The Use of Formal Charges

A provisional Lewis structure may contain the correct bonding framework, but the distribution of the valence electrons may not be the one that gives the maximum stability. The correct stereochemistry is predicted by the valence shell configuration using VSEPR theory, as shown in Chapter 6. A concept called formal charge (FC) can be used to predict which structure of a number of alternative structures is the most reasonable for a Lewis structure. The formal charge (FC) on any atom in a Lewis structure can be defined as  

Half the electrons in a bond are assigned to each atom in the bond. 

Both electrons of an unshared pair are assigned to an atom. 

A few simple examples demonstrate the application of the calculation of FC for each of the separate atoms  

These examples illustrate the application of FC to simple examples which can then be extended to double and tripe bonds  

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Formal charges can be used to help in the assessment of resonance structures and molecular topology. The use of formal charges is presented here as a simplified method of describing structures, just as the Bohr atom is a simple method of describing electronic configurations in atoms. Both of these methods are incomplete and newer approaches are more accurate, but they can be useful as long as their limitations are kept in mind. 

Example 3.4 illustrates the use of formal charges to choose the correct Lewis structure for formaldehyde. 

Describe the strategy for writing Lewis structures, and the use of formal charges for determining the plausibility of a given Lewis structure. 

Plus (+) and minus (-) signs are often used to indicate the presence of formal charges on atoms in molecules. Assigning formal charges to specific atoms is a bookkeeping technique that makes it possible to keep track of the valence electrons around an atom and offers some clues about chemical reactivity. 

The concept of formal charge has a much wider applicability than this short discussion might imply. In particular, it can be used to predict situations in which conventional Lewis structures, written in accordance with the octet rule, may be incorrect (Table 7.2). 

The concept of formal charges is a very useful one that is essentially a way of keeping track of electrons. In order to determine the formal charge on each atom in a structure, we must first apportion the electrons among the atoms. This is done according to the following procedure ... 

The distribution of formal charges in the carbon skeleton is determined by the functional groups (or heteroatoms) present on it. In this context is very useful to use the "Lapworth model" of alternating polarities. 

Many of the tertiary bonds reported by Preiser et al. (1999) are likely artefacts of their calculations since these were based on the use of formal ionic charges. Substituting a more physically reasonable value for the formal ionic charge will reduce the total flux starting at the cation and eliminate many of the tertiary bonds around the highly charged cations where most tertiary bonds were found. However, there are some cases where tertiary bonds undoubtedly do occur and these can provide important information about the crystal chemistry. 

The final method of determining charge, and the one most useful to us, is the method of formal charges. 

The two resonance structures shown for SO2 contribute equally to the actual structure, as did the three resonance structures shown for C032-. However, resonance structures do not always contribute equally, and it necessary to have some way of estimating their relative contributions. One of the most useful tools for this is provided by the concept of formal charges. 

In this way the concept of formal charge may be used to distinguish the most stable structure of alternative Lewis structures of simple inorganic molecules, cations, and anions, including oxyacids and oxyanions. 

Let us illustrate the concept of formal charge using the ozone molecule (O3). Proceeding by steps, as we did in Examples 9.3 and 9.4, we draw the skeletal structure of O3 and then add bonds and electrons to satisfy the octet rule for the two end atoms ... 

In the Lewis structure we now have a useful tool that can predict internal atomic arrangement and positions of lone pairs inside molecules and composite ions. Furthermore we can have valuable information about the bond orders (whether we are dealing with single, double or triple bonds) and we can get information about eventual resonance structures. The concept of formal charge could also be useful in judging which of more possible Lewis structures are the most realistic. 

The sulfate ion can be represented with four S—O bonds or with two S—O and two S=0 bonds, (a) Which representation is better from the standpoint of formal charges (b) What is the shape of the sulfate ion, and what hybrid orbitals of S are postulated for the cr bonding (c) In view of the answer to part (b), what orbitals of S must be used for the tt bonds What orbitals of O (d) Draw a diagram to show how one atomic orbital from S and one from O overlap to form a tt bond. 

Second, there are significant sources of errors in both the force field and continuum dielectric model used to approximate solvation effects. As an example, polarization effects are neglected in force fields based on fixed partial charges. A recent extension of the LIECE model has emphasized the importance of quantum mechanics to capture these effects, in particular for sets of inhibitors vith significant differences in the number of formal charges [114]. 

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