Trivalent atom

Again, experiments show that it is generally true that a trivalent atom with three p orbitals as valence orbitals forms a pyramidal molecule. The bonding is called p3 bonding (read, p three ). 

Asl3 and Bils. Heyworth and Braekken4) in their studies of the hexagonal crystals Asls and Bils assigned to them structures in which each arsenic or bismuth atom is surrounded by six equidistant iodine atoms, the interatomic distances reported being As—I = 2.97 A and Bi—I = 3.09 A. As in the case of eulytite, we believe that the trivalent atoms are displaced towards three and away from three of these six atoms, until the smallest interatomic distances become As — 7 = 2.54 A and Bi — I = 2.84 A5). 

In our design, divalent Ca was chosen to partially substitute the trivalent atoms, and La and Ce were selected for a trivalent element because their ionic size (rLas+ = l.SOA rce3+=l-48A) was close to that of Ca (rca2+ = T48A) [21]. Like La, the Ce element also generally shows a formal -i-3 oxidation state in in-termetallics. Erom the reactions of the elements, we have identified as major phases the electron-precise/deficient alloys, Ln5.xCaxGe4 (Ln=La, Ce x=3.37,... 

Zeolites consist of linked tetrahedra of Si04 and AIO4. The substitution of the aluminum by other trivalent atoms such as B, Ga, and In in order to modify the surface acidity of such solids has aroused considerable interest [34,213-215],... 

The bridgeheads in a bimacrocycle must connect three bridges each. Therefore they must be trifunctional. This is given when trivalent atoms are used and also when trisubstituted groups are incorporated. 

Similarly, if we treat the nitrogen in nitromethane as a trivalent atom, the index is 1, which is compatible... 

On the supposition that the total number of unit cells keeps invariable and no aluminum atoms are lost during the boronation, the composition of unit cell and the population of vacancies can be estimated as listed in composition of unit cell (I) in Table 2. It can be seen that the vacancies occupy about 30-50% of total T sites after the boronation. However, it should be noted that the population of vacancies thus obtained by chemical analysis is only a bulk average result. The composition on the surface of crystallites is actually different from that in the bulk because the dissolution of silicon starts first from the outer surface, so that the vacancies on the surface are much more than those in the interior of crystallites. Such a large number of vacancies on the surface will result in corrosion and dissolution of the surface parts of crystal particles. Therefore, the number of unit cells in the sample after the boronation is actually less than that before the boronation, whereas boron atoms in each unit cell should be more than those shown in composition of unit cell (1) in Table 2. On the other hand, if all the 64 T sites are occupied by silicon and trivalent atoms, we can give another set of compositions as shown in composition of unit cell (II) in Table 2. The real composition of a unit cell should be between these two sets of compositions, that is, the 64 T sites are neither occupied completely nor vacated so severely that the collapse of the framework occurs. It can also be seen that the introduction of boron atoms is so limited that there are no more than 1.5 atoms per unit cell even though the repeated boronation is performed. 

The porous volumes measured by N2 adsorption are listed in Table 3. After the boronation, the total porous volumes (Vt) of the samples increase, corresponding to the increase of benzene adsorption capacity mentioned above. This should be resulted from the following aspects (1) The average mass of zeolite crystallite decrease and the number of crystal particles in unit weight of sample increases after the boronation owing to a limited introduction of trivalent atoms and Na+cations as counterions, as well as a severe dissolution of silicon. Thus, the total porous volume (mL/g) and the adsorption capacity increase. (2) The transformation of pore size occurs during the boronation. As shown in Table 3, the mesoporous volumes increase and the microporous volumes decrease after the boronation, meaning that some micropores are developed into mesopores due to the removal of silicon from the framework. This is also one of the important reasons why the total porous volumes as well as the adsorption capacities increase after the boronation. 

Fig.6. Relationship between structure vacancies and Si(3Si, 10H) sites (O) Si, ( ) trivalent atom, ( ) vacancy...

The Fermi level is a theoretical energy of electrons in a semiconductor, such that the probability of occupation of the VB and CB is 50%. In an intrinsic semiconductor this Fermi level is about half-way between the VB and the CB, but it can be displaced substantially in doped semiconductors. An intrinsic semiconductor would be for example a crystal of pure Si or Ge, all tetravalent atoms being linked together in a three-dimensional array. In a doped semiconductor of n -type some of the Si atoms are replaced by pentavalent atoms such as As, and these will release electrons into the CB. A p -type semiconductor, however, contains some trivalent atoms like A1 which are electron deficient. The Fermi level moves closer to the CB in the n-doped semiconductor, while it comes closer to the VB in the p-type semiconductor (Figure 3.46). 

Figure 3.45 In a doped semiconductor some tetravalent atoms are replaced by trivalent atoms (p-type) or pentavalent atoms (n-type)...

Figure 3.46 In a p-type semiconductor the electrons are held by the trivalent atoms and their overall energy (the Fermi level) is lowered. In an n-type semiconductor electrons are released by the pentavalent atoms and the Fermi level comes closer to the conduction band. SC = semiconductor...

The addition of a trivalent atom (e.g. boron) to silicon leads to an empty electron state, or positive hole, which can be ionized from the effective single negative charge — e on the B atom. The ionization energy is again about 0.01 eV, as might be expected. Therefore the doping of silicon with boron leads to the... 

Similarly, if we treat the nitrogen in nitromethane as a trivalent atom, the index is 1, which is compatible with Figure 1.12. If we treat phosphorus in triphenylphosphine oxide as trivalent, the index is 12, which fits the Lewis structure in Figure 1.12. As an example, let us consider the molecular formula Ci3H9N204BrS. The index of hydrogen deficiency would be 13 - 10/2 + 2/2 + 1 = 10 and a consistent structure would be... 

For an organic compound the first step is usually to find the molecular formula, probably from the mass spectrum, and to calculate the number of double bond equivalents (DBEs). An acyclic saturated hydrocarbon has the formula where M = 2N+2. Each double bond or ring in the molecule reduces the value of M by two. So if M = 2N the molecule has one DBE we cannot tell from the formula whether it is in the form of a ring or unsaturation. A benzene ring corresponds to 4 DBEs three double bonds and a ring. The presence of oxygen or other divalent elements does not affect the value of M. Each monovalent atom such as chlorine can be treated as a proton for the purpose of calculation, while one proton has to be subtracted for each trivalent atom such as nitrogen. 

The preparation of trivalent nitrogen was achieved in 2004 by Eremets et al. in a diamond cell at 1150 000 bar and 2000 K [47-49]. The crystallographic data for the trivalent nitrogen is cubic, lattice parameter a = 3.4542(9) A. A three-dimensional structure which consisted of trivalent nitrogen atoms (Fig. 9.5) was found. The N—N bond length at l.lMbar is 1.346 A, and the NNN angle is 108.8°. The nitrogen atoms form screws of trivalent atoms which are connected to form a three-dimensional network. 

Some of these derivatives, such as Prussian blue, are of considerable commercial importance on account of their characteristic deep colour. As a general rule, the derivatives which are devoid of colour contain iron in one stage of oxidation only within the molecule, whilst the coloured compounds possess divalent and trivalent atoms of iron respectively. It would appear, therefore, that the colour is in some way connected with the presence of similar atoms in more than one stage of oxidation. Thus, ferrous potassium ferrocyanide, Fe"K2[Fe (CN)6], is white, the iron atoms in the positive and negative radicles respectively being divalent. Upon oxidation, however, Prussian blue, Fe K[Fe (CN)e] is obtained, the iron atom of the negative radicle remaining divalent, whilst the positive iron ion is trivalent. 

ReCle octahedra which form a quasi-infinite chain by sharing one vertex at each metal center. Such dimeric units are observed in the complex salts A3M2X9 with trivalent atoms (M = Tl, V, Nb, Cr, Mo, W). ... 

If H = number of univalent atoms (H, halogen), N = number of trivalent atoms (N, P), and C = number of tetravalent atoms, then... 

Acid sites are associated with framework A1 or other trivalent atoms. The number of the acid sites is proportional to the concentration of framework A1 or other trivalent atom. The strength of the acid sites in most zeolites is inversely proportional to the concentration of framework A1 up to about a silica/alumina ratio of 10. The nature of the heteroatom also affects acid strength. A1 zeolites are much more acidic than Ga- or Fe-zeolites. B-zeolites have very weak acidity. ALPO4-S have no exchangeable cations and therefore no acidity. 

The substitution of —CH= by —N= in aromatic rings has been one of the most successful applications of classical isosterism (see Section III.D.). Interchange of trivalent atoms are found also in non aromatic rings. For example the 4-dimethylamino-antipyrine and its carba-isostere are about equally active as antipyretics (Figure 15.7). 

Na one gets a much larger s character (=50%) than for Li. Moreover, for Na a d character of about 10% is found. One obtains almost the same 1-like distribution for Zn and Cd as for Na. For the trivalent atoms the s-amplitudes increase with increasing atomic numbers. This is a general trend observed in molecular systems ... 

Equally important isosteric replacements involve trivalent atoms and groups. These include -N= and -C=, especially in ring compounds, and they will be discussed in the appropriate chapters. 

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