Dewar benzene is a valence isomer of benzene, to which it reverts on heating.  [c.130]

Heavily substituted valence bond isomers of  [c.130]

Hume-Rothery s rule The statement that the phase of many alloys is determined by the ratio.s of total valency electrons to the number of atoms in the empirical formula. See electron compounds.  [c.206]

The term resonance has also been applied in valency. The general idea of resonance in this sense is that if the valency electrons in a molecule are capable of several alternative arrangements which differ by only a small amount in energy and have no geometrical differences, then the actual arrangement will be a hybrid of these various alternatives. See mesomerism. The stabilization of such a system over the non-resonating forms is the resonance energy.  [c.344]

VSEPR theory See valency, theory of. vulcanite See ebonite.  [c.423]

Correlations have been found between certain absorption patterns in the infrared and the concentrations of aromatic and paraffinic carbons given by the ndA/method (see article 3.1.3.). The absorptions at 1600 cm due to vibrations of valence electrons in carbon-carbon bonds in aromatic rings and at 720 cm (see the spectrum in Figure 3.8) due to paraffinic chain deformations are directly related to the aromatic and paraffinic carbon concentrations, respectively. )  [c.60]

There have been numerous reviews of photoelectrochemical cells for solar energy conversion see Refs. 181-184 for examples. Figure V-14 shows a typical illustrative scheme for a cell consisting of an n-type semiconductor electrode as anode and an ordinary metal electrode as cathode separated by an electrolyte solution. The valence and conduction bands of the semiconductor are bent near the interface, and as a consequence, the electron-hole pair generated by illumination should separate, the electron going into the bulk semiconductor phase and thence around the external circuit to the metal electrode and the hole migrating to the interface to cause the opposite chemical reaction. Current flow, that is, electricity, is thus generated. As the diagram in Fig. V-14 indicates, there are a number of interrelations between the various potentials and energies. Note the approximate alignment of the solid-state and electrochemical energy scales, the former having vacuum as the reference point and the latter having the H /Hj couple as the reference point.  [c.204]

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection.  [c.275]

The principal use of Auger spectroscopy is in the determination of surface composition, although peak positions are secondarily sensitive to the valence state of the atom. See Refs. 2, 82, and 83 for reviews.  [c.306]

In photoelectron spectroscopy monoenergetic x-radiation ejects inner (Ir, Is, Ip, etc.) electrons. The electron energy is then Eq - Ej, where Eq is the x-ray quantum energy and E, is that of the ith type of electron. The energy of the ejected electrons is determined by means of an electron spectrometer, thus obtaining a spectrum of both the primary photoelectrons and Auger electrons. The method is more accurate than Auger spectroscopy, and because of this, one can observe that a given type of electron has an energy that is dependent on the valence state of the atom. Thus for Ir sulfur electrons, there is a chemical shift of over 5 V, the ionization energy increasing as the valence state of sulfur varies from -2 to +6. The effect is illustrated in Fig. VIII-11 for the case of aluminum, showing how it is possible to analyze for oxidized aluminum on the surface.  [c.308]

MDS Metastable deexcitation spectroscopy [119] Same as PI Surface valence-electron states  [c.314]

Cationic surfactants may be used [94] and the effect of salinity and valence of electrolyte on charged systems has been investigated [95-98]. The phospholipid lecithin can also produce microemulsions when combined with an alcohol cosolvent [99]. Microemulsions formed with a double-tailed surfactant such as Aerosol OT (AOT) do not require a cosurfactant for stability (see, for instance. Refs. 100, 101). Morphological hysteresis has been observed in the inversion process and the formation of stable mixtures of microemulsion indicated [102].  [c.517]

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60).  [c.581]

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and  [c.690]

Reciprocal lattice vectors are usefiil in defining periodic fimctions. For example, the valence charge density, p (r), can be expressed as  [c.106]

The first reliable energy band theories were based on a powerfiil approximation, call the pseudopotential approximation. Within this approximation, the all-electron potential corresponding to interaction of a valence electron with the iimer, core electrons and the nucleus is replaced by a pseudopotential. The pseudopotential reproduces only the properties of the outer electrons. There are rigorous theorems such as the Phillips-Kleinman cancellation theorem that can be used to justify the pseudopotential model [2, 3, 26]. The Phillips-Kleimnan cancellation theorem states that the orthogonality requirement of the valence states to the core states can be described by an effective repulsive  [c.108]

Core el echo ns Valence elecirons  [c.108]

Figure Al.3.10. Pseudopotential model. The outer electrons (valence electrons) move in a fixed arrangement of chemically inert ion cores. The ion cores are composed of the nucleus and core electrons. Figure Al.3.10. Pseudopotential model. The outer electrons (valence electrons) move in a fixed arrangement of chemically inert ion cores. The ion cores are composed of the nucleus and core electrons.
With the density fiinctional theory, the first step in the constmction of a pseudopotential is to consider the solution for an isolated atom [27]. If the atomic wavefiinctions are known, tire pseudo-wavefiinction can be constmcted by removing the nodal stmcture of the wavefiinction. For example, if one considers a valence  [c.111]

The sensitive layer of the systems under investigation eonsists of a mixture of BaFBr with Eu dotation. Other systems are available in the mean time too. X-ray- or y-quants initiate transitions of electrons in the crystal lattice. Electrons are excited from the valence band to the conduction band [2]. Electrons from the conduction band are trapped in empty Br -lattice places. They can return to the valence band via the conduction band after an excitation by  [c.468]

An important group of electrical phenomena concerns the nature of the ion distribution in a solution sunounding a charged surface. To begin with, consider a plane surface bearing a uniform positive charge density in contact with a solution phase containing positive and negative ions. The electrical potential begins at the surface as said decreases as one proceeds into the solution in a manner to be determined. At any point the potential if/ determines the potential energy ze of an ion in the local field where z is the valence of the ion and e is the charge on the electron. The probability of finding an ion at a particular point will depend on the local potential tluough a Boltzmann distribution, g-ze4iikT jjj analogy (q the distribution of a gas in a gravitational field where the potential is mgh, and the variation of concentration with altitude is given by  [c.169]

Electrolytes have a flocculating effect on charge stabilized sols, and the flocculation value may be expressed as the concentration needed to coagulate the sol in some given time interval. There is a roughly 10- to 100-fold increase in flocculation effectiveness in going from mono- to di- to trivalent ions this is due in part to the decreasing double-layer thickness and partly to increasing adsorption of ions into the Stem layer [39, 91]. The effect of valence can be stated by the Schulze-Hardy rule, where the critical flocculation concentration decreases as z" [9] (see Section VI-4). There is an order of effectiveness of ions within a given valence known as the Hofineister series. See Section VI-4 for more on such observations.  [c.190]

Fig. XVin-6. Curve-fitted Mo XPS 3d spectra of a 5 wt% Mo/Ti02 catalyst (a) in the oxidic +6 valence state (b) after reduction at 304°C. Doublets A, B, and C refer to Mo oxidation states +6, +5, and +4, respectively [37]. (Reprinted with permission from American Chemical Society copyright 1974.) Fig. XVin-6. Curve-fitted Mo XPS 3d spectra of a 5 wt% Mo/Ti02 catalyst (a) in the oxidic +6 valence state (b) after reduction at 304°C. Doublets A, B, and C refer to Mo oxidation states +6, +5, and +4, respectively [37]. (Reprinted with permission from American Chemical Society copyright 1974.)
On the other hand, an oxide such as NiO is oxygen-rich, in the sense that occasional Ni ions are missing, electroneutrality being preserved by some of the nickel being in the +3 valence state. These Ni ions t e electrons from the otherwise filled conduction bands, thus again providing the condition needed for electrical conductivity. Oxygen adsorption according to Eq. XVlIl-31 can draw on the electrons in the slightly depleted band (or, alternatively, can produce unlimited additional Ni ) and so should be able to proceed to monolayer formation. Furthermore, since adsorption will make for more vacancies in a nearly filled band, electrical conductivity should rise. Again, the predictions are borne out experimentally [183].  [c.718]

This ionic potential is periodic. A translation of r to r + R can be acconnnodated by simply reordering the sunnnation. Since the valence charge density is also periodic, the total potential is periodic as the Hartree and exchange-correlation potentials are fiinctions of the charge density. In this situation, it can be shown that the wavefiinctions for crystalline matter can be written as  [c.101]

Since the pseudopotential does not bind the core states, it is a very weak potential. Simple basis functions can be used to describe the pseudo-wavefiinctions. For example, a simple grid or plane wave basis will yield a converged solution [25]. The simplicity of the basis is important as it results in an unbiased, flexible description of the charge density. Also, since the nodal structure of the pseudo-wavefunctions has been removed, the charge density varies slowly in the core region. A schematic model of tire pseudopotential model is illustrated in figure Al.3.10. The pseudopotential model describes a solid as a sea of valence electrons movmg in a periodic background of cores (composed of nuclei and inert core electrons). In this model many of the complexities of all-electron calculations, calculations that include the core and valence electrons on an equal footing, are avoided. A group IV solid such as C with 6 electrons per atom is treated in a similar fashion to Sn with 50 electrons per atom since both have 4 valence electrons per atom. In addition, the focus of the calculation is only on the accuracy of the valence electron wavefunction in the spatial region away from the chemically inert core.  [c.108]

Since and depend only on die valence charge densities, they can be detennined once the valence pseudo- wavefiinctions are known. Because the pseudo-wavefiinctions are nodeless, the resulting pseudopotential is well defined despite the last temi in equation Al.3.78. Once the pseudopotential has been constructed from the atom, it can be transferred to the condensed matter system of interest. For example, the ionic pseudopotential defined by equation Al.3.78 from an atomistic calculation can be transferred to condensed matter phases without any significant loss of accuracy.  [c.112]

See pages that mention the term Valency : [c.78]    [c.110]    [c.114]    [c.148]    [c.151]    [c.285]    [c.287]    [c.297]    [c.310]    [c.311]    [c.339]    [c.415]    [c.415]    [c.415]    [c.415]    [c.415]    [c.415]    [c.415]    [c.468]    [c.174]    [c.242]    [c.51]    [c.108]    [c.110]   
See chapters in:

Hazardous chemicals handbook Изд.2  -> Valency

Modern inorganic chemistry (1975) -- [ c.20 , c.28 , c.29 , c.30 , c.31 , c.32 , c.33 , c.34 , c.35 , c.36 , c.37 , c.38 , c.39 , c.40 , c.41 , c.42 ]

Hazardous chemicals handbook Изд.2 (2002) -- [ c.20 , c.23 ]