Editing Dinitrogen tetroxide (section)

==Structure and properties==
Dinitrogen tetroxide could be regarded as two [[nitro group]]s (-NO<sub>2</sub>) bonded together. It forms an [[Chemical equilibrium|equilibrium mixture]] with [[nitrogen dioxide]].<ref>{{Cite journal |doi = 10.1021/ic50008a020|title = Dimers of Nitrogen Dioxide. II. Structure and Bonding|journal = Inorganic Chemistry|volume = 2|issue = 4|pages = 747–752|year = 1963|last1 = Bent|first1 = Henry A.}}</ref>  The molecule is planar with an N-N bond distance of 1.78{{nbsp}}Å and N-O distances of 1.19{{nbsp}}Å. The N-N distance corresponds to a weak bond, since it is significantly longer than the average N-N single bond length of 1.45{{nbsp}}Å.<ref>{{cite book |last1 = Petrucci |first1 = Ralph H. |last2 = Harwood |first2 = William S. |last3 = Herring |first3 = F. Geoffrey |date=2002 |title = General chemistry: principles and modern applications |url = https://archive.org/details/generalchemistry00hill |url-access = registration |edition=8th |location=Upper Saddle River, N.J |publisher=Prentice Hall |isbn = 978-0-13-014329-7 |lccn=2001032331 |oclc=46872308 |page=[https://archive.org/details/generalchemistry00hill/page/420 420] }}</ref> This exceptionally weak σ bond (amounting to overlapping of the ''sp''<sup>2</sup> hybrid orbitals of the two NO<sub>2</sub> units<ref>{{Cite book|last=Rayner-canham|first=Geoff|url=https://www.worldcat.org/oclc/1026755795|title=Descriptive inorganic chemistry.|date=2013|isbn=978-1-319-15411-0|edition=6th|pages=400|oclc=1026755795}}</ref>) results from the simultaneous delocalization of the bonding electron pair across the whole N<sub>2</sub>O<sub>4</sub> molecule, and the considerable electrostatic repulsion of the doubly occupied molecular orbitals of each NO<sub>2</sub> unit.<ref>{{Cite journal|last1=Ahlrichs|first1=Reinhart|last2=Keil|first2=Frerich|date=1974-12-01|title=Structure and bonding in dinitrogen tetroxide (N2O4)|url=https://doi.org/10.1021/ja00832a002|journal=Journal of the American Chemical Society|volume=96|issue=25|pages=7615–7620|doi=10.1021/ja00832a002|issn=0002-7863|url-access=subscription}}</ref>

Unlike NO<sub>2</sub>, N<sub>2</sub>O<sub>4</sub> is [[diamagnetism|diamagnetic]] since it has no unpaired electrons.<ref name=HW>Holleman, A. F.; Wiberg, E. (2001) ''Inorganic Chemistry''. Academic Press: San Diego. {{ISBN|978-0-12-352651-9}}.</ref>  The liquid is also colorless but can appear as a brownish yellow liquid due to the presence of NO<sub>2</sub> according to the following equilibrium:<ref name=HW/>
: N<sub>2</sub>O<sub>4</sub> ⇌ 2 NO<sub>2</sub>{{Pad|2em}}({{Math|Δ''H'' {{=}} +57.23 kJ/mol}})

Higher temperatures push the equilibrium towards nitrogen dioxide.  Inevitably, some dinitrogen tetroxide is a component of [[smog]] containing nitrogen dioxide.

Solid {{chem2|N2O4}} is white, and melts at −11.2&nbsp;°C.<ref name=HW/>