Resonance

Key points: ResonancebetweenLewis structures lowers the calculated energy of the molecule and distributes the bonding character of electrons over the molecule; Lewis structures with similar energies provide the greatest resonance stabilization. A single Lewis structure is often an inadequate description of the molecule. The Lewis structure of O₃ (4), for instance, suggests incorrectly that one O-O bond is different from the other, whereas in fact they have identical lengths (128 pm) intermediate between those of typical single O-O and double O=O bonds (148 pm and 121 pm, respectively). This deficiency of the Lewis description is overcome by introducing the concept of resonance,

in which the actual structure of the molecule is taken to be a superposition, or average, of all the feasible Lewis structures corresponding to a given atomic arrangement. Resonance is indicated by a double-headed arrow, as in

At this stage we are not indicating the shape of the molecule. Resonance should be pictured as a blending of structures, not a flickering alternation between them. In quantum mechanical terms, the electron distribution of each structure is represented by a wavefunction, and the actual wavefunction, of the molecule is the superposition of the individual wavefunctions for each contributing structure:1

The overall wavefunction is written as a superposition with equal contributions from both structures because the two structures have identical energies. The blendedstructure of two or more Lewis structures is called a resonance hybrid. Note that resonance occurs between structures that differ only in the allocation of electrons; resonance does not occur between structures in which the atoms themselves lie in different positions. For instance, there is no resonance between the structures SOO and OSO. Resonance has two main effects:

1. Resonance averages the bond characteristics over the molecule.

2. Theenergy of a resonance hybrid structure is lower than that of any single contributing structure. The energy of the O3 resonance hybrid, for instance, is lower than that of either individual structure alone. Resonance is most important when there are several structures of identical energy that can be written to describe the molecule, as for O₃ . In such cases, all the structures of the same energy contribute equally to the overall structure. Structures with different energies may also contribute to an overall resonance hybrid but, in general, the greater the energy difference between two Lewis structures, the smaller the contribution of the higher energy structure. The BF3 molecule, for instance, could be regarded as a resonance hybrid of the structures shown in (5), but the first structure dominates even though the octet is incomplete. Consequently, BF₃ is regarded primarily as having that structure with a small admixture of double-bond character. In contrast, for the NO₃ ion (6), the last three structures dominate, and we treat the ion as having partial double-bond character.

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مواضيع ذات صلة

Polyatomic molecules

Homonuclear diatomic molecules

The hydrogen molecule

The basic shapes

Polarizability

Electronegativity

Electron affinity

Atomic and ionic radii

The format of the periodic table

The periodic table

Penetration and shielding

The angular variation of atomic orbitals