Properties. Valency

2 Crystallographic Properties. Valency Crystallographic Properties of Bulk Samples 

YbSe crystallizes in the cubic NaCl structure type, but stoichiometry and preparation conditions have a substantial influence on the lattice constants, and distortions of the cubic unit cell are sometimes observed. 

The Yb chalcogenides exhibit anomalous compressibilities in the 150 to 200 kbar region (see p. 401), but the structure remains NaCl type in the entire pressure range. The anomaly is attributed to the Yb Yb valence change, Jayaraman et al. [1, p. 2515]. This is in agreement with optical studies, which indicate the semiconductor-metal transition in the region of 175 kbar, Narayanamurti etal. [2]. 

YbSe films on a Pyrex substrate show a preferential (200) orientation. Epitaxial growth is observed on films deposited on (111) faces of CaFs. The films have cubic NaCl structure with a = 5.93 A, Paparoditis, Suryanarayanan [10], Suryanarayanan et al. [11] a = 5.86 A is reported for YbSe films deposited on glass, quartz, or KBr substrates, Lashkarev et al. [12]. 

An Yb valence of v = 2.0 at 300 K is deduced from the X-ray L,, absorption spectrum of YbSe, in agreement with the value obtained from the lattice constant, Bauchspiess et al. [13]. Based on magnetic susceptibility measurements, YbSe contains some trivalent Yb and may be written Yb 83Ybgt2Se, Obolonchik et al. [14]. The existence of trivalent Yb in YbSe is also assumed by Abrikosov et al. [15]. 

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Figure 2. The Lewis structure routine will draw on the PROPERTY VALENCY, which is the number of electrons each vertex contributes to the Lewis structure. It assigns a pair of electrons to each LINK, and satisfies the octet requirement.

A. Benoit, J. X. Boucherle, J. Flouquet, F. Holtzberg, J. Schwelzer, C. Vettier, Thulium Selen-ide and Cerium Compounds Properties, Valence Fluctuations Solids St. Barbara Inst. Theor. Phys. Conf., Santa Barbara, Calif., 1981, pp. 197/207. 

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... 

Semiconductors are poor conductors of electricity at low temperatures. Since the valence band is completely occupied, an applied electric field caimot change the total momentum of the valence electrons. This is a reflection of the Pauli principle. This would not be true for an electron that is excited into the conduction band. However, for a band gap of 1 eV or more, few electrons can be themially excited into the conduction band at ambient temperatures. Conversely, the electronic properties of semiconductors at ambient temperatures can be profoundly altered by the... 

Several factors detennine how efficient impurity atoms will be in altering the electronic properties of a semiconductor. For example, the size of the band gap, the shape of the energy bands near the gap and the ability of the valence electrons to screen the impurity atom are all important. The process of adding controlled impurity atoms to semiconductors is called doping. The ability to produce well defined doping levels in semiconductors is one reason for the revolutionary developments in the construction of solid-state electronic devices. 

Is 2s 2p 3s 3p 3d 4s. If the 3d states were truly core states, then one might expect copper to resemble potassium as its atomic configuration is ls 2s 2p 3s 3p 4s The strong differences between copper and potassium in temis of their chemical properties suggest that the 3d states interact strongly with the valence electrons. This is reflected in the energy band structure of copper (figure Al.3.27). 

Mendeleef drew up a table of elements considering the chemical properties, notably the valencies, of the elements as exhibited in their oxides and hydrides. A part of Mendeleefs table is shown in Figure 1.2 -note that he divided the elements into vertical columns called groups and into horizontal rows called periods or series. Most of the groups were further divided into sub-groups, for example Groups... 

In essence, a BE-matrix li.sts all the valence clcctron.s of the atoms in a molecule, both the ones involved in bonds and those associated as free electrons with an atom. A BE-matrix has a scries of interesting mathematical properties that directly... 

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

Drawing-, text-, and structure-input tools are provided that enable easy generation of flow charts, textual annotations or labels, structures, or reaction schemes. It is also possible to select different representation styles for bond types, ring sizes, molecular orbitals, and reaction arrows. The structure diagrams can be verified according to free valences or atom labels. Properties such as molecular... 

A is a parameter that can be varied to give the correct amount of ionic character. Another way to view the valence bond picture is that the incorporation of ionic character corrects the overemphasis that the valence bond treatment places on electron correlation. The molecular orbital wavefimction underestimates electron correlation and requires methods such as configuration interaction to correct for it. Although the presence of ionic structures in species such as H2 appears coimterintuitive to many chemists, such species are widely used to explain certain other phenomena such as the ortho/para or meta directing properties of substituted benzene compounds imder electrophilic attack. Moverover, it has been shown that the ionic structures correspond to the deformation of the atomic orbitals when daey are involved in chemical bonds. 

The trends in chemical and physical properties of the elements described beautifully in the periodic table and the ability of early spectroscopists to fit atomic line spectra by simple mathematical formulas and to interpret atomic electronic states in terms of empirical quantum numbers provide compelling evidence that some relatively simple framework must exist for understanding the electronic structures of all atoms. The great predictive power of the concept of atomic valence further suggests that molecular electronic structure should be understandable in terms of those of the constituent atoms. 

The energy obtained from a calculation using ECP basis sets is termed valence energy. Also, the virial theorem no longer applies to the calculation. Some molecular properties may no longer be computed accurately if they are dependent on the electron density near the nucleus. 

There have been some attempts to compute nonlinear optical properties in solution. These studies have shown that very small variations in the solvent cavity can give very large deviations in the computed hyperpolarizability. The valence bond charge transfer (VB-CT) method created by Goddard and coworkers has had some success in reproducing solvent effect trends and polymer results (the VB-CT-S and VB-CTE forms, respectively). 

Extended Hiickel gives a qualitative view of the valence orbitals. The formulation of extended Hiickel is such that it is only applicable to the valence orbitals. The method reproduces the correct symmetry properties for the valence orbitals. Energetics, such as band gaps, are sometimes reasonable and other times reproduce trends better than absolute values. Extended Hiickel tends to be more useful for examining orbital symmetry and energy than for predicting molecular geometries. It is the method of choice for many band structure calculations due to the very computation-intensive nature of those calculations. 

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