Noble gases London forces

One of the more profound manifestations of quantum mechanics is that this curve does not accurately describe reality. Instead, because the motions of electrons are correlated (more properly, the electronic wave functions are correlated), the two atoms simultaneously develop electrical moments that are oriented so as to be mutually attractive. The force associated with tills interaction is referred to variously as dispersion , the London force, or the attractive van der Waals force. In the absence of a permanent charge, the strongest such interaction is a dipole-dipole interaction, usually referred to as an induced dipole-induced dipole interaction, since the moments in question are not permanent. Such an interaction has an inverse sixtli power dependence on the distance between the two atoms. Thus, the potential energy becomes increasingly negative as the two noble gas atoms approach one another from infinity. 

The London force is always attractive. It holds the molecules of hydrocarbons together, so that gasoline is a liquid at normal temperatures. It holds 12 molecules together as a solid at ordinary temperatures and N2 molecules as a solid at very low temperatures. Even noble gas atoms can have instantaneous dipole moments and so condense to a liquid. 

Noble gases (group 18 elements) have no color or odor and exist as individual gas atoms that experience London forces. These... 

When a nonpolar molecule (or a noble gas atom) encounters an ion, its electron density is temporarily distorted resulting in an induced dipole that will be attracted to the ion. Intermolecular attractions due to induced dipoles in a nonpolar molecule are known as London forces or Van der Waals interactions. These are very weak intermolecular forces. 

The heats of vaporization are measures of the work that must be done to overcome interatomic attractive forces. Since there are no ordinary electron-pair interactions between noble gas atoms, these weak forces (of the van der Waals or London type) are proportional to the polarizability and inversely proportional to the ionization enthalpies of the atoms they increase therefore as the size and diffuseness of the electron clouds increase. 

London dispersion forces the forces, existing among noble gas atoms and nonpolar molecules, that involve an accidental dipole that induces a momentary dipole in a neighbor. (16.1) Lone pair an electron pair that is localized on a given atom an electron pair not involved in bonding. (13.9)... 

Even molecules without dipole moments must exert forces on each other. We know this because all substances—even the noble gases—exist in the liquid and solid states at very low temperatures. There must be forces to hold the atoms or molecules as close together as they are in these condensed states. The forces that exist among noble gas atoms and nonpolar molecules are called London dispersion forces. To understand the origin of these forces, consider a pair of noble gas atoms. Although we usually assume that the electrons of an atom are uniformly distributed about the nucleus,... 

The motions of the atoms must be greatly slowed down before the weak London dispersion forces can lock the atoms into place to produce a solid. This explains, for instance, why the noble gas elements have such low freezing points (see Table 14.2). 

London dispersion forces are relatively weak forces that arise among noble gas atoms and in nonpolar molecules. London forces arise horn instantaneous dipoles that develop when one atom (or molecule) momentarily distorts the electron cloud of another atom (or molecule). London forces are typicaUy weaker than either permanent dipole-dipole forces or covalent bonds. 

Even noble-gas atoms and molecules that are nonpolar experience a weak intermolecular attraction. In any atom or molecule—polar or nonpolar—the electrons are in continuous motion. As a result, at any instant, the electron distribution may be slightly uneven. The momentary, uneven charge creates a positive pole in one part of the atom or molecule and a negative pole in another. This temporary dipole can then induce a dipole in an adjacent atom or molecule. The two are held together for an instant by the weak attraction between the temporary dipoles, as illustrated in Figure 5.13. The intermolecular attractions resulting from the constant motion of electrons and the creation of instantaneous dipoles are called London dispersion forces, after Fritz London, who first proposed their existence in 1930. 

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