublimation condition match when the vapor pressure of iodine gets higher than total enviromental gas pressure and .Iodine doesn't melt because sublimation occurs at temperature that is slightly above than room temperature.

T(melting) for iodine ≈ 386K (113°C)

[Enthalpy]

The same kind of bonds let a compound condensate and solidify, so "only van der Waals" and similar reasons would tell why a compound both melts and evaporates easily, not why it sublimates, that is goes directly from solid to gas - why it evaporates about as easily as it melts.

My partial understanding is that in a solid, malecules attract an other through several points at once because they are properly organized, while in a liquid, only one or few points attracts them at a given instant.

In this image of liquids and solids, molecules that are unsymmetric can touch an other with an unfavourable angle, attract an other a bit, and make a liquid, but are less probable to have the favourable orientation that lets them make a solid. As well, deformable molecules can attract an other at one or few atoms, but have rarely the proper conformation to fit optimally an other and make a solid.

So stiff and symmetric molecules solidify almost as easily as they condense, have a small liquid range and can under some pressures sublimate, while deformable and unsymmetric ones condense easily but need a much colder temperature to solidify, showing a wide liquid range.

This fits observation rather well - keeping in mind that no theory up to now predicts melting points. very symmetric molecules like cubane, adamantane, benzene... have a small liquid range (or sublimate depending on the pressure, total or partial) while unsymmetric and deformable ones like phytane have a wide liquid range.