Valence electrons number

The uncertainties given are calculated standard deviations. Analysis of the interatomic distances yields a selfconsistent interpretation in which Zni is assumed to be quinquevalent and Znn quadrivalent, while Na may have a valence of unity or one as high as lj, the excess over unity being suggested by the interatomic distances and being, if real, presumably a consequence of electron transfer. A valence electron number of approximately 432 per unit cell is obtained, which is in good agreement with the value 428-48 predicted on the basis of a filled Brillouin polyhedron defined by the forms 444, 640, and 800. ... 

Hume-Rothery phases (brass phases, electron compounds ) are certain alloys with the structures of the different types of brass (brass = Cu-Zn alloys). They are classical examples of the structure-determining influence of the valence electron concentration (VEC) in metals. VEC = (number of valence electrons)/(number of atoms). A survey is given in Table 15.1. 

Before studying some examples more closely, let us consider some cases which are not listed in Table 13. There are numerous compounds SnX2 which are definitely monomeric but are nevertheless no carbene analogs since their valence electron number at the tin atom is at least eight. These compounds contain chelating ligands which can stabilize the carbenoid tin atom due to intramolecular Lewis acid-base interactions as shown by structure A and B (see also Chapter 3). 

Valence Electronic Number of 2c-2e B B Number of Number of Aromaticity/... 

Formal Charge = (number of valence electrons) - (number of nonbonding electrons +... 

On the basis of the Periodic Table, topics of intermetallic systematics will be presented in the next chapter. In the present chapter the Periodic Table will be revisited and its structure and subdivisions summarized. In relation also to some concepts previously presented, such as electronegativity, Mendeleev number, etc. described in Chapter 2, typical property trends along the Table will be shown. Strictly related concepts, such as Periodic Table group number, average group number and valence-electron number will be considered and used in the description and classification of intermetallic phase families. 

Comments on some trends and on the Divides in the Periodic Table. It is clear that, on the basis also of the atomic structure of the different elements, the subdivision of the Periodic Table in blocks and the consideration of its groups and periods are fundamental reference tools in the description and classification of the properties and behaviour of the elements and in the definition of typical trends in such characteristics. Well-known chemical examples are the valence-electron numbers, the oxidation states, the general reactivity, etc. As far as the intermetallic reactivity is concerned, these aspects will be examined in detail in the various paragraphs of Chapter 5 where, for the different groups of metals, the alloying behaviour, its trend and periodicity will be discussed. A few more particular trends and classification criteria, which are especially relevant in specific positions of the Periodic Table, will be summarized here. 

Notice that for the alkali and alkaline earth metals and all the elements to the right of the 10th group the valence-electron numbers are 1, 2 and, respectively assumed to be given by the group number minus 10. For the transition elements, various assumptions have been used in some cases. 

In a first hypothesis (s and p electrons, according to Hume-Rothery), the following valence-electron numbers have been assumed ... 

Figure 4.39. The phase diagrams of the binary Cu systems with Zn, A1 and Sn are shown. In order to highlight their similarities the phase equilibria have been plotted with dilferent scales of the atomic composition in order to have the same extension for a scale based on the average v.e.c. in the alloys (number eta of electrons per atom). Notice the consequent change of length of the at.% scale. The values 1, 2, 3, 4 have been assumed for the valence electron numbers of the metals Cu, Zn, A1 and Sn. 

A general agreement may be noticed this may highlight the relevance, and the limits, of the role played by the average valence-electron number per atom... 

Pettifor s structure maps additional remarks. We have seen that in a phenomenological approach to the systematics of the crystal structures (and of other phase properties) several types of coordinates, derived from physical atomic properties, have been used for the preparation of (two-, three-dimensional) stability maps. Differences, sums, ratios of properties such as electronegativities, atomic radii and valence-electron numbers have been used. These variables, however, as stressed, for instance, by Villars et al. (1989) do not always clearly differentiate between chemically different atoms. 

Hp n, 3, and are the numbers of polyhedron vertices at which 1, 2, 3 or 4 polyhedron edges (M-M bonds) meet, respectively. Polyhedra with five or more edges per vertex are generally not electron precise (for this reason no numbers n, occur in the equations). Therefore, the expected valence electron numbers for some simple polyhedra are ... 

Fig. 6. Intermediates proposed by TAMREAC for the two initial steps of the ethylene dimerization. The two numbers below each complex correspond to the complex number and to the valence electron number, respectively. Numbers in parentheses on, or below, the arrows correspond to the reaction numbers (Tables II, III, and IV). TBY = trigonal bipyramidal geometry.

Add the number of valence shell electrons for each atom. If the compound is an anion, add the charge of the ion to the total electron count because anions have "extra" electrons. If the compound is a cation, subtract the charge of the ion (an easy way to remember is that the number of valence electrons for groups 1 7 2 is the group number, e.g.,, H-1,Ca-2,etc. and for groups 13-18 it is group number minus 1, e.g AI-13, valence electron number is 3). 

This formula allows to tackle a number of different problems concerning the relation of crystal structure, valence electron number, and basic structure type as given in [15]. The basic defect-free structure type to the compound AaB(,D[Pg.471]

It seems to be a sort of a rule in these Zintl phases that whenever there is a trigonal prismatic cation coordination then the silicon atoms form planar fragments and vice versa. It is not yet understood why this is so. To none of the Si-Si-bonds in SrMgSi2 a i-bonding contribution can be assigned neither in terms of bond distances nor by valence electron numbers and counting rules. 

Pt from laboratory wastes have been described. Platinum can be successfully electroplated, at least in thin films Many of the Pt alloys follow the Hume-Rothery structural rules, in which the structure adopted can be related to the average valence electron number for the alloy. Pt-Ir alloy (80-20) is especially useful for its ductility. 

Total number of atomic orbitals Total number of valence electrons Number of i-centre bonds... 

By combining the core count a of an atom with its valence electron number 2 , the electronegativity ETA measure E for the ith atom was defined as... 

Fig. 39. A compilation of data for diatomic bond strengths of meta] oxides (solid data points) and carbides (open points) against valence electron number in the transition scries. Circles for first row elements, squares for second row and triangles for third row.

Comment Check to see that the number of valence electrons (number of dots) is the same on both sides of the equation (24). 

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