Placing electrons

Now that you ve counted how many electrons you have in the game, it s time to make your play. In the following steps, you make a Lewis structure for xenon oxytetrafluoride (XeOF ). 

Determine the total number of valence electrons. Xe is in Group 18 

With those 24 electrons placed, only 8 remain. Six go to oxygen to satisfy the octet rule, and the last two go to Xe. 

Place your bonds. Fluorine wants one bond with three lone pairs, whereas oxygen wants two bonds with two lone pairs. This step is explained in more detail in the upcoming section Price tags in black ties Formal charges.  

Here are some good general rules to follow when double-checking your final 

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Turning to the orbital part of we consider the electrons in two different atomic orbitals Xa and x.b as, for example, in the 1x 2/) configuration of helium. There are two ways of placing electrons 1 and 2 in these orbitals giving wave functions Z (1)Z6(2) and Z (2)Z6(1) but, once again, we have to use, instead, the linear combinations... 

The area of photoinduced electron transfer in LB films has been estabUshed (75). The abiUty to place electron donor and electron acceptor moieties in precise distances allowed the detailed studies of electron-transfer mechanism and provided experimental support for theories (76). This research has been driven by the goal of understanding the elemental processes of photosynthesis. Electron transfer is, however, an elementary process in appHcations such as photoconductivity (77—79), molecular rectification (79—84), etc. 

In Fig. 9.1, orbitals below the dashed reference line are bonding orbitals when they are filled, the molecule is stabilized. The orbitals that fall on the reference line are nonbonding placing electrons in these orbitals has no effect on the total bonding energy of the molecule. The orbitals above the reference line are antibonding the presence of electrons in these orbitals destabilizes the molecule. The dramatic difference in properties of cyclobutadiene (extremely unstable) and benzene (very stable) is explicable in terms of... 

With these two assumptions, we can propose the electronic arrangement of lowest energy for each atom. We do so by mentally placing electrons successively in the empty orbitals of lowest energy. The electron orbital of lowest energy is the Is orbital. The single electron of the hydrogen atom can occupy this orbital. In the helium... 

Remembering how we placed electrons in the lowest empty orbitals, two per orbital, we can now generalize concerning the number of valence... 

Remember the spatial arrangement of the p or- atom has partially filled valence orbitals. Elec-bitals Each one protrudes along one of the tron sharing can occur, placing electrons close three cartesian axes (as shown in Figure 15-9). to two nuclei simultaneously. Hence a stable If the electrons have the orbital occupancy of bond can occur. This is shown in representations 20), then two electrons occupy the p orbital (22) and (23). 

If we were to place a piece of zinc metal into an aqueous copper(II) sulfate solution, we would see a layer of metallic copper begin to deposit on the surface of the zinc (see Fig. K.5). If we could watch the reaction at the atomic level, we would see that, as the reaction takes place, electrons are transferred from the Zn atoms to adjacent Cu2 r ions in the solution. These electrons reduce the Cu2+ ions to Cu atoms, which stick to the surface of the zinc or form a finely divided solid deposit in the beaker. The piece of zinc slowly disappears as its atoms give up electrons and form colorless Zn2+ ions that drift off into the solution. The Gibbs free energy of the system decreases as electrons are transferred and the reaction approaches equilibrium. However, although energy is released as heat, no electrical work is done. 

The last electron could be placed in any of the 3 p orbitals, because these three orbitals are equal in energy. The final electron also could be given either spin orientation. By convention, we place electrons in unfilled orbitals starting with the left-hand side, with spins pointed up. 

The first four steps in our procedure lead to a provisional Lewis structure that contains the correct bonding framework and the correct number of valence electrons. Although the provisional stmcture is the correct structure in some cases, many other molecules require additional reasoning to reach the optimum Lewis structure. This is because the distribution of electrons in the provisional structure may not be the one that makes the molecule most stable. Step 3 of the procedure places electrons preferentially on outer atoms, ensuring that each outer atom has its full complement of electrons. However, this step does not always give the optimal configuration for the inner atoms. Step 5 of the procedure addresses this need. 

Let us consider the possible relations of LS and HS potential energy surfaces as shown schematically in Fig. 9. As long as the zero-order or diabatic surfaces are considered, the eleetrons remain localized on the particular spin state, no eleetron transfer being possible. In order that a conversion between the LS and HS state takes place, electronic coupling of the states is required. This coupling effectively removes the degeneracy at the interseetion of the zero-order surfaces... 

The following are rules that apply to drawing resonance structures. Remember that resonance relates to different ways of placing electrons in the structures, not ways of arranging the atoms themselves. 

To the extent that back donation occurs, it places electron density in the a orbital, which weakens the... 

Place electrons into the orbitals in order of increasing energy level. 

The nucleus is held together very tightiy—so tightly, in fact, that it takes a nuclear reaction to split one. But the electrons aren t held as tightiy, so it s easier to add an electron to an atom or take away an electron from an atom. And those electrons don t always like to stay in one place. Electrons have a negative charge and are attracted... 

The next topic of our consideration is the ion-radical incipiency. Generally, the mechanism of the ion-radical generation in frozen solution is as follows. Irradiation drives electrons out from a solvent. An organic precmsor (P) transforms into an ion-radical. At first glance, two reactions might be expected to take place electron capture (P -F e P ) and electron detachment (P + e P+ -F 2e). In fact, an indirect redox process takes place, with solvent participation. The example in Scheme 2.41 visualizes 2-methyltetrahydrofman (MeTHF) participation in the redox process, when P is a substance of electron affinity higher than that of the solvent. 

Electron-transfer processes play many very important roles in chemistry and biology. Because the present work is focused on electron-transfer events occurring within positively charged gas-phase peptides as they occur in ETD and ECD mass spectrometry experiments, it is not appropriate or feasible to review the myriad of other places electron-transfer reactions occur in chemistry. Chapter 10 of the graduate level textbook by Schatz and Ratner [12] gives a nice introduction to the main kinds of electron-transfer events that chemists usually study as well as to the theoretical underpinnings. They also give, at the end of Chapter 10, several literature references to selected seminal papers on these subjects. 

The different substituents have different electron donating/accepting properties and hence affect electron density and the acidity of the carboxyl group through which adsorption takes place. Electron accepting groups on the aromatic ring weaken the carboxylate-oxide surface bond. 

The use of doubly excited CSFs is thus seen as a mechanism by which VF can place electron pairs, which in the single-configuration picture occupy the same orbital, into... 

Lastly, one must occupy the MOs with the correct number of electrons. A neutral dicoordinated carbon atom has two valence electrons and a neutral uncoordinated oxygen atom has six, for a total of eight. Place electrons into the MOs two at a time. The HOMO is seen to be the higher nonbonding MO, no, and the LUMO is n 0. 

However, electronic factors can also have an effect. It was found that placing electron-withdrawing groups on the aryl substituents at silicon led to an increase in the proportion of A-alkene formed, as shown by the data in Table 14202. 

Thus the two key features to be identified in the target molecule are a cyclohexene ring and an appropriately placed electron-withdrawing substituent. It should be noted that both structural features may be constituent parts of a larger, more complex molecule. 

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