Helium intermolecular forces

Primitive considerations convince us that such secondary forces exist. For example, gases consist of disordered molecules, whether they be polyatomic like chlorine or ether vapour, or single atoms like helium or mercury vapour. But all gases, even helium, ultimately condense to liquids — and then to solids — if they are cooled and/or compressed sufficiently. When the molecules are forced into close proximity and have their kinetic energies diminished, the weak intermolecular forces are able to take control. Liquefaction results. The strength of these forces can be measured by the latent heat necessary to evaporate the liquid, or to sublime the solid. The equation,... 

Ion-Induced Dipole and Dipole-Induced Dipole Interactions Thus far in this section, aU of the species we have considered have been charged or polar. What types of intermolecular forces are present when one or both interacting species is a neutral atom or nonpolar molecule As an example, consider the interaction of an ion or an polar molecule with an atom (such as helium) or nonpolar molecule. If we place an ion or polar molecule near the atom (or nonpolar molecule), the electron distribution of the atom will be distorted by the electrostatic force exerted by the charge of the ion or the dipole moment of the polar molecule, causing the formation of a dipole on the atom (or nonpolar molecule) (Figure 4.29). 

Liquids are more ordered than gases because of the stronger intermolecular forces and the lower mobility of the liquid particles. According to the kinetic-molecular theory of liquids, the particles are not bound together in fixed positions, but move about constantly. This particle mobility explains why liquids and gases are referred to as fluids. A fluid is a substance that can flow and therefore take the shape of its container. Most liquids naturally flow downhill because of gravity. However, some liquids can flow in other directions as well. For example, liquid helium near absolute zero is able to flow uphill. 

Gibby C.W., Tanner C.C., Masson I., The pressure of gaseous mixtures II. Helium and hydrogen and their intermolecular forces . Proceedings of the Royal Society of London A, vol. 122, pp. 283 - 304,1929. 

By contrast, consider the behavior of heUum, which displays a positive deviation from the ideal behavior. This is because hehum has very weak intermolecular forces and their effect on lowering the pressure (relative to ideal gas) is small compared to the effect of particle size on increasing the volume. Therefore PV/RT is greater than predicted from the ideal gas law for helium. 

Noble gases with small atoms, such as helium and neon, approach ideal gas behaviour. This is because the intermolecular forces are so small. 

Explain why the intermolecular forces in a sample of helium and neon are very small. 

The rest of Section I contains a summary of the general concepts used throughout this article. Section II contains a statistical-mechanical derivation of Maxwell s equations in matter. Section III deals with the problem of the ponderomotive force in a dielectric. In Section IV we give the theory of refraction of light in a medium of isotropic molecules, having a polarizability which is a function of intermolecular distances. We also calculate in this section the polarizability as a function of intermolecular distances in a very simple case (helium gas). Finally, Section V is concerned with the theory of light scattering. 

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