Partial ionicity

Chemisorption occurs when the attractive potential well is large so that upon adsorption a strong chemical bond to a surface is fonued. Chemisorption involves changes to both the molecule and surface electronic states. For example, when oxygen adsorbs onto a metal surface, a partially ionic bond is created as charge transfers from the substrate to the oxygen atom. Other chemisorbed species interact in a more covalent maimer by sharing electrons, but this still involves perturbations to the electronic system. 

Since both Si—O and Si—CHj bonds are thermally stable it is predictable that the polydimethylsiloxanes (dimethylsilicones) will have good thermal stability and this is found to be the case. On the other hand since the Si—O bond is partially ionic (51%) it is relatively easily broken by concentrated acids and alkalis at room temperature. 

Chemists refer to the bond in a molecule like sodium chloride as ionic , meaning that its electron pair resides entirely on chlorine. At the other extreme is the covalent bond in the hydrogen molecule, where the electron pair is shared equally between the two hydrogens. Intermediate cases, such as the bond in hydrogen fluoride which is clearly polarized toward fluorine, are generally referred to as polar covalent bonds (rather than partially ionic bonds). Are these situations really all different or do they instead represent different degrees of the same thing ... 

Cova lent bond with. partial ionic character... 

The simplest explanation for the hydrogen bond is based upon the polar nature of F—H, O—H, and N—H bonds. In a molecule such as H20, the electron pair in the O—H bond is displaced toward the oxygen nucleus and away from the hydrogen nucleus. This partial ionic character of the O—H bond lends to the hydrogen atom some positive character, permitting electrons from another atom to approach closely to the proton even though the proton is already bonded. A second, weaker link is formed. 

A more general and fundamental view is obtained by a consideration of (a) the number of electrons involved in the partial ionic equation representing the reaction, and (b) the change in the oxidation number of a significant element in the oxidant or reductant. Both methods will be considered in some detail. 

We can now apply our knowledge of partial ionic equations to the subject of equivalents. The standard oxidation-reduction process is H H+ + e, where e represents an electron per atom, or the Avogadro number of electrons per mole. If we know the change in the number of electrons per ion in any oxidation-reduction reaction, the equivalent may be calculated. The equivalent of an oxidant or a reductant is the mole divided by the number of electrons which 1 mole of the substance gains or loses in the reaction, e.g. ... 

For convenience of reference the partial ionic equations for a number of oxidising and reducing agents are collected in Table 17A.1... 

Palladium, D. of as dimethylglyoximate. (g) 463 as nioximate, (g) 474 by EDTA, (ti) 329 Paper chromatography 229 see Thin layer chromatography Parallax errors due to, 85 Parallel determinations 132 Partial ionic equations 850, (T) 851 Partition chromatography 13. 217 Partition coefficient 162 Patton and Reeder s indicator 317, 328 Peptisation 419. 421... 

It has been pointed out4 that bonds between non-identical atoms may be considered to resonate between a covalent and an ionic structure, the bond in this way having partially covalent and partially ionic character. The resonance energy of this effect, which is usually essentially the same for a given bond in different molecules, is included in the values given for the bond energies in the nonresonating molecules discussed. 

HN3, with the structure H N N N making the largest contribution to the normal state, cannot be discussed until methods are developed for evaluating the energies of partially ionic bonds. 

In the foregoing treatment the assumption of additivity of interatomic distances in the compounds under discussion has been tacitly made. Examination of Table IV shows that this assumption is approximately substantiated by experiment. The agreement between the observed distances and the calculated radius sums is excellent in most cases. Aside from those just discussed, the exceptional crystals are AIN and SiC with observed interatomic distances slightly smaller than the radius sums. It seems doubtful that these deviations are to be attributed to a partially ionic character of the bonds, and the number of other factors which might conceivably be operative is so large that no single one can be selected with confidence as responsible. 

A number of years ago an equation was proposed13 for calculating the partial ionic character of a a bond between two atoms A and B from their electronegativity difference sa — b... 

Partial Ionic Character op Bonds in Relation to Electronegativity Difference... 

The charges on the oxygen atoms due to partial ionic character of the bonds to the metal atoms in the silicates and other salts should be taken into consideration in making this calculation. These charges lead to further decreases in the Si-O, P-O, S-O, and Cl-0 distances, of amount depending on the nature of the metal and the structure of the crystals. Because of uncertainties in the system of equations used in this paper, this refinement in the calculation has not been carried out. 

Ionic Bonding, Partial Ionic Character, and Electronegativity... 

The discussion of interatomic distances is less simple for intermetallic compounds than for pure metals among the complicating factors are the partial ionic character of bonds, the transfer of electrons, with consequent changes in valency, and the preferential use of the valencies of an atom in the formation of strong bonds rather than weaker ones. These factors, which of course participate in minimizing the energy of the system, usually operate to decrease the interatomic distances. Then-effects may be illustrated by some examples. 

In the above discussion the effect of difference in electronegativity of unlike atoms on bond length (usually a decrease) has been ignored. There is the possibility also of a small change in bond length between unlike atoms, such as of a metal and a metalloid, that reflects the difference in the nature of the overlapping orbitals, in addition to the effects of partial ionic character and of electron transfer. I believe that a thorough... 

For sphalerite and wurtzite, for example, the discussion of partial ionic character as described above for molyde-nite leads to the resultant average charges +0.67 for sulfur and—0.67 for zinc. The distribution of the sulfur atoms is calculated to be 12% S2 (quadricovalent), 50 percent S+, 32 percent S°, 6 percent S-, 0.2% S2-. The observed bond length 2.34 A with the sulfur radius 1.03 A and the Schomaker-Stevenson correction 0.05 A leads to 5 = 1.36 A for zinc (quadricovalent Zn2-). The increase by 0.05 A over the value 1.309 A for sp3 bonds of Zn° is reasonable as the result of screening of the nucleus by the extra electrons. 

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