There was a very informative thread a while back that dealt with, among other things, why helium does not easily become a solid at normal atmospheric pressure, regardless of the temperature.
However, this brings up a question:
Why does hydrogen, a “smaller” atom", become a solid at normal atmospheric pressure? I basically understand why helium doesn’t. Why isn’t hydrogen affected the same way? Is it because hydrogen doesn’t normally naturally exist as a single atom, but as H[sub]2[/sub], which would then make it much larger?
In a related question:
I seem to recall from a high school chemistry textbook, which gave the various melting points and boiling points of elements, that carbon sublimes (and possibly other elements do, too). With a little Googling, I’ve seen some references that liquid carbon can exist under some extreme situations, but why doesn’t it exist “naturally” when you raise the temp.
Jusgt a WAG but since Helium is an inert element the atoms can’t form bonds with any others to make a solid structure. I once read a reference to ultracooled helium as a “monatomic powder.”
[sup]4[/sup]He will go solid at pressures above 24 atmospheres and T < 1.8°K.
[sup]3[/sup]He solidifies at pressures above 33 atmospheres and T < 0.5°K.
You’re on the right track with hydrogen. Hydrogen has one electron in a two-electron inner shell, thus making it chemically unstable. This instablity causes it to bond with other atoms, including itself… it also makes hydrogen molecules slightly more cohesive than helium. (To stretch the truth a little, you can think of the hydrogen molecules as longer than they are wide and slightly dumbelled at both ends, which explains why they can’t bounce off each other quite as freely as helium atoms do. There is also an electrostatic element IIRC.) More cohesion makes it possible for hydrogen to become a solid if the energy of molecular motion (temperature) is low enough.
For carbon, (excluding the combustion factor which seems kinda beside the point,) carbon is a much more highly bondable atom than hydrogen - it has four electrons in it’s eight-electron shell, the second electron shell out, which means that it ideally wants to ‘share’ electrons in four covalent bonds with other nearby atoms. If the only element around is carbon, that turns into a tendency to form involved lattices, which fix the carbon in place, making it solid. If enough heat is applied to break the lattices, it is generally enough to send carbon particles flying all over the place and bouncing off each other - carbon gas.
Because carbon has such a great tendency to share so many electrons, to get liquid carbon… (individual particles of carbon always staying in contact with neighbors but always changing which neighbors they are around… to get carbon to ‘mingle’, essentially,) you have to combine a relatively high degree of heat with a high amount of pressure… get the carbon particles moving so fast that they break apart their lattices, but cram so much carbon into such a small space that they HAVE to be touching other particles… you can force them to mingle that way. But it requires considerably more than atmospheric pressure.