Editing Naphthalene (section)

===Molecular geometry===
The molecule is planar, like benzene. Unlike benzene, the carbon–carbon bonds in naphthalene are not of the same length. The bonds C1−C2, C3−C4, C5−C6 and C7−C8 are about 1.37 Å (137 pm) in length, whereas the other carbon–carbon bonds are about 1.42 Å (142 pm) long. This difference, established by [[X-ray diffraction]],<ref>{{cite journal|last1=Cruickshank|first1=D. W. J.|last2=Sparks|first2=R. A.|title=Experimental and Theoretical Determinations of Bond Lengths in Naphthalene, Anthracene and Other Hydrocarbons|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|date=18 October 1960|volume=258|issue=1293|pages=270–285|doi=10.1098/rspa.1960.0187|bibcode=1960RSPSA.258..270C|s2cid=96765335}}</ref> is consistent with the [[valence bond theory|valence bond]] model in naphthalene and in particular, with the theorem of [[cross-conjugation]]. This theorem would describe naphthalene as an [[Aromaticity|aromatic]] benzene unit bonded to a [[diene]] but not extensively [[Conjugated system|conjugated]] to it (at least in the [[ground state]]), which is consistent with two of its three [[Resonance (chemistry)|resonance]] structures.

:[[Image:Naphthalene resonance structure.svg|400px|Resonance structures of naphthalene]]

Because of this resonance, the molecule has [[bilateral symmetry]] across the plane of the shared carbon pair, as well as across the plane that bisects bonds C2-C3 and C6-C7, and across the plane of the carbon atoms. Thus there are two sets of equivalent hydrogen atoms: the ''alpha'' positions, numbered 1, 4, 5, and 8, and the ''beta'' positions, 2, 3, 6, and 7. Two [[isomer]]s are then possible for mono-substituted naphthalenes, corresponding to substitution at an alpha or beta position.

:[[File:Naftalene reattivita.PNG|180px|Alpha vs beta symmetry-equivalent positions]]


[[File:Azulen.svg|thumb|left|120px|Azulene]]
[[Structural isomer]]s of naphthalene that have two fused aromatic rings include [[azulene]], which has a 5–7 fused ring system, and [[Bicyclo(6.2.0)decapentaene|Bicyclo[6.2.0]decapentaene]] which has a fused 4–8 ring system.<ref>{{cite journal | doi = 10.1021/jo00215a018 | title = Theoretical determination of molecular structure and conformation. 14. Is bicyclo[6.2.0]decapentaene aromatic or antiaromatic? |author1=Dieter Cremer |author2=Thomas Schmidt |author3=Charles W. Bock | journal = J. Org. Chem. | date = 1985 | volume = 50 | issue = 15 | pages = 2684–2688}}</ref>

The point group symmetry of naphthalene is ''D<sub>2h</sub>''.
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