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Resonance

Resonance. AP Chemistry Bonding Notes 4. Some molecules are not adequately described by a singe Lewis Structure. Some structures with multiple bonds can have similar structures with multiple bonds between different pairs if atoms.

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Resonance

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  1. Resonance AP Chemistry Bonding Notes 4

  2. Some molecules are not adequately described by a singe Lewis Structure. • Some structures with multiple bonds can have similar structures with multiple bonds between different pairs if atoms. • For example Ozone experimentally has two identical bonds, whereas, the Lewis Structure requires one single ( longer) and one double bond ( shorter) Resonance Strucutres

  3. Resonance structures are attempts to represent real structure that is a mix of several extreme possibilities • Resonance structures are Lewis structures that differ only with respect to placement of the electrons • The “true” arrangement is a blend or hydrid of the resonance structures • Example in ozone is a hybrid with intermediate character. • Common example O3,NO3, SO3,NO2, and benzene Resonance Strucutres

  4. Benzene belongs to an important class of organic molecules called aromatic compounds. • Benzene C6H6 is a cyclic structure- six atoms of carbon in a ring structure with alternating double bonds • Experimentally the C-C bonds are the same length and it is a planar molecular • Benzene is often represented as a hexagon with a ring inside to represent the alternating single and double bonds. Benzene

  5. There are three classes of exceptions to the octet rule • Molecules with an odd number of electrons • Molecules in which one atom has less than an octet • Molecules in which one atom has more than an octet Exceptions to the octet rule

  6. The vast majority of molecules have an even number of electron, and complete pairing of electrons occur. • Generally, molecules such as ClO2, NO, and NO2 have odd numbers of electrons Odd numbers of Electrons

  7. Molecules with less tan an octet are also relatively rare • They are most often encountered in compounds of boron or beryllium • The most typical example is BF3 Less than an Octet

  8. This is the largest class of exceptions • Atoms from the third period onward can accommodate more than an octet • Examples • PCl5, SF4,AsF6- More than an Octet

  9. Elements from the third period and beyond have unfilled d orbitals that can be used to accommodate additional electrons • Size also plays a role • The larger the central atom, the larger the number of atoms that can surround it • The size of the surrounding atoms is important • Expanded octets occur often when the atoms bound to the central atom are the smallest and most electronegative atoms (F,Cl,O)

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