|MadSci Network: Chemistry|
From: Gareth Morris Grade: grad (non-science) City: London, State/Prov.: No state entered. Country: UK Area: Chemistry Message ID Number: 982604638.Ch I was thinking about the spaces between atoms and it occurred to me that you could squash the space if there was enough force.Gareth,
Your question made me think and look about and as usual there is a “no” and “yes” aspect to it. First I will refer you to a good physical chemistry Text of your choice. I recommend Robert A Alberty, Physical chemistry. The topics to read about are: Van der Waals, London, or dispersion Forces; hydrogen bonding, dipole interactions, Lennard-Jones diagrams. These describe forces between MOLECULES or between sections of larger molecules. There is a lot to know in these and related subjects. The reason I mention this is that the first thing that happens when pressure is applied to a molecular system such as water is that the distance between the molecules is reduced and if there are any rotations or phase changes etc. that can happen to reduce the stress [LeChatelier’s principle] they will happen. This is what happens in the water-normal ice [Ice Ih or hexagonal ice]. This ice is less dense that water so if a pressure is applied to a system of this ice and water the ice simply melts [or possibly vaporizes if the pressure is from an inert gas] to reduce the pressure and the freezing point lowers. This is the “no” answer to your question and is explained in these two links, especially the second.
859148048.Ph ice is described here" http://skua.caltech.edu/hermann/ice.htm
The next link describes it all with a good phase diagram. http://www.ill.fr/AR-97/page/05chemi.htm
It is rather difficult to compress the space between atoms in a simple molecule because there is no space; there are electrons in their lowest orbitals. These orbitals usually occupy the smallest possible volume and while they are compressible the energy required is fantastically high and there are no readily available relaxation processes that allow a lower volume. One possible mechanism is positive ion formation with ejection of an electron [piezoelectric effect] since positive ions are usually much smaller than the neutral atom or molecule. Topics to look up here are Vibrational and electronic spectra of molecules and the Franck-Condon Principle. These show that pressure, while raising the energy of a molecule, doesn’t usually permit excitation of that molecule to higher energy states because the higher states are of greater volume. So what can happen?
Molecules can rearrange to forms that are of less volume; the classic example is the graphite to diamond transformation. Crystal forms can rearrange to more compact forms. Finally in water there are weak bonds, hydrogen bonds, that have longer than normal bond lengths. These are compressible. All these things happen in ice at extremely high pressures. It also happens that some of these ice forms are stable at temperatures above zero C to really quite high temp possibly even to the critical point. The second and third links describe the processes well. The third link has a good phase diagram describing the many phases of ice and several models of some of the structures. This is the “yes” answer to your question, however, the compression still is not really taking place in the atom but in the longer bonds between molecules. However, at least one form might contain hydronium[H3O+]] ions which means that the space between the H and O atoms was compressed to force a rearrangement.
High pressure chemistry is still a field with many opportunities. Water is a excellent study because of its simplicity combined with complex behavior.
Try the links in the MadSci Library for more information on Chemistry.