2.6: Resonance and Formal Charge

Learning Targets/ Content Objectives

SAP-4.B Represent a molecule with a Lewis diagram that accounts for resonance between equivalent structures or that uses formal charge to select between nonequivalent structures.

Essential Knowledge

SAP-4.B.1 In cases where more than one equivalent Lewis structure can be constructed, resonance must be included as a refinement to the Lewis structure. In many such cases, this refinement is needed to provide qualitatively accurate predictions of molecular structure and properties.
SAP-4.B.2 The octet rule and formal charge can be used as criteria for determining which of several possible valid Lewis diagrams provides the best model for predicting molecular structure and properties.
SAP-4.B.3 As with any model, there are limitations to the use of the Lewis structure model, particularly in cases with an odd number of valence electrons.

Important Equation(s)

Formal charge= # valence electrons - (# lone electrons + # bonds connected)

(the above equation should be memorized, it is not on the equation sheet)

Formal Charge

When drawing Lewis Dot diagrams for some molecules and polyatomic ions, there can be several structures drawn. Formal Charge is a method that can help determine which structure is most valid.

TO CALCULATE THE FORMAL CHARGE FOR EACH ATOM IN THE MOLECULE:

Determine the number of valence electrons of the atom.
Subtract the sum of the lone electrons and bonds connected to the atom from the valence electrons.

TO CHECK YOUR FORMAL CHARGE CALCULATION:

The sum of the formal charges of each atom should add up to zero for a molecule or add up to the charge for a polyatomic ion.

The most valid structure will have:

Formal Charges as close to zero for all atoms. (Most preferable is “no formal charge.”)
Negative Formal Charges will be placed on the most electronegative atom.

RESONANCE STRUCTURES

Sometimes when drawing a Lewis Structure you might find that there is more than one arrangement of bond/electrons that are equally valid; you might find that a double (or triple) bond could be placed in one of two or more places in a molecule or ion. Look at the NO₂− ion shown; both images are valid so both must be included when drawing a Lewis structure.

Resonance structures are present simultaneously; they do not flip between them. The electrons can be described as delocalized. The actual structure is an average of the resonance structures. As result, the bond lengths are an average of the bonds present. In the NO₂− ion the bond length is close to the average between a single and a double bond, a 1.5 bond.

To Represent Resonance: Draw all the structures of the molecule, keeping the arrangement of the atoms the same but changing the location of the electrons. Double arrows are drawn between the structures indicating that the structures are superimposed to make a hybrid.

Drawing Resonance Structures Step by Step: See examples

Check Your Understanding

Calculate the formal charge for each atom in the Hydrogen Cyanide compound, HCN.

We use the equation for formal charge= Valence electrons - (# of lone electrons + # of bonds)

for Hydrogen, 1-(0+1)= 0
for Carbon, 4-(0+4)= 0
for Nitrogen, 5-(3+2)=0

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