Van der Waais forces

Forces found between haiogen moiecuies are van der Waais forces, which are due to temporariiy induced dipoies caused by poiarization of eiectron ciouds. ... 

Therefore, greater poiarization of eiectron cioud wouid cause greater attractive force (van der Waais force), resuiting in higher boiiing points. ... 

Historical deveiopment of van der Waais forces. The Lennard-Jones potentiai. intermoiecuiar forces. Van der Waais forces between surfaces and coiioids. The Hamaker constant. The DLVO theory of coi-loidal stability. 

Historical development of van der Waais forces and the Lennard-Jones potential... 

FIGURE 21.23 The van der Waals radii of the carbon and nitrogen atoms superimposed on an outiine of the moiecuiar structure of cyanuric triazide, C3N12, to show the voiume of space from which each moiecuie exciudes the others. Van der Waais forces in the moiecuiar crystai hoid the moiecuies in contact in a pattern that minimizes empty space. The thin white iines emphasize the 3-foid symmetry of the pattern. 

Electrostatic interactions cannot account for all of the non-bonded interactions in a system. The rare gas atoms are an obvious example all of the multipole moments of a rare gas atom are zero and so there can be no dipole-dipole or dipole-induced dipole interactions. But there clearly must be interactions between the atoms, how else could rare gases have liquid and solid phases or show deviations from ideal gas behaviour Deviations from ideal gas behaviour were famously quanfatated by van der Waais, thus the forces that give rise to such deviations are often referred to as van der Waais forces. 

Van der Waais forces opemie between molecules, and are classified as- (i) dii.wle dipole inleraclions or Keesom forces. 

So far, we have discussed the bonding within covalent molecules (intramolecular bonding). In this section, we will describe the forces that hold covalent molecules (and some neutral atoms) together in the liquid and solid states (intermolecular bonding). Intermolecular forces have the general name van der Waais forces, after the Dutch scientist Johannes van der Waais (1837-1923). The different types of intermolecular forces discussed are shown in Table 5.2. 

Dispersion (London-Van der Waais) Forces. Dispersion forces are formed by mutual induction of atomic dipoles due to the electromagnetic field between the nucleus and electrons of the atom. 

In 1873 van der Waais pointed out that real gases do not obey the ideal gas equation PV = RT and suggested that two correction terms should be included to give a more accurate representation, of the form (P + ali/) V - b) = RT. The term a/v corrects for the fact that there will be an attractive force between all gas molecules (both polar and nonpolar) and hence the observed pressure must be increased to that of an ideal, non-interacting gas. The second term (b) corrects for the fact that the molecules are finite in size and act like hard spheres on collision the actual free volume must then be less than the total measured volume of the gas. These correction terms are clearly to do with the interaction energy between molecules in the gas phase. 

Nonbonded interactions are the forces between atoms that aren t bonded to one another they may be either attractive or repulsive. It often happens that the shape of a molecule may cause two atoms to be close in space even though they are separated from each other by many bonds. Induced-dipole/induced-dipole interactions make van der Waals forces in alkanes weakly attractive at most distances, but when two atoms are closer to each other than the sum of their van der Waals radii, nuclear-nuclear and electron-electron repulsive forces between them dominate the Evan der waais term-The resulting destabilization is called van der Waals strain. 

SECTION 10.9 Departures from ideal behavior increase in magnitude as pressure increases and as temperature decreases. The extent of nonideality of a real gas can be seen by examining the quantity PV = RT for one mole of the gas as a function of pressure for an ideal gas, this quantity is exactly 1 at all pressures. Real gases depart from ideal behavior because the molecules possess finite volume and because the molecules experience attractive forces for one another. The van der Waais equation is an equation of state for gases that modifies the ideal-gas equation to account for intrinsic molecular volume and intermolecular forces. 

The details of chemical bonding are left to other books in this series, but it is useful in this text to summarize the main features of the ways in which atoms can bond to each other in elementary forms and in compounds. In the pure elements, the bonding may be described as either metallic or covalent, with small covalent molecules held together by van der Waais intermolecuiar forces in their liquid and solid states. In addition, because compounds are discussed at some length in subsequent chapters, the basis of ionic bonding is described. 

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