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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 - non-bonding electrons -( 0.5 x shared electrons)


EXAMPLE:: Assign a formal charge to the nitrogen in the compound shown below

ammonium

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 non-bonding electrons, and the number of shared electrons.
  • Nitrogen is in group five; thus, it has five valence electrons
  • Number of non-bonding electrons: there are no non-bonding 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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