Question

formal charge of :h=c=h:

Answer 1

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: 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. Be sure to show all nonbonded electrons, as these influence formal charges.
The best Lewis structure for the hydronium ion is shown below. The brackets indicate the
positive charge belongs to the entire molecule.
Step 2. Assign the formal charge to each atom. Formal charge is calculated using this
formula:
H O H
H