Mothballs are composed of naphthalene, C₁₀H₈, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:
(c) Not all of the C—C bond lengths in naphthalene are equivalent. Based on your resonance structures, how many C—C bonds in the molecule do you expect to be shorter than the others?

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Draw the Lewis structure of naphthalene, C_{10}H_{8}, showing all atoms and bonds.

Identify the resonance structures of naphthalene by moving electrons to form different double bond arrangements.

Count the number of C—C bonds in each resonance structure.

Determine which C—C bonds are double bonds in more than one resonance structure, as these will be shorter due to increased bond order.

Conclude how many C—C bonds are expected to be shorter based on their presence as double bonds in multiple resonance structures.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Resonance Structures

Resonance structures are different ways of drawing the same molecule that illustrate the delocalization of electrons. In naphthalene, the electrons in the C—C bonds can be shared among multiple bonds, leading to bond lengths that are not all equivalent. Understanding resonance helps predict the stability and reactivity of the molecule.

Bond Lengths and Strengths

Bond length refers to the distance between the nuclei of two bonded atoms. In molecules with resonance, some bonds can be shorter or longer than others due to the distribution of electron density. In naphthalene, the C—C bonds that participate in resonance are typically shorter and stronger than those that do not.

Delocalization of Electrons

Delocalization of electrons occurs when electrons are not associated with a single atom or bond but are spread over several atoms. In naphthalene, this delocalization results from the overlapping p-orbitals of the carbon atoms, leading to a stabilization of the molecule and affecting the bond lengths of the C—C bonds.

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Textbook Question

Mothballs are composed of naphthalene, C₁₀H₈, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:(a) Draw all of the resonance structures of naphthalene. How many are there?

Textbook Question

Mothballs are composed of naphthalene, C₁₀H₈, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:

(b) Do you expect the C—C bond lengths in the molecule to be similar to those of C—C single bonds, C ═C double bonds, or intermediate between C—C single and C ═C double bonds?

Textbook Question

(a) Which of these compounds is an exception to the octet rule: carbon dioxide, water, ammonia, phosphorus trifluoride, or arsenic pentafluoride?

Textbook Question

(b) Which of these compounds or ions is an exception to the octet rule: borohydride (BH4-), borazine (B3N3H6, which is analogous to benzene with alternating B and N in the ring), or boron trichloride?