Electron accountancy

The nonlocalized or mobile electrons account for the many unique features of metals. Since metallic bonds do not have strong directional character, it is not surprising that many metals can be easily deformed without shattering their crystal structure. Under the influence of a stress, one... 

It is now known that the view of electrons in individual well-defined quantum states represents an approximation. The new quantum mechanics formulated in 1926 shows unambiguously that this model is strictly incorrect. The field of chemistry continues to adhere to the model, however. Pauli s scheme and the view that each electron is in a stationary state are the basis of the current approach to chemistry teaching and the electronic account of the periodic table. The fact that Pauli unwittingly contributed to the retention of the orbital model, albeit in modified form, is somewhat paradoxical in view of his frequent criticism of the older Bohr orbits model. For example Pauli writes,... 

When coupled with a suitable matching half-equation, the electron accountancy will remove the inadmissible term. For example ... 

To indicate resonance forms, we use a doubleheaded arrow between the contributing structures. This arrow is reserved for resonance structures and never used elsewhere. The difference between the two structures is that the electrons in the n bonds have been redistributed, and we can illustrate this by use of another type of arrow, a curly arrow. This arrow is used throughout chemistry to represent the movement of two electrons. In the benzene case, a cyclic movement of electrons accounts for the apparent relocation of double bonds, though there are two ways we might show this process both are equally satisfactory. 

The Na+ ions are surrounded by a pool of delocalized electrons, which acts as the glue that holds the metallic atoms together. This arrangement is referred to as the metallic bond. This mobile pool of electrons accounts for the characteristic properties of metals. For example, because the electrons are loosely attached, rigid bonds are not formed and atoms can easily be shaped because the electrons move freely throughout the structure. 

The systems of valent states and oxidation states introduced by chemists are not merely electron accounting systems. They are the systems which allow us to understand and predict which ratios of elements will form compounds and also suggests what are the likely structures and properties for these compounds (3). In the case of highly covalent compounds, the actual occupancy of the parent orbitals may seem to be very different than that implied from oxidation states if ionicity were high. Nonetheless, even some physicists have recognized the fundamental validity and usefulness of the chemist s oxidation state approach where the orbitals may now be described as symmetry or Wannier orbitals (6). 

Electron Theory of Matter. An atom is pictured as a nucleus around which are arranged electrons, or negative charges, equal in number to the net charge of nucleus. The nuclear charge and the number of external (outer) electrons vary from element to element, increasing by one for every increase in the atomic number of the elements. The total number of external electrons is thus identical with the atomic number of the element. These electrons account for valency and some other properties of elements. They can be liberated... 

Formulating credible mechanisms, i.e. accounting for all the electron movements and bonding changes in an acid- or base-catalysed reaction, is a difficult task although the framework on which the final mechanism is based is relatively easy to write. For this reason, and in the absence of evidence suggesting otherwise, a mechanism is conventionally illustrated as a series of steps as this enables the electron accounting to be most easily determined. This is illustrated by the mechanism for acid-catalysed acetal hydrolysis (Scheme 11.1a) shown... 

Akeypart in the analysis and proposal was electron accountancy - on the basis of the usual propensity for the adoption of a noble gas outer shell configuration - and all of the proposed intermediates have 16- or 18-electron configurations except the metallocyclobutane (26) in pathway II which has 14. On this basis, one might expect that pathway I, the non-dissociative pathway, would predominate over pathway II. In the presence of a large excess of Cy3P, which was used only to simplify the kinetic analysis, this is clearly the case. However, under the... 

There is an alternative pathway to II, in which the phosphine dissociates before the alkene group coordinates pathway III. On the basis of electron accountancy alone, this should be viewed as unfavourable as it involves two 14-electron intermediates (26 and 27). However, it should be noted that the mechanism-derived rate equation for reaction via pathways I/III rather than I/II would be equally consistent with the empirical rate equation. 

An advantage of VSEPR is its foundation upon Lewis electron-pair bond theory. No mention need be made of orbitals and overlap. If you can write down a Lewis structure for the molecule or polyatomic ion in question, with all valence electrons accounted for in bonding or nonbonding pairs, there should be no difficulty in arriving at the VSEPR prediction of its likely shape. Even when there may be some ambiguity as to the most appropriate Lewis structure, the VSEPR approach leads to the same result. For example, the molecule HIO, could be rendered, in terms of Lewis theory as ... 

What good is the r-value It approximates how closely the calculated activity values match the experimental values. As has already been stated, r2 describes the fraction of variability in the experimental data that is captured by the calculated data. Based on Equation 12.20, electronics account for about 50% of the variance. Based on Equation 12.21, steric effects account for over 80% of the variance. That gives a total of 130%, which is impossible. Clearly something is breaking down in these analyses, and, unfortunately, this type of inconsistency is common in QSAR analysis. 

Electron delocalization is most extreme in solid and liquid metals. Here valence electrons are spread over not two, four, or six atoms, but over the entire crystal network. The extreme mobility of electrons accounts for their typically high electrical and thermal conductivity. Non-loealized electrons carry an electric current more effectively than any other species, and, because of their slight weight, are particularly quick to respond to thermal disturbance. 

The complex should remind you of a bromonium ion, and rightly so, because its reactions are really rather similar. Even relatively feeble nucleophiles such as water and alcohols, when used as the solvent, open the mercurinium ion and give alcohols and ethers. In the next scheme, the mercury(II) is supplied as mercury(II) acetate, Hg(OAc)2, which we shall represent with two covalent Hg-0 bonds (simply because it helps with the arrows and with electron-accounting to do so). Unsurprisingly, water attacks at the more substituted end of the mercurinium ion. 

Let s address the question now as it equally applies to the 8- and 18-electron rules. Compounds that follow the rules are classified as normal and define the electronic accounting favored when no other factors are of overriding importance. The rules give the experimental chemist a simple method to rationalize and predict compound stoichiometry and connectivity. The rules permit logical categorization of the myriad of compounds via similarities in an electron count. Both facilitate more rapid development of a field such as cluster chemistry where both structure and composition can seem intimidating in the absence of an organizing principle. 

One disadvantage of all energy optimized basis sets is the fact that they primarily depend on the wave function in the region of the inner shell electrons. The 1 s-electrons account for a large part of the total energy, and minimizing the energy will tend to make the basis set optimum for the core electrons, and less than optimum for the valence electrons. However, chemistry is mainly dependent on the valence electrons ... 

Consists structurally of large sheets of fused benzene-like hexagonal units. A n-bonding network of delocalized electrons accounts for the high electrical conductivity of graphite. 

The a-bonding network of localized electrons accounts for the electrical insulating properties of diamond. 

It is at this point that we depart from the terminology used by Bockris and Reddy (Ref. 3, p. 1007) in their often-cited and generalized discussion of transfer coefficients [Eqs. (la) and (lb)] (i.e., and y ) and introduce the related terms y. and y p. The difference between these sets of electron-number parameters is that in the latter, an electron transferred in a step that occurs, say, v times (i.e., it has a stoichiometric number v greater than 1) is counted only once and not the v times it actually has to occur for one turnover of the overall reaction. This added complication of the electron accounting has the advantage of showing more clearly how stoichiometric coefficients and numbers enter into experimentally obtainable transfer coefficients and hence can demonstrate one of the links between mechanism and experiment. 

CuII(S2), Ba02 as BalJ(02) and OsQi as Os 04. In the last case, the real charge on Osv is less than one. Some refer to formal oxidation states as a chemist s simple-minded way of electron accounting. However, such a system of formal oxidation states is highly useful in predicting the number and character of unpaired electrons in a system. 

Without any doubt, the ability of unsaturated species to add reversibly two electrons, and the ability of saturated species to lose reversibly two electrons account for their chemical propensity to add/remove the two-electron donor CO ligand. 

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