The brackets indicate that the positive charge belongs to the entire molecule.
Formal Charges
Additional reading recommendation: You may find Chapter 1 of Pushing Electrons : A Guide for Students of Organic Chemistry, 3/e by Daniel P. Weeks (Harcourt College Publishers; ISBN 0-03-0206936) 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. In order to make a rough estimate of chemical and physical properties, chemists need a way to rapidly estimate the electron-rich and electron-poor sites of a molecule or ion.
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 gives 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: The procedure to determine formal charges on the atoms of an ion or molecule has three steps. The process is illustrated using hydronium ion (H3O+), an ion very frequently encountered in organic and biochemical reaction mechanisms.
Step 1. Draw the best Lewis structure for the molecule, including all unpaired electrons. (Text Section 1.2) Be sure to show all unbonded electrons, as these influence formal charges.
The best Lewis structure for the hydronium ion is:
The brackets indicate that the positive charge belongs to the entire molecule.
Step 2. Assign the formal charge to each atom. Formal charge is calculated as follows:
FC = GN - UE -1/2BE Where:
The formal charge on hydrogen is calculated as follows. Hydrogen has one valence electron (GN = 1), no unshared electrons (UE = 0), and two shared electrons in the oxygen-hydrogen covalent bond (BE = 2). Thus, the calculated formal charge on hydrogen is zero. Because each hydrogen atom in this molecule is identical, each hydrogen atom has the same formal charge of zero. Any hydrogen bearing one covalent bond always has a formal charge of zero.
The formal charge on oxygen is calculated as follows. Oxygen has six valence electrons (GN = 6), two unshared electrons in one lone pair (UE = 2), and six shared electrons in three oxygen-hydrogen covalent bonds (BE = 6). Thus, the calculated formal charge on oxygen is +1. This indicates the oxygen atom bears the majority of the positive charge of this ion.
Step 3. Check your work. The sum of the formal charges of all atoms must equal the overall charge on the structure. For hydronium ion, the sum of the formal charges on the hydrogen atoms (3 x zero), plus one for the oxygen, gives a total charge of +1, which agrees with the overall charge.
After you practice this procedure with a few structures, you will begin to notice patterns in formal charge distribution. Becoming familiar with these patterns will help you avoid having the tedious task of calculating formal charge for every atom of every structure you encounter.
Formal Charge Patterns:
1. The best Lewis structure or resonance-contributing structure has the least number of atoms with formal charge.
2. Equivalent atoms have the same formal charge. For example, all the hydrogen atoms of methane (CH4) are equivalent and therefore have the same formal charge. All six hydrogens of ethane (H3C-CH3) have the same formal charge, as do the two carbon atoms.
3. Formal charges other than +1, 0, or -1 are uncommon except for metals.
4. The vast majority of organic structures are made up of a small set
of atoms with a limited number of bonding possibilities. Recognizing
these cases will allow you to avoid formal charge calculations most of
the time, and speed your understanding of how charge influences reactions
and properties of molecules. These patterns are summarized in the
table below.
| Element | FC = -1 | FC = 0 | FC = +1 |
| hydrogen |
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| carbon |
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| nitrogen |
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| oxygen |
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halogen
(X = F, Cl, Br, I) |
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Exercises: Calculate the formal charge on each atom in the following Lewis structures.
