Identifying Formal Charges

A summary of commonly encountered formal charges and the bonding situations in which they occur is given in Table 2.2. Although only a bookkeeping device, formal charges often give clues about chemical reactivity, so it s helpful to be able to identify and calculate them correctly. 

Octet expansion (expansion of the valence shell to more than eight electrons) can occur in elements of Period 3 and later periods. These elements can exhibit variable covalence and be hypervalent. Formal charge helps to identify the dominant resonance structure. 

Determine the formal charge on each atom in the following molecules. Identify the structure of lower energy in each pair. 

Consider the bonding in CH2=CHCHO. (a) Draw the most important Lewis structure. Include all nonzero formal charges, (b) Identify the composition of the bonds and the hybridization of each lone pair—for example, by writing o(H ls,C2s/ 2). 

Which atoms in proUmated mcthylamine iCHs NH ) and protonated methyl alcohol iCHjOH . ) carry A formal charge Uae SpartanView to examine the electrostatic potential map fur each ion, and identify the most positive atom in each. Do the for-m l charge asrtgnnneiiu agree with the e lectriwtatk potential maps ... 

In perovskites of the type AA BB Oe, ordering of the B-site ions may occur if the differences between ionic size and formal charge is sufficiently large. The crystal chemistry of these complex perovskites has been reviewed recently. More than 300 materials can be identified as double perovskites so only a few compounds will be described here. Attention... 

Oxidation numbers must not be confused with the formal charges on Lewis dot diagrams (see Section 3.8). They resemble formal charges to the extent that both are assigned, by arbitrary conventions, to symbols in formulas for specific purposes. The purposes differ, however. Formal charges are used solely to identify preferred Lewis diagrams. Oxidation nnmbers are nsed in nomenclatnre, in... 

Assign formal charges and identify resonance diagrams for a given Lewis diagram (Section 3.8, Problems 51-58). 

Write down the Lewis structure of the reactants, complete with formal charges, and draw any major resonance forms. Look for leaving groups, polarized single and multiple bonds, acids and bases. Classify into generic sources and sinks and then rank them. The reaction usually occurs between the best source and sink. Above all, note if the medium is acidic or basic. In basic media, find the best base, and then locate any acidic hydrogen within range (not more than 10 p Ta units above the pATabH of Ihe base). In acidic media, identify the best sites for protonation. Likewise, do not create a species that is more than 10 units more acidic than your acid. Understand what bonds have been made or broken, but do not lock into an arbitrary order as to which occurred first. 

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