Normal valence compound

In normal-valence compounds of non-metallic atoms each atom forms covalent bonds to a number given by its valence one for the halogens, two for oxygen, three for nitrogen, and four for carbon. 

N, 0, F, P, S, and Cl the bond orbitals for normal valence compounds lead to about the same radii as tetrahedral orbitals, whereas in atoms below these in the periodic system normal valence bonds involve orbitals which approach p-orbitals rather closely, and so lead to weaker bonds, and to radii larger than the tetrahedral radii. This effect should be observed in Br, Se, and As, but not in Ge, and in I, Te, and Sb, but not Sn. For this reason we have added 0.03 A to the tetrahedral radii for As and Se and... 

To an increasing weight of the chemical bond factor (ionic and/or covalent bonding) will correspond, as an extreme case, the formation of valence compounds. According to Parthe (1980), a compound CmAn can be called a normal valence compound if the number of valence electrons of cations (ec) and anions (eA) correspond to the relation... 

If we consider only the s andp block elements without the noble gases, the number of valence electrons of the elements is included between 1 and 7. In this case, considering that no anions are formed from the elements of groups 1,2 and 3, the following formulae can be deduced for the normal valence compounds, formed in binary systems with large electronegativity difference between elements ... 

According to the conventional simple chemical definition, the valence compounds are those in which individual atoms are assumed to reach a filled valence shell by accepting, donating or sharing electrons. This definition was first applied to the compounds (normal valence compounds) where the cations may donate the exact number of electrons to complete the valence shells (particularly the octet shells) of every anion. 

Some general comments on the solid-state chemistry ( From a molecular view on solids to molecules in solids ) have been reported by Simon (1995) emphasis was especially placed on the structural chemistry of metal-rich compounds formed by the metals in groups 1 to 6 and it was underlined that it is largely based on discrete and condensed clusters. In the chemistry of metals in low oxidation states, the residual valence electrons can be used for metal—metal bonding. Metal-rich compounds lie between normal valence compounds and the elemental metals themselves, with respect to their compositions, and often also with respect to their structures fragments of usual metal structures (close-packed, b.c.c., etc.) are often component units in the structures of metal-rich compounds. 

Compounds with the 16th group elements. Among the various phases formed with these non-metal elements, the normal valence compounds, NaCl type, may be mentioned. Some of them have been described as point compounds (for instance BaS, BaSe, BaTe) a few others as corresponding to a solid solution range (for instance, EuS 43-50 at.% S, EuTe 50-57 at.% Te). 

FeS2, 3 2PO). The structure of CaC2 (Section 5.1.2) is similar to that of pyrite, but with elongation in one direction because of alignment of C ions. Intermetallic compounds with the CaC2 structure are listed in Table 9.3. Thus intermetallic compounds can follow the rules for "normal valence" compounds or those of the metallic state. The "normal valence" compounds have lower CN than found in cep or hep structures and are expected to be less "metallic" in terms of electrical conductance, etc. 

An ordered cation-deficient derivative of the NaCl structure was found in the orthorhombic SC2S3 type (168). The unit cell contains twelve rocksalt units (a = 2 ao, b = ]/2 ao, c = 3J/2 o) The cation vacancies are three-dimensionally distributed without pronounced directional preference. In each sulfur-centered octahedron two scandium atoms are missing. SC2S3, ScaSe3, Y2S3 and Y2Se3 (213) Eire normal valence compounds with non-metallic properties. 

Very similar coordinations are found in the orthorhombic structure of PdS (Fig. 52). Thus, these compounds are normal valence compounds with divalent cations, like PdCla. All known representatives of the PtS and PdS structure are diamagnetic semiconductors (55). 

Mooser, E., and W. B. Pearson (1959). On the crystal chemistry of normal valence compounds. Acta Cryst. 12, 1015-22. 

By analogy with the normal layer structures (large polarizable anions) we should expect the anti structures to occur for the large polarizable cations such as Ag" ", Cs", Tl" , and Pb ". However, the situation is more complicated because not all these compounds are normal valence compounds there is clearly some delocalization of electrons in a number of them. It is interesting to compare the oxides of Cs ... 

Between electron compounds and normal valence compounds, which are characterized by relatively small numbers of structures, there lie large groups of intermetallic compounds in which the structural principles are less clear. In the structures to be described in this section the importance of geometrical factors is becoming evident, a special feature of these structures being the high coordination numbers which range from 12 to 16. 

The chemistry of the compounds of the early transition metals in low oxidation states is full of examples of the occurrence of metal - metal bonds. These compounds show unusual compositions in terms of the traditional valence rules metal - rich compounds contain more metal atoms than one expects for a normal valence compound (example Nbelii instead of Nbis). The transition metal elements on the left In the periodic system, and of those mainly the 4d and 5d elements, are capable of using the excess valence electrons, not needed to complete the octets of the anions, to fbnn metal -metal bonds. 

Calculation of VEC allows to classify a compound as a polyanionic, normal or polycationic valence compound. Polyanionic valence compounds are characterized by anion - anion bonds. In the normal valence compounds there are neither anion - anion nor cation - cation bonds and in the poiycationic valence compounds some valence electrons are used for cation - cation bonds and/or lone electron pairs on the cations. 

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