Metallic solids forces

We have seen that the pure elements may solidify in the form of molecular solids, network solids, or metals. Compounds also may condense to molecular solids, network solids, or metallic solids. In addition, there is a new effect that does not occur with the pure elements. In a pure element the ionization energies of all atoms are identical and electrons are shared equally. In compounds, where the most stable electron distribution need not involve equal sharing, electric dipoles may result. Since two bonded atoms may have different ionization energies, the electrons may spend more time near one of the positive nuclei than near the other. This charge separation may give rise to strong intermolecular forces of a type not found in the pure elements. 

Whereas the quasi-chemical theory has been eminently successful in describing the broad outlines, and even some of the details, of the order-disorder phenomenon in metallic solid solutions, several of its assumptions have been shown to be invalid. The manner of its failure, as well as the failure of the average-potential model to describe metallic solutions, indicates that metal atom interactions change radically in going from the pure state to the solution state. It is clear that little further progress may be expected in the formulation of statistical models for metallic solutions until the electronic interactions between solute and solvent species are better understood. In the area of solvent-solute interactions, the elastic model is unfruitful. Better understanding also is needed of the vibrational characteristics of metallic solutions, with respect to the changes in harmonic force constants and those in the anharmonicity of the vibrations. 

The molecules (or atoms, for noble gases) of a molecular solid are held In place by the types of forces already discussed In this chapter dispersion forces, dipolar interactions, and/or hydrogen bonds. The atoms of a metallic solid are held in place by the delocalized bonding described in Section 10-. A network solid contains an array of covalent bonds linking every atom to its neighbors. An ionic solid contains cations and anions, attracted to one another by electrical forces as described in Section 8-. 

In substitutional metallic solid solutions and in liquid alloys the experimental data have been described by Epstein and Paskin (1967) in terms of a predominant frictional force which leads to the accumulation of one species towards the anode. The relative movement of metallic ion cores in an alloy phase is related to the scattering cross-section for the conduction electrons, which in turn can be correlated with the relative resistance of the pure metals. Thus iron, which has a higher specific resistance than copper, will accumulate towards the anode in a Cu-Fe alloy. Similarly in a germanium-lithium alloy, the solute lithium atoms accumulate towards the cathode. In liquid alloys the same qualitative effect is observed, thus magnesium accumulates near the cathode in solution in bismuth, while uranium, which is in a higher Group of the Periodic Table than bismuth, accumulated near the anode in the same solvent. 

Condensed matter can be classified by the nature of the forces that hold it together ionic solids covalent solids metallic solids molecular solids. 

In conduction, heat is conducted by the transfer of energy of motion between adjacent molecules in a liquid, gas, or solid. In a gas, atoms transfer energy to one another through molecular collisions. In metallic solids, the process of energy transfer via free electrons is also important. In convection, heat is transferred by bulk transport and mixing of macroscopic fluid elements. Recall that there can be forced convection, where the fluid is forced to flow via mechanical means, or natural (free) convection, where density differences cause fluid elements to flow. Since convection is found only in fluids, we will deal with it on only a limited basis. Radiation differs from conduction and convection in that no medium is needed for its propagation. As a result, the form of Eq. (4.1) is inappropriate for describing radiative heat transfer. Radiation is... 

Metallic solids are excellent electrical and thermal conductors. They are ductile and malleable. When a piece of metal is forced to take a different shape, it continues to hold together since its electrons can shift to bond the metallic atoms in their new positions. 

The atoms in a metallic solid are held together by forces created when many atoms share electrons. These forces create a type of chemical bond known as a metallic bond. 

There can be slightly different forces holding particles together within a solid. Ionic solids, metallic solids, network atomic solids, molecular solids, and amorphous solids each use a different force or combination of forces to hold molecules or atoms together. 

Solids are usually described according to the forces that hold the particles together. The four types of solids are molecular solids, ionic solids, covalent network solids, and metallic solids. 

Crystalline solids can be classified into five categories based on the types of particles they contain atomic solids, molecular solids, covalent network solids, ionic solids, and metallic solids. Table 13-4 summarizes the general characteristics of each category and provides examples. The only atomic solids are noble gases. Their properties reflect the weak dispersion forces between the atoms. 

Metallic solids Recall from Chapter 8 that metallic solids consist of positive metal ions surrounded by a sea of mobile electrons. The strength of the metallic bonds between cations and electrons varies among metals and accounts for their wide range of physical properties. For example, tin melts at 232°C, but nickel melts at 1455°C. The mobile electrons make metals malleable—easily hammered into shapes—and ductile—easily drawn into wires. When force is applied to a metal, the electrons shift and thereby keep the metal ions bonded in their new positions. Read Everyday Chemistry at the end of the chapter to learn about shape-memory metals. Mobile electrons make metals good conductors of heat and electricity. Power lines carry electricity from power plants to homes and businesses and to the electric train shown in Figure 13-21a. 

One particularly vivid example is the cluster of 55 atoms bound by Lennard-Jones forces, effectively Ar55 or by metallic binding forces. Besides its solid and liquid phases, this cluster (and others of similar size and constitution) can exhibit a surface-melted phase [15, 24, 25], Strictly, as animations show, the term surface melting is not really accurate. In the phase that shows a high mobility of the atoms in the surface layer, the actual motion of almost all of those atoms is a large-amplitude, very anharmonic vibration, while at least one atom is promoted to move rather freely around the shell as a floater the average is about one surface atom in 15 becomes a floater. The floater atom exchanges positions occasionally with an atom in the outer shell, so that, over time, all the surface atoms are, at some time, a floater. This process allows all the surface atoms to permute their positions and eventually to occupy all the surface sites - as one would expect of a liquid. 

London forces (6.2) melting point (6.3) metallic bond (6.3) metallic solid (6.3) molar volume (6.1) molecular solid (6.3) normal boiling point (6.2) partial pressure (6.1) pressure (6.1) standard temperature and pressure (STP) (6.1) surface tension (6.2) surfactant (6.2) van der Waals forces (6.2) vapor pressure of a liquid (6.2) viscosity (6.2)... 

In a liquid bath used for melting, there may be slow melting of submerged metal solids because of poor liquid-to-solid heat transfer. (See fig. 3.29.) Heating from the top down in a liquid bath depends on conduction or convection. Some stirring or pumping velocity can be supplied to add forced convection heat transfer. The pumping equipment can be expensive to buy and to maintain. 

Metallic solids, also simply called metals, consist entirely of metal atoms. The bonding in metals is too strong to be due to dispersion forces, and yet there are not enough valence electrons to form covalent bonds between atoms. The bonding, called metallic... 

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