Formal Charge Definition Formal charge is the net electrostatic charge that would reside on an atom if all electrons in covalent bonds were equally shared. It is easily determined and offers a useful approximation to the actual charge distribution in a molecule. Attributes A formal charge is assigned to an atom. The magnitude is always an integral number of electron charges, e.g. 0, 1, 2 etc. The sign is positive(+) when the atom is electron deficient or negative(-) when the atom is electron rich. Calculation of formal charge 1. Assume bond pairs are shared equally by the atoms they join. 2. Assume unshared (lone) pair electrons reside solely on the atom to which they are assigned. 3. F. C. = Valence electrons in free atom - Valence electrons assigned to atom in a structural formula. This gives the formula F. C. = Periodic group number - u.p. electrons - 1/2( b.p. electrons) Formal charges in trimethylamine oxide Trimethylamine oxide provides an example. The structural formula below is incomplete because it lacks formal charges. Formal charge on nitrogen: The nitrogen atom has no unshared pair (u.p.) electrons and 8 bond pair (b.p.) electrons. F.C. = 5 - 0 - 1/2(8) = +1 Formal charge on oxygen: The oxygen atom has 6 unshared pair (u.p.) electrons and 2 bond pair (b.p.) electrons. F.C. = 6 - 6 - 1/2(2) = -1 Formal charge on carbon: Each of the 3 equivalent carbon atoms has no unshared pair (u.p.) electrons and 8 bond pair (b.p.) electrons in 3 C-H bonds and 1 C-N bond. F.C. = 4 - 0 - 1/2(8) = 0 You should verify that the formal charge on the hydrogens is zero. The complete structural formula with formal charges is Formal charges in common structures You will see common structures countless times as you learn organic chemistry. Thus, it pays to learn their formal charges, instead of calculating the charge each time you encounter them. Joshua McKenna Formal Charge
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