What you'll be able to do: Rank competing Lewis structures and justify which contributes most to the resonance hybrid.
Best after: Formal Charge: Definition and Calculation, Resonance Structures
Several structures can be legal at once. Formal charge is the tie breaker chemists actually use.
These are recommended, not required. You can start this lesson at any time.
Apply them in order. First, prefer the structure with formal charges closest to zero overall. Second, prefer negative formal charge on the more electronegative atom and positive on the less electronegative. Third, avoid placing like charges on adjacent atoms, and avoid charges larger than one unit when a reasonable alternative exists.
When contributors are not equivalent, the better ranked structure contributes more to the hybrid. For the thiocyanate ion SCN⁻, the structure with the negative charge on the more electronegative nitrogen contributes more than the one with it on sulfur, and the structure with a +1 on nitrogen contributes least of all.
A complete answer names the structure, gives the formal charges, and states the rule used. For example: this structure is preferred because all formal charges are zero, while the alternative places +1 on oxygen, the most electronegative atom present.
Minimise the magnitude of formal charges.
Put negative formal charge on the more electronegative atom.
Avoid adjacent like charges and charges beyond one unit where possible.
The contributor with -1 on nitrogen outranks the one with -1 on sulfur.
FC = V - N - B/2
Two structures for N₂O with connectivity N-N-O are possible: a triple bonded N to N with a single N-O, or a double bond on both sides. Which contributes more?
Try it first: Compute formal charges on all three atoms in each structure.
0 of 4 steps revealed.
For CO₂, compare O=C=O with a structure holding one C-O single bond and one C triple bond O.
Try it first: Compute the charges for both.
0 of 3 steps revealed.
Why it's wrong: There are no available orbitals, so the structure is invalid however good the charges look.
Check instead: Keep C, N, O and F at eight electrons.
Why it's wrong: Only equivalent structures contribute equally; unequal ones are weighted by the ranking rules.
Check instead:
Why it's wrong: The charges must sum to the ionic charge, so some must be nonzero.
Check instead: Aim for the best placement rather than for zeros.
No practice questions are available for this topic yet. You can still practice the whole unit.
The preferred Lewis structure is the one with formal charges closest to zero, with any negative formal charge on the most electronegative atom, and with like charges kept apart. These rules also rank unequal resonance contributors, deciding which one dominates the hybrid.
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