Formal Charge Evaluation and Resonance Hybrids

All flashcards

Formal charge

A bookkeeping charge assigned to an atom by dividing bonding electrons equally; it is not necessarily the atom’s actual charge.

Formal-charge formula

$FC=V-N-\frac{B}{2}$, where $V$ is neutral-atom valence electrons, $N$ is nonbonding electrons, and $B$ is bonding electrons.

Formal-charge sum

The sum of atomic formal charges must equal the total charge of the molecule or ion.

Resonance forms

Lewis structures with the same atom arrangement but different placements of electrons.

Resonance hybrid

The actual electronic structure represented by the combined contribution of valid resonance forms.

Quiz questions

1. What does formal charge represent in a Lewis structure?Answer: A bookkeeping charge calculated by dividing bonding electrons equally between bonded atoms.
2. A neutral atom has six valence electrons, two lone pairs, and two bonds. What is its formal charge?Answer: Zero, because $6-4-2=0$.
3. Two otherwise similar Lewis structures differ in whether negative formal charge is on carbon or oxygen. Which placement is generally preferred?Answer: On oxygen, because it is more electronegative than carbon.
4. Which description identifies resonance forms of a molecule or ion?Answer: Structures with the same atom arrangement but different electron placements.
5. What does the resonance hybrid represent?Answer: The actual electronic structure represented by the combined resonance forms.