Net charge of atoms

Qi and Qj are the net charges of atoms i and j Nvai(i) and Nvai(j) their number of valence electrons. Cexch and pexCh are empirical parameters. Some additional refinements exist within SIBFA as explicit addition of lone pairs for the exchange term [50],... 

Two approaches have been applied to estimate the net charges of atoms from the observed electron-density distribution in crystals. The first method is a direct integration of observed density in an appropriate region around an atom (hereafter abbreviated as DI method) (64). The second is the so-called extended L-shell method (ELS method) (19, 81) in which a valence electron population of an atom is calculated by a least-squares method on the observed and calculated structure amplitudes. 

This method may be useful to estimate the approximate net charge of atoms, because they are easily calculated by the conventional least-squares method. However, it seems that the ELS method does not necessarily yield accurate net charges of the atoms. The method is... 

The values of i calculated fromEq. 2.105 on v(M) in different molecules, give the average (for 700 molecular halides and chalcogenides) of e = 0.5e, in agreement with Pauling s famous Electroneutrality Rule [480,481] which states that net charges of atoms in stable molecules and crystals should not exceed Yi, even though later he softened this limitation to 1 [475]. This principle later has been proved theoretically, confirmed experimentally and now plays a key role in the description of electronic structure of molecules and crystals. 

Figure C2.12.1. Origin of ion exchange capacity in zeolites. Since every oxygen atom contributes one negative charge to the tetrahedron incoriDorated in the framework, the silicon tetrahedron carries no net charge while the aluminium tetrahedron carries a net charge of-1 which is compensated by cations M.

The evolution of the mean ir net charge of the five atoms of the ring as a function of the calculation method is reported in Fig. Tl. The sophistication of the method corresponds rougly to a leveling of the charges except for sulfur. The ab initio model, being unique, is tentatively reported in Fig. I-l. 

Fig. I-l. Variation of the mean v net charge of the five atoms of thiazole ring as a function of the calculation method employed.

COMPARISON BETWEEN ir NET CHARGES OF NITROGEN AND CARBON ATOMS IN THE a- AND p-POSITIONS IN PYRIDINE AND THIAZOLE (PPP-ir METHOD)... 

The chemical shifts of in natural abundance have been measured for thiazole and many derivatives (257,258). They are given in Tables 1-37 and T38. These chemical shifts are strongly dependent on the nature of the substituent CNDO/2 calculations have shown (184) that they correlate well with the ((t+tt) net charge of the atom considered. As a consequence, the order of the resonance signals is the same for protons and for carbon atoms. 

The interaction between a substituent and the ring carbon to which it is bonded could be related to some electronic characteristics of the unsubstituted ring and especially to the net charge of its various sites. In that respect the rr-net charges diagram discussed in Section 1.5 indicates that the electron-withdrawing power of the ring-carbon atoms will decrease in the order, 2>4>5. 

Fitting atomic charges to give a best match to a calculated electrostatic potential. The constraint is that the sum of atomic charges should equal the net charge of the molecule. 

As stated above, a typical zeolite consists of silicon and aluminum atoms that are tetrahedrally joined by four oxygen atoms. Silicon is in a +4 oxidation state therefore, a tetrahedron containing silicon is neutral in charge. In contrast, aluminum is in a +3 oxidation state. This indicates that each tetrahedron containing aluminum has a net charge of -1, which must be balanced by a positive ion. 

Zeolites are crystalline alumina-silicates having a regular pore structure. Their basic building blocks are silica and alumina tetrahedra. Each tetrahedron consists of silicon or aluminum atoms at the center of the tetrahedron with oxygen atoms at the comers. Because silicon and aluminum are in a +4 and +3 oxidation state, respectively, a net charge of -1 must be balanced by a cation to maintain electrical neutrality. 

Note that the other electrons do not block the influence of the nucleus they simply provide additional repulsive coulombic interactions that partly counteract the pull of the nucleus. For example, the pull of the nucleus on an electron in the helium atom is less than its charge of +2e would exert but greater than the net charge of +e that we would expect if each electron balanced one positive charge exactly. 

In summary, despite the structural diversity exhibited by the silicates, their silicon atoms always have tetrahedral geometry. In addition, every outer oxygen atom contributes a net charge of -1 to the structure, while every inner oxygen atom is electrically neutral and has an Si—O—Si bond angle close to 109.5°. 

H3 PO4 Phosphoric acid is a covalent compound with a net charge of zero. Each hydrogen atom has an oxidation number of+1 (Guideline 3), and each oxygen has an oxidation number of-2 (Guideline 4). Now add the contributions from these atoms 3(+l) + 4(-2) = -5. For the oxidation numbers to sum to zero (Guideline 2), the phosphorus atom of phosphoric acid must have an oxidation number of +5. 

The form of Eq. (3.13) indicates that this term is the sum of Coulomb potentials arising from the net charge of each atom of molecule A and that of each atom of molecule B. Therefore, cq is significant in the interaction of polar molecules, causing a long-range force. 

The number of positive charges must equal the number of negative charges, since the atom has a net charge of 0. The number of positive charges, as shown in the table, is equal to the number of protons. The number of negative charges, also from the table, is equal to the number of electrons. Therefore, in an uncombincd atom, the number of protons must equal the number of electrons. 

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