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Part II
Interactions of Atoms: Formal Charge,
Resonance and VSEPR
Formal Charge (FC)
the hypothetical charge on an atom in a
molecule or polyatomic ion
not a real charge due to differences in proton
and electron numbers
assumes that electrons are shared equally
between atoms
ignores differences in electronegativity
the closer to zero, the better
Formal Charge (FC)
Assists in writing correct Lewis formulas
FC = (group number) - [(number of bonds)+(number of unshared e-)]
1. Calculation of Formal Charge for an atom
2. An atom in a Lewis formula that has the same number of
bonds as its periodic group number has a formal charge of zero
3. In a molecule, the sum of the formal charges is zero
4. In an ion, the sum of the formal charges is equal to the real
charge
Lewis Dot Formulas: Limitations of
the Octet Rule/Using Formal Charge
Limitation 4
: Covalent compounds or polyatomic ions
with an odd number of electrons
NO
2
Step 1: What is the reasonable symmetrical skeleton?
Step 2: What are N, A and S?
Step 3: Where do we place the shared electrons?
Step 4: What do we do with the rest of the electrons?

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