Nitrogen dioxide is the chemical compound N O₂. It is one of the several nitrogen oxides. This reddish-brown gas has a characteristic sharp, biting odor. N O₂ is one of the most prominent air pollutants and a poison by inhalation.

1 Formation and occurrence
2 Reactions
3 Structure and bonding
4 Safety and pollution considerations
5 See also
6 Reference
7 External links

[edit] Formation and occurrence

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Nitric oxide (NO), also a common pollutant, oxidizes in air to the dioxide:

2NO + O₂ → 2NO₂

NO₂ is generated by the action of nitric acid on a variety of metals, such as copper or silver.

2HNO₃ + Ag → AgNO₃ + N O₂ + H₂O

"Red fuming nitric acid" owes its red color to the presence of NO₂.

NO₂ is generated in biological settings from decomposition of peroxynitrite (ONOO⁻), a potent oxidizing and nitrating agent formed from the reaction of nitric oxide with superoxide.

[edit] Reactions

Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide.

2 NO₂ ↔ N₂O₄ ΔG = 45.53 kJ/mol

The equilibrium favors NO₂ at higher temperatures. Solid NO₂ can be obtained from NO₂ by very rapid cooling (for example with liquid nitrogen), although it commonly contains N₂O₄.

At −50 °C the crystals of N₂O₄, which is diamagnetic, are colorless, but they become honey-yellow at the melting point. The vapour at −10 °C is pale yellow and deepens as the temperature rises.

[edit] Structure and bonding

NO₂ is a radical, having one unpaired electron, which renders this molecule paramagnetic. Low energy electronic transitions give rise to the visible color of this molecule. The molecule is nonlinear with bond distances and angles intermediate between those for the corresponding anion, nitrite, and the cation, nitronium.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref>

Species	O-N-O angle (°)	N-O distance (Å)
NO₂⁺	180	1.154
NO₂	134	1.197
NO₂⁻	115	1.236

[edit] Safety and pollution considerations

Nitrogen dioxide is toxic by inhalation. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential for overexposure.

Long-term exposure to NO₂ at concentrations above 40–100 µg/m³ causes adverse health effects [1]. The most important source of NO₂ is internal combustion engines, which emit nitrogen oxides near people.

This map, depicting results of satellite measurements, illustrates nitrogen dioxide as large scale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthful levels. Nitrogen dioxide plays a role in atmospheric chemistry, including the formation of tropospheric ozone.

A recent study by researchers at the University of California, San Diego, suggests a link between NO₂ levels and Sudden Infant Death Syndrome [2].

[edit] See also

Nitrous oxide or N₂O, "laughing gas", a linear molecule, isoelectronic with CO₂ but with a nonsymmetric arrangement of atoms (NNO)
Nitric oxide or NO, a problematic pollutant, related to CO but with one additional electron.
NO_x = all of the above in unspecified proportions but tending toward NO₂.

More esoteric nitrogen oxides include N₂O₅ and the blue species N₂O₃.

Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite (NO₂⁻), nitrate (NO₃⁻), nitronium or NO₂⁺, and nitrosonium or NO⁺. NO₂ is intermediate between nitrite and nitronium: