Continuous valency rule

Considering in Fig, 38 the relative position of these curves and those for 1—2 and 1— 3, we easily recognise the sequence of the so-called " continuous valency rule . 

We now turn to another salt rule, the continuous valency rule in which the same salt symbols occur as in the double valency rule, but in which they are arranged in another way. 

Fig. 14. Continuous valency rule in the in-fluence of added salts on the electrophoretic Velocity of complex coacervate drops (gelatin and gum arabic).

If however one compares several salts with each other and at equal concentrations (in mini eq. per 1.) it is to be expected that the coacervate drops will always come out relatively more positive with a salt of the type 2—1 than with 1—1, conversely always relatively more negative with a salt of the type 1—2 than with 1—1. If one extends this ai ument to still other terms on either side, one has the above given " continuous valency rule. ... 

Under certain circumstances the continuous valency rule can also occur in experiments in which the suppressive action of neutral salts is investigated. (See fig. 15A and C). 

Abscissae salt concentration in m. eq. per 1. The double valency rule makes its appearance in the actual suppressive action. Preceding this in small concentrations a bunching out according to the continuous valency rule (see text). 

To avoid misunderstanding we must add that if on adding small concentrations of salt a bunching according to the continuous valency rule is first produced (such as for example Fig. 15A and C), nevertheless at higher concentrations, at which one is concerned with the real suppression, the curves are arranged according to the double valency rule. 

In 2f, p. 349 we already saw from qualitative experiments that salts in general exert two kinds of action on the one hand a suppressive action in which the so-called double valency rule holds and a displacement of the optimum mixing proportion in which the so-called continuous valency rule holds. 

The electrophoretic velocity of the drops is influenced by added salts according to the continuous valency rule ... 

And as a third characteristic of the complex nature of the flocculation the continuous valency rule is encountered in the imfluence of added indifferent salts on the electrophoretic velocity of the flocculi ... 

In Fig. 24a the G + A + n shell is already abolished before the remainii G +N + a coacervate drop is transformed into a hollow sphere. In Fig. 24b on the other hand the enclosed G -h N + a coacervate drop can transform into a hollow sphere while retaining the peripheral coacervate shell. The differences between the salts used must be associated with displacements in the material composition of the coexistit] coacervates, in fact we know that salts do this (p. 365) and that the salts arrange themselves in this case in the sequence of the so-called continuous valency rule (p. 452-455). 

If the selected space group of the starting structure has high symmetry, the number of free parameters will be smaller than the number of constraints and it is not possible for all the predicted distances to be realized. If the deviations are small, the structure may be stable, but if they are large, the structure will relax or, in extreme cases, be so unstable that it cannot be prepared. Relaxation may involve only a small adjustment to the bond lengths so that the valence sum rule continues to be obeyed (at the expense of the equal valence rule), or it may involve a reduction in the symmetry as is found in the case of CaCrF5 described above. If the symmetry is reduced, the number of free variables is increased and the atomic coordinates may be under-determined. In this case, the constraints on the sizes of non-bonding distances become important. 

Since the electron density is a continuous function across the interatomic surface, the two atoms that form the surface must have the same distribution of electrons over this face. The most stable structures will be those which require the least amount of redistribution of electron density when the free atoms come together, that is, they will be formed between atoms that have similar surface electron densities. This idea is related to the valence matching principle (Rule 4.2) which states that the most stable bonds are formed between ions that have similar bonding strengths. The bonding strength is thus related to the surface electron density of the ion. 

If one organic compound has dominated the historical literature of the last few years, that compound must be benzene. Most probably, this is because its structure in some respects marks a transition from the most austere form of classical organic chemistry, in which carbon was tetravalent and tetrahedral, to a continuing series of changes from oscillating molecules, through partial valencies to MO descriptions, and Huckel s rules of aromaticity. It is the case par excellence of a single substance whose history intersects all major streams of chemical theory - except perhaps the periodic law - and which also has enormous industrial and economic importance. 

It s important to realize that formal charge is a useful, but not perfect, tool for assessing the importance of contributions to a resonance hybrid. You ve already seen that it does not predict an important resonance form of NO2. In fact, recent theoretical calculations indicate that, for many species with central atoms from Period 3 or higher, forms with expanded valence shells and zero formal charges may be less important than forms with higher formal charges. But we will continue to apply the formal charge rules because it is usually the simplest approach consistent with experimental data. 

The strong influence of Zintl on the description of chemical bonding in compounds at the border of salts and intermetallics led to the nomenclature Zintl ion f ° for soluble polyanions (as part of a polyanionic salt ) and Zintl phase f for compounds with anionic substructures obeying the (8 — N) rule. Further development and the perception that the salt-metal transition is not abrupt led to a continuous extension of these terms. Soluble polycations, discrete units, and low-dimensional substructures in Zintl phases are called Zintl ions. These ions commonly consist of metal- or semi metal-atoms, or of atoms of semiconducting elements. Clearly, they must be distinguished from classical ions as elucidated by a comparison of SnTe4 and the iso(valence)electronic ion S04 . 

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