Ionic sizes definition

Ionic Size. The size of an ion is a somewhat hazy concept because the modern notion of atoms pictures the electron distribution to extend to infinity. Nevertheless, it is true that there are definite distances established between the centers of atoms in a compound. It is thus natural to attempt to conceive of the distance between, say, Na+ and CT in solid sodium chloride as being made up as a sum of two contributions, one from the negative ion and the other the positive ion. This amounts to defining the sizes of ions in such a manner that in each ionic compound the sum of the ionic radii equals the observed interionic distance at equilibrium. 

It follows from the definition cited that the size of the zeta potential depends on the structure of the diffuse part of the ionic EDL. At the outer limit of the Helmholtz layer (at X = X2) the potential is j/2, in the notation adopted in Chapter 10. Beyond this point the potential asymptotically approaches zero with increasing distance from the surface. The slip plane in all likelihood is somewhat farther away from the electrode than the outer Helmholtz layer. Hence, the valne of agrees in sign with the value of /2 but is somewhat lower in absolute value. 

The problem of the definition of charge and consideration of the sizes of reactants (the diameters of the reactant ions and the activated complex are assumed equal in the derivation of (2.184)) is most acute with reactions of metalloproteins. Probably the most nsed expression for the effect of ionic strength on such reactions is ... 

There exist a number of membranes which are permeable to some ions and not to others, and these give rise to definite potential differences when the membrane is interposed between two solutions of the electrolyte. These membranes must be considered capillary in structure and selective permeability attributed to selective ionic adsorption, or in some cases to restriction imposed by the size of the capillaries. 

Equation 6.1 is valid for a macroscopic particle moving in a continuous medium. In electrophoresis where the analyte ion moves in the media where particle size is comparable with that of the analyte size, this is definitely not the case. Also, analyte ions are not spherical and the term of the ionic radius, the value of which is difficult to estimate, becomes ambiguous. Thus, even in... 

The GVB and SC methods provide wave functions that are, of course, much more compact than the corresponding valence—CASSCF one (e.g., only 14 spin-coupling modes for methane with the SC method, and a single one with the GVB method). Owing to this difference in size, the GVB and SC methods cannot be expected to include the totality of the nondynamical correlation, even if these two methods treat well, by definition, the left—right correlation for each bond of the molecule. Physically, this is because the various local ionic... 

The ionic substitutions are again governed by definite criteria known as Hume-Rothery rales. Size of the atoms is the most important factor in these rales. Substitution of one atom by another in a crystal structure is most likely when their ionic radii are within 15% it is less likely when sizes differ by 15-30%, and unlikely beyond that range. Note that these substitutions must also maintain overall charge balance, because the crystal structure must be neutral. 

Unfortunately, the calculated values of yt cannot be confirmed by direct experiment, because in principle all experimental methods yield the mean activity coefficient y rather than the individual ionic values. By use of the definition given in (2-16), the experimentally determined value can be apportioned to give nd y. This procedure is theoretically justified only at high dilution, where the DHLL is valid because the limiting slope of log y plotted against /n is found experimentally to be O.SZ Zb, as required by (2-17). At higher values of n the ion-size parameter a must be introduced. 

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