Editing Naphthalene (section)

==Physical properties==
A naphthalene molecule can be viewed as the fusion of a pair of [[benzene]] rings. (In [[organic chemistry]], rings are ''fused'' if they share two or more atoms.) As such, naphthalene is classified as a benzenoid [[polycyclic aromatic hydrocarbon]] (PAH).<ref>{{Cite web|url=https://www.epa.gov/sites/production/files/2014-03/documents/pahs_factsheet_cdc_2013.pdf |archive-url=https://web.archive.org/web/20141130115013/http://www2.epa.gov/sites/production/files/2014-03/documents/pahs_factsheet_cdc_2013.pdf |archive-date=2014-11-30 |url-status=live|title=Polycyclic Aromatic Hydrocarbons (PAHs)|quote=Naphthalene is a PAH that is produced commercially in the US}}</ref>

The eight carbon atoms that are not shared by the two rings carry one hydrogen atom each. For purpose of the standard [[IUPAC]] nomenclature of derived compounds, those eight atoms are numbered 1 through 8 in sequence around the perimeter of the molecule, starting with a carbon atom adjacent to a shared one. The shared carbon atoms are labeled 4a (between 4 and 5) and 8a (between 8 and 1).<ref>''Blue Book'', P-14.4 NUMBERING</ref>

===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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=== Electrical conductivity ===
Pure crystalline naphthalene is a moderate insulator at room temperature, with [[resistivity]] of about 10<sup>12</sup> [[ohm|Ω]] [[metre|m]]. The resistivity drops more than a thousandfold on melting, to about 4 × 10<sup>8</sup> Ω m. Both in the liquid and in the solid, the resistivity depends on temperature as ''ρ'' = ''ρ''<sub>0</sub> exp(''E''/(''kT'')), where ''ρ''<sub>0</sub> (Ω⋅m) and ''E'' (eV) are constant parameters, ''k'' is the Boltzmann constant (8.617 × 10<sup>−5</sup> eV/[[kelvin|K]]), and ''T'' is absolute temperature (K). The parameter ''E'' is 0.73 in the solid. However, the solid shows semiconducting character below 100 K.<ref name="born1962">{{Cite journal |last=Bornmann |first=John A. |year=1962 |title=Semiconductivity of Naphthalene |url=https://pubs.aip.org/jcp/article/36/6/1691/205625/Semiconductivity-of-Naphthalene |journal=The Journal of Chemical Physics |language=en |volume=36 |issue=6 |pages=1691–1692 |bibcode=1962JChPh..36.1691B |doi=10.1063/1.1732805 |issn=0021-9606}}</ref><ref name="sche1978">{{Cite journal |last1=Schein |first1=L. B. |last2=Duke |first2=C. B. |last3=McGhie |first3=A. R. |year=1978 |title=Observation of the Band-Hopping Transition for Electrons in Naphthalene |url=https://link.aps.org/doi/10.1103/PhysRevLett.40.197 |journal=Physical Review Letters |language=en |volume=40 |issue=3 |pages=197–200 |bibcode=1978PhRvL..40..197S |doi=10.1103/PhysRevLett.40.197 |issn=0031-9007}}</ref>