Representing MoleculesResonance

Exceptions to the Octet RuleFormal Charge

Bond Length•The average distance between two bonded atoms•The distance between nuclei at their minimum potential energy

Bond Dissociation Energy• Also called “bond energy”

• The energy required to break a bond and form neutral isolated atoms

Trends in Bond Energy• C-C vs. C=C vs. C≡C

– Higher bond energy for multiple bonds compared to single bonds

• Triple bonds are shorter on average than single bonds– There can be a lot of variation in bond

length, depending on other bonds for those atoms

Bond Energy & Bond Length• Higher bond dissociation energy is

linked to lower chemical reactivity

• Longer bond lengths lower bond dissociation energies– “Short bonds are strong bonds”

Ozone, O3

•Draw a Lewis structure for ozone (the oxygen atoms are in a line)

Both structures are equally correct!

Resonance Structures• Sometimes, more than one correct

Lewis structure can be drawn for a compound– A single Lewis structure may be

inadequate for describing the situation– “resonance” structures

• Same arrangement of atoms, same numbers of single and multiple bonds

• Link the multiple structures with a double headed arrow

Resonance• The actual structure is a “blend”,

somewhere in between the Lewis structures

• Alternating single/double bonds would have different bond lengths– In the real molecule, all the bond lengths

are identical!

Problem• A problem to try:

– Draw resonance structures for the nitrate ion, NO3

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Exceptions to the Octet Rule• Elements in row 3 or below on the

periodic table can have expanded octets– i.e., they can have more than 8

electrons, because they may use d orbitals in bonding

• B, Be may be “electron deficient”– Fewer than 8 electrons around central

atom

Nonequivalent Lewis Structures

• If a molecule or polyatomic ion exceeds the octet rule, or if different skeletal arrangements (i.e. isomers) are possible, non-equivalent Lewis structures may be drawn.

• Million dollar question: Which Lewis structure best describes the actual bonding situation?– Use “formal charge” to evaluate

Formal charge• FC = # valence electrons – (# of lone

pair electrons) – (# bonds)– Calculated for each atom– Sum of formal charges must equal

overall charge of species

• The most appropriate Lewis structures have:– Lowest formal charges (zero is best)– Negative formal charge on the most

electronegative element

Calculate formal charges• Which is the most appropriate

structure for the molecule CCl2O?

Problem• Below are two different Lewis

structures for nitrous acid (HNO2).

H-O-N=O: H-N=O

:O:

• Which is the better Lewis structure based only on formal charge arguments?

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