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Chemistry LibreTexts

2.4 Resonance

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  • Page ID
    67055
  • Objective

    After completing this section, you should be able to understand the concept of resonance.

    Key Terms

    Make certain that you can define, and use in context, the key term below.

    • resonance form

    Resonance StructuresEdit section

    Sometimes, even when formal charges are considered, the bonding in some molecules or ions cannot be described by a single Lewis structure. Such is the case for ozone (O3), an allotrope of oxygen with a V-shaped structure and an O–O–O angle of 117.5°. 

    O3Edit section

    1. We know that ozone has a V-shaped structure, so one O atom is central:

    88758c95f785542fa2f8da9a77c28e5f.jpg

    2. Each O atom has 6 valence electrons, for a total of 18 valence electrons.

    3. Assigning one bonding pair of electrons to each oxygen–oxygen bond gives

    6e707fb4bcf79f13d3a017e163650dd8.jpg

    with 14 electrons left over.

    4. If we place three lone pairs of electrons on each terminal oxygen, we obtain

    0dba2719a1c946313e50598da82f3343.jpg

    and have 2 electrons left over.

    5. At this point, both terminal oxygen atoms have octets of electrons. We therefore place the last 2 electrons on the central atom:

    324d4b4f680e700765c50b3e5ae1b825.jpg

    6. The central oxygen has only 6 electrons. We must convert one lone pair on a terminal oxygen atom to a bonding pair of electrons—but which one? Depending on which one we choose, we obtain either

    db9cf47bed755c41dd8c100bc67b7c41.jpg

    Which is correct? In fact, neither is correct. Both predict one O–O single bond and one O=O double bond. If the bonds were of different types (one single and one double, for example), they would have different lengths. It turns out, however, that both O–O bond distances are identical, 127.2 pm, which is shorter than a typical O–O single bond (148 pm) and longer than the O=O double bond in O2 (120.7 pm).Edit section

    Equivalent Lewis dot structures, such as those of ozone, are called resonance structures . The position of the atoms is the same in the various resonance structures of a compound, but the position of the electrons is different. Double-headed arrows link the different resonance structures of a compound:

    746dfd0700de81b298835a0e84e128a9.jpg

    Before the development of quantum chemistry it was thought that the double-headed arrow indicates that the actual electronic structure is an average of those shown, or that the molecule oscillates between the two structures.  Today we know that the electrons involved in the double bonds occupy an orbital that extends over all three oxygen molecules, combining p orbitals on all three.

    ozone.png

    Figure 5.3.4: The resonance structure of ozone involves a molecular orbital extending all three oxygen atoms.In ozone, a molecular orbital extending over all three oxygen atoms is formed from three atom centered pz orbitals. Similar molecular orbitals are found in every resonance structure.Edit section

    We will discuss the formation of these molecular orbitals in the next chapter but it is important to understand that resonance structures are based on molecular orbitals not averages of different bonds between atoms. We describe the electrons in such molecular orbitals as being delocalized, that is they cannot be assigned to a bond between two atoms.Edit section

    Note

    When it is possible to write more than one equivalent resonance structure for a molecule or ion, the actual structure involves a molecular orbital which is a linear combination of atomic orbitals from each of the atoms.

    CO32−Edit section

    Like ozone, the electronic structure of the carbonate ion cannot be described by a single Lewis electron structure. Unlike O3, though, the Lewis structures describing CO32− has three equivalent representations.Edit section

    1. Because carbon is the least electronegative element, we place it in the central position:

    62631ab04cf387e8f3a8baccfe446f9a.jpg

    2. Carbon has 4 valence electrons, each oxygen has 6 valence electrons, and there are 2 more for the −2 charge. This gives 4 + (3 × 6) + 2 = 24 valence electrons.

    3. Six electrons are used to form three bonding pairs between the oxygen atoms and the carbon:

    Edit section
    a24107f743253b06a88b174c9e95a1b0.jpg

    4. We divide the remaining 18 electrons equally among the three oxygen atoms by placing three lone pairs on each and indicating the −2 charge:

    3db195f257b28a739f1b1d7ffa50877c.jpg

    5. No electrons are left for the central atom.Edit section

    6. At this point, the carbon atom has only 6 valence electrons, so we must take one lone pair from an oxygen and use it to form a carbon–oxygen double bond. In this case, however, there are three possible choices:

    8fe84f925f60984bb8ba98b378d0b9ee.jpg

    As with ozone, none of these structures describes the bonding exactly. Each predicts one carbon–oxygen double bond and two carbon–oxygen single bonds, but experimentally all C–O bond lengths are identical. We can write resonance structures (in this case, three of them) for the carbonate ion:

    2cde63f59c8c431b668b7be757af0420.jpg

    As the case for ozone, the actual structure involves the formation of a molecular orbital from pz orbitals centered on each atom and sitting above and below the plane of the CO32− ion.  Edit section

    Example 2.4.1

    Benzene is a common organic solvent that was previously used in gasoline; it is no longer used for this purpose, however, because it is now known to be a carcinogen. The benzene molecule (C6H6) consists of a regular hexagon of carbon atoms, each of which is also bonded to a hydrogen atom. Use resonance structures to describe the bonding in benzene.

    Given: molecular formula and molecular geometry

    Asked for: resonance structures

    Strategy:

    A Draw a structure for benzene illustrating the bonded atoms. Then calculate the number of valence electrons used in this drawing.

    B Subtract this number from the total number of valence electrons in benzene and then locate the remaining electrons such that each atom in the structure reaches an octet.

    C Draw the resonance structures for benzene.

    Solution:

    A Each hydrogen atom contributes 1 valence electron, and each carbon atom contributes 4 valence electrons, for a total of (6 × 1) + (6 × 4) = 30 valence electrons. If we place a single bonding electron pair between each pair of carbon atoms and between each carbon and a hydrogen atom, we obtain the following:

    ba6cb60e595d3bbb3a83517d9e8b4669.jpg

    Each carbon atom in this structure has only 6 electrons and has a formal charge of +1, but we have used only 24 of the 30 valence electrons.

    B If the 6 remaining electrons are uniformly distributed pairwise on alternate carbon atoms, we obtain the following:

    0465b1775e6980116b7582aed7577764.jpg

    Three carbon atoms now have an octet configuration and a formal charge of −1, while three carbon atoms have only 6 electrons and a formal charge of +1. We can convert each lone pair to a bonding electron pair, which gives each atom an octet of electrons and a formal charge of 0, by making three C=C double bonds.

    C There are, however, two ways to do this:

    ff5776f9f311c1157b67d84e3468a7c4.jpg

    Each structure has alternating double and single bonds, but experimentation shows that each carbon–carbon bond in benzene is identical, with bond lengths (139.9 pm) intermediate between those typically found for a C–C single bond (154 pm) and a C=C double bond (134 pm). We can describe the bonding in benzene using the two resonance structures, but the actual electronic structure is an average of the two. The existence of multiple resonance structures for aromatic hydrocarbons like benzene is often indicated by drawing either a circle or dashed lines inside the hexagon:

    c7d7af4d5e17d7590132bd3fd07bfe57.jpg

    This combination of p orbitals for benzene can be visualized as a ring with a node in the plane of the carbon atoms. 

     

     

     

    Exercise 2.4.1

    The sodium salt of nitrite is used to relieve muscle spasms. Draw two resonance structures for the nitrite ion (NO2).

    Answer:

    1e6377431bae8a12f8127b8b1bf7588c.jpg

    Resonance structures are particularly common in oxoanions of the p-block elements, such as sulfate and phosphate, and in aromatic hydrocarbons, such as benzene and naphthalene.

    Exercises

    Questions

    Q2.4.1

    Draw the resonance structures for the following molecule:

    Solutions

    S2.4.1

    Contributors