formalcharge - Formal Charges Additional reading...

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Formal Charges Additional reading recommendation : You may find Chapter 1 of Pushing Electrons (Weeks) to be a useful tool for mastering the fundamentals of Lewis structure, including assignment of formal charges. Discussion : Ions bear a positive or negative charge. If the ion is polyatomic (is constructed of more than on atom), we might ask which atom(s) of the ion carry the charge? Knowledge of charge distribution (identification of atoms that are electron rich or electron poor) can be useful to interpret many facets of organic chemistry, including how and why reactions occur (mechanisms) or how molecules interact with each other, a feature which strongly influences physical and biological properties. Valence electrons do not "belong" to any one atom in a molecule or ion. Quantum mechanics tells us that electrons are shared by a few neighboring atoms, or even by the whole molecule. Because atoms differ in electronegativity and hybridization, it is inaccurate to assume that these electrons are shared equally. (The rare exception is a pure covalent bond between two identical atoms such as in molecular chlorine, Cl-Cl.) Because of this uneven sharing atoms have fractional, rather than integer, charges. Calculations to determine exact electron distribution and atomic charges require complex computer programs and often many hours of computer time. Chemists need a way to rapidly estimate the electron rich and electron poor sites of a molecule or ion to allow a rough estimate of chemical and physical properties. Chemists have developed a very simple bookkeeping method to determine if an atom within a molecule or ion is neutral, or bears a positive or negative charge. The method provides integer charges only. Because this method provides some indication of charge distribution, it is an excellent starting point for determining electron distribution within a molecule or ion, and hence give us a starting point to predict chemical and physical properties. These assigned integer charges are called formal charges. A formal charge is a comparison of electrons "owned" by an atom in a Lewis structure versus the number of electrons possessed by the same atom in its unbound, free atomic state. Procedure
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This note was uploaded on 10/15/2010 for the course CHEM 213 taught by Professor Johnson during the Spring '10 term at Lyndon State.

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formalcharge - Formal Charges Additional reading...

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