Forces also melting points, relation

The polarity of molecules also creates attractive forces between molecules that cause the molecules to stick together. These attractive forces are called Intermolecular Forces. The physical properties of melting point, boiling point, vapor pressure, evaporation, viscosity, surface tension, and solubility are related to the strength of attractive forces between molecules. 

When a molecule takes part in a reaction, it is properties at the molecular level which determine its chemical behaviour. Such intrinsic properties cannot be measured directly, however. What can be measured are macroscopic molecular properties which are likely to be manifestations of the intrinsic properties. It is therefore reasonable to assume that we can use macroscopic properties as probes on intrinsic properties. Through physical chemical models it is sometimes possible to relate macroscopic properties to intrinsic properties. For instance 13C NMR shifts can be used to estimate electron densities on different carbon atoms in a molecule. It is reasonable to expect that macroscopic observable properties which depend on the same intrinsic property will be more or less correlated to each other. It is also likely that observed properties which depend on different intrinsic properties will not be strongly correlated. A few examples illustrate this In a homologous series of compounds, the melting points and the boiling points are correlated. They depend on the strengths of intermolecular forces. To some extent such forces are due to van der Waals interactions, and hence, it is reasonable to assume a correlation also to the molar mass. Another example is furnished by the rather fuzzy concept nucleophilicity . What is usually meant by this term is the ability to donate electron density to an electron-deficient site. A number of measurable properties are related to this intrinsic property, e.g. refractive index, basicity as measured by pK, ionization potential, HOMO-LUMO energies, n — n ... 

We have seen (Section 12-15) how the presence of strong attractive forces between gas molecules can cause gas behavior to become nonideal when the molecules get close together. In liquids and solids the molecules are much closer together than in gases. As a result, properties of liquids, such as boiling point, vapor pressure, viscosity, and heat of vaporization, depend markedly on the strengths of the intermolecular attractive forces. These forces are also directly related to the properties of solids, such as melting point and heat of fusion. Let us preface our study of these condensed phases with a discussion of the types of attractive forces that can exist between molecules and ions. 

When it is obvious that the solubility of a solid depends on intemiolecular forces between solute and solvent, it is pethaps not as obvious that this solubility also depends on how far the system temperature is removed from the solid s melting point and on the solute s anthalpy of fusion. These dependencies become clear when we relate the standard-stale fogacity of the solute in the solid phase 10 that in die liquid phase, as indicated below. 

Metallic Solids Most metallic elements crystallize in one of the two closest packed structures (Figure 12.30). In contrast to the weak dispersion forces in atomic solids, powerM metallic bonding forces hold atoms together in metallic solids. The properties of metals— high electrical and thermal conductivity, luster, and malleability— result from their delocalized electrons (Section 9.1). Melting points and hardnesses of metallic solids are also related to packing efficiency and number of valence electrons. We discuss bonding models that explain these metallic properties in the next two subsections. 

Low temperature (in relation to the melting point of the material) creep of metals is usually controlled by dislocation movements, because their structures contain sufficient active slip systems and have small Peierls stresses (the force needed to bring about dislocation movement) [4-8]. Deformation can also be controlled by dislocation climb, a process requiring vacancy diffusion. At high temperatures, deformation in metals is usually controlled by diffusion creep mechanisms that do not involve dislocation movement. In ceramics, however, diffusion creep may be the dominant mechanism under most processing conditions due to the small number of slip planes, the high Peierls stresses, and to the need to move stoichiometric amounts of the different atomic species present in the material (both anions and cations for an ionic compound). 

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