Resonance & Resonance & Formal ChargeFormal Charge

Chapter 8 part 4Chapter 8 part 4

                      

  

ResonanceResonance What if more than What if more than

one valid Lewis dot one valid Lewis dot structure is possible?structure is possible?

Consider Nitrate ion.Consider Nitrate ion. Nitrogen bound to 3 Nitrogen bound to 3

oxygen atoms, one oxygen atoms, one with a double bond.with a double bond.

But is that the only But is that the only possible structure possible structure that obeys the octet that obeys the octet rule?rule?

Possible structures for Possible structures for NONO33 --

Another illustrationAnother illustration The readers of The readers of

the AP do not the AP do not recognize this recognize this method of writing method of writing the resonance the resonance structure, but I structure, but I think it is think it is important to be important to be familiar with it.familiar with it.

More ExamplesMore Examples

Why the Resonance Why the Resonance model ?model ?

A word about bond A word about bond length:length:

Experimentally it has Experimentally it has been shown:been shown:

Single bonds are Single bonds are longer than double longer than double bonds bonds

Double bonds are Double bonds are longer than triple longer than triple bonds.bonds.

But…But…

The Resonance model The Resonance model explains this…explains this…

Since there is more Since there is more than one correct than one correct position for the double position for the double bond, the model for bond, the model for resonance allows that resonance allows that double bond to shift double bond to shift locations. The locations. The resonance bond is resonance bond is shorter than a single shorter than a single bond and larger than bond and larger than a double bond.a double bond.

QuestionQuestion Which of the Which of the

two resonance two resonance structures has structures has the shorter the shorter bond length bond length and …and …

Why?Why?

Odd Electron MoleculesOdd Electron Molecules Relatively few Relatively few

molecules formed molecules formed with nonmetals with nonmetals contain odd contain odd numbers of numbers of electrons.electrons.

These examples These examples usually involve N.usually involve N.

These are not very These are not very stable.stable.

Formal ChargeFormal Charge The difference between the number of The difference between the number of

valence electrons on the free atom valence electrons on the free atom and the number of valence electrons and the number of valence electrons assigned to the atom in the molecule.assigned to the atom in the molecule.

Need to determine the valence Need to determine the valence electron of the free atom.electron of the free atom.

Need to determine the number of Need to determine the number of valence electrons assigned to the valence electrons assigned to the atom in the molecule.atom in the molecule.

Formal chargeFormal charge Assigning electrons:Assigning electrons: Lone pairs and single electrons on Lone pairs and single electrons on

the atom each count for that atom.the atom each count for that atom. Bonded electrons are shared and Bonded electrons are shared and

therefore half go to each atom bound.therefore half go to each atom bound. Subtract this number form the Subtract this number form the

valence number.valence number. The result and its sign is the atom’s The result and its sign is the atom’s

formal charge.formal charge.

ExampleExample In The Sulfate ion, In The Sulfate ion,

what are the what are the possible Lewis dot possible Lewis dot structures?structures?

Which one is the Which one is the most likely?most likely?

Use formal charge.Use formal charge.

Final example:Final example: Give the possible Lewis dot Give the possible Lewis dot

structures for XeOstructures for XeO33. Which one is . Which one is the most likely given the formal the most likely given the formal charge?charge?

SummarySummary To calculate the formal charge of an To calculate the formal charge of an

atom:atom:1.1. Take the sum of the lone pair Take the sum of the lone pair

electrons and one half the shared electrons and one half the shared electrons of the atom in the electrons of the atom in the molecule.molecule.

2.2. Subtract the number of valence Subtract the number of valence electrons on the free neutral atom.electrons on the free neutral atom.

Summary continuedSummary continued The sum of the formal charge of all The sum of the formal charge of all

atoms in a given molecule or ion must atoms in a given molecule or ion must equal the overall charge of that species.equal the overall charge of that species.

If nonequivalent Lewis structures exist If nonequivalent Lewis structures exist for a species, those with formal charges for a species, those with formal charges closest to zero and with any negative closest to zero and with any negative formal charges on the most formal charges on the most electronegative atoms are considered to electronegative atoms are considered to best describe the bonding in the best describe the bonding in the molecule.molecule.