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So, a simple question: what makes neon a gas; what makes water liquid; what makes plutonium-238 a solid at room temperature? Of course, the phase is determined by the excitation of the molecules, but what determines that excitation?

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It's all in the intermolecular forces (and the air pressure). Intermolecular forces are the forces of attraction and repulsion between molecules to neighboring particles. To understand how they determine physical properties like enthalpies of vaporization, fusion, etc..., you need to think about what states of matter are and how they affect the molecules. When you heat up water to a temperature (average kinetic energy) of 90 °C, the energy within the molecules isn't strong enough to overcome the intermolecular bonds (and the atmospheric pressure above the sample) and become a gas. However, it does have enough energy to overcome the bonds to a certain extent and thus can remain in a liquid state instead of in a solid state. This principle applies to any substance. In order for there to be a change in state of matter, the energy within the molecules has to overcome the intermolecular bonds and atmospheric pressure.

As mentioned several times above, another factor is the atmospheric pressure. A common experiment is to put water in a vacuum and watch it boil, something many first-time-experimenters wouldn't expect. The reason water is able to vaporize at room temperature is because it doesn't have the atmosphere to prevent it from vaporizing and therefore, in order to reach an equilibrium, the water is able to vaporize with much less energy than normally required at STP (standard temperature/pressure).

As a direct response to your question about neon, the reason that the noble gases are usually gases unless at extreme temperatures has to do with the weak intermolecular forces they have (van der Waals forces, or more specifically, london dispersion forces). Because the noble gases don't usually bond intramolecularly with anything else, including themselves, and have full energy levels, they have extremely weak intermolecular forces allowing them to almost always be in the gas phase.

I've listed some helpful links below if you want to take your research to the next step.

  1. http://en.wikipedia.org/wiki/Intermolecular_force
  2. http://wps.prenhall.com/esm_hillpetrucci_genchem_4/16/4219/1080299.cw/index.html
  3. http://www.youtube.com/watch?v=A30lguyG0WM
  4. http://chemed.chem.purdue.edu/genchem/topicreview/bp/intermol/intermol.html
  5. http://en.wikipedia.org/wiki/Van_der_Waals_force
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