Why is formal charge used?

1 Answer
May 23, 2016

Answer:

Because it gives us an idea of electronic structure.

Explanation:

Of course, formal charge is a formalism. That is it does not have any real existence, but the concept can be useful to understand structure and bonding. We are introduced very early on to the idea that #"covalent bonding"# results from the sharing of electrons, and #"ionic bonding"# from the transfer of electrons. Thus, the neutral molecule methane, #CH_4#, has no charge separation, and the ionic species #NaCl#, can be represented as #Na^(+)Cl^(-)#.

To keep methane as an example, the methane molecule has #10# electrons in total: #6# from #C#, and #4# from #H#. For carbon, 2 of its electrons are inner core, and are not conceived to participate in bonding. The remaining #4# carbon electrons are conceived to lie in the #4xxC-H# bonds; the other #4# electrons derive from the hydrogen atoms. These 10 negative charges (the electrons) are balanced by the 10 positive nuclear charges present in the carbon and hydrogen nuclei, and methane is thus a neutral molecule.

Now consider methyl lithium, which is a real molecule, and might be represented as #{H_3C^(delta-)Li^(delta+)}_4#; an equally valid representation of the molecule is as #H_3C^(-)Li^+#, the which is an ionic representation. The carbon atom is FORMALLY associated with 7 electrons, and thus this bears a formal negative charge. Such a formalism helps us to rationalize the reactivity of methyl lithium, both as a base, and as a powerful nucleophile.