JizzaDaMan said:
I have heard that the melting/freezing point of helium is lower than that of hydrogen. Is this correct? if so why?
My thoughts on this are that hydrogen has the lowest mass and therefore requires less energy than helium to be in a liquid or gaseous state, so it appears counter intuitive that the melting point of helium is lower than that of hydrogen.
Melting point of nitrogen is also lower than that of carbon.
The thermal movement of molecules is not working agains mass. True, heavier molecules move slower; but they also have more inertia. The main effect is that the thermal movement works against binding energy. Carbon atoms in graphite are bound by strong covalent bonds and extreme heat is needed to break them; strong covalent bonds also bond two nitrogen atoms to a molecule, but the bonds between nitrogen molecules are feeble London forces.
The London forces which bind to each other molecules of nitrogen, or paraffin, or hydrogen, or atoms of helium or neon, are forces which only connect electron clouds. It is only some higher order effects which depend on mass of the nuclei inside the clouds.
Helium, being an inert gas, has very small, tightly bound electron clouds. They are very hard to polarize and thus have weak London forces, compared to the bigger, looser and more polarizable electron clouds of hydrogen molecules.
The higher order effects come from zero point movements of the nuclei inside. Just like electrons cannot fall onto nuclei but have to undergo orbital movements and spread out as electron clouds, nuclei cannot occupy a single position in a molecule or crystal. Both hydrogen and deuterium atoms need some space to move around in crystal - but since the electrons follow the nuclei in zero point movements, this weakens the bonds which the electrons make. And weakens them more for protium than for deuterium, whose nuclei are more massive and have lower amplitude of zero point motion.
Also, boiling points generally have more regular systematic trends than melting points, because they are not so dependent on specifics of crystal structure.