Formal charges
Calculating
the formal charge on an atom in a molecule is a fairly simple
task. After some time, calculating formal charges should
become second nature and you should be able to assign formal
charges on atoms very rapidly. To calculate the formal charge
on an atom, you use the following equation:
formal
charge = group number  nonbonding electrons (
0.5 x shared electrons)

EXAMPLE::
Assign a formal charge to the nitrogen
in the compound shown below
To calculate
the formal charge on nitrogen there are three pieces of information
that you need to know: the group number (number of valence
electrons), the number of nonbonding electrons, and the number
of shared electrons.

Nitrogen is in group five; thus, it has five valence electrons

Number of nonbonding electrons: there are no nonbonding
electrons, so this is zero

Number of shared electrons: there are four bonds, and
there are two electrons in each bond, so this number is
eight
formal
charge = 5  0  0.5(8) = +1
Thus, the nitrogen has a formal charge of +1
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