Nitrogen oxide

Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:

• Nitric oxide, also known as nitrogen monoxide (NO), nitrogen(II) oxide
• Nitrogen dioxide (NO₂), nitrogen(IV) oxide
• Nitrous oxide (N₂O), nitrogen(−I,III) oxide
• Nitrosylazide (N₄O), nitrogen(−I,0,I,II) oxide
• Oxatetrazole (N₄O)
• Dinitrogen trioxide (N₂O₃), nitrogen(II,IV) oxide
• Dinitrogen tetroxide (N₂O₄), nitrogen(IV) oxide
• Dinitrogen pentoxide (N₂O₅), nitrogen(V) oxide
• Trinitramide (N(NO₂)₃ or N₄O₆), nitrogen(0,IV) oxides
• Dinitramide (N(NO₂)₃⁻)
• Nitrite (NO−
  2)
• Nitrate (NO−
  3)
• Nitronium (NO+
  2)
• Nitrosonium (NO+
  )
• peroxonitrite (ONO−
  2)

The compounds NO and NO₂ are radicals. In addition, there are several anions of nitrogen oxides. The most stable of these is the nitrate anion:

• Nitrate (NO₃⁻), trioxonitrate(V) ion

but other nitrogen oxoanions include hyponitrite, nitrite, peroxonitrite, trioxodinitrate (hyponitrate), and nitroxylate.

In atmospheric chemistry, air pollution, and related fields, nitrogen oxides refers specifically to NO_x (NO and NO₂).^[1][2]

Only the first three of these compounds can be isolated at room temperature. N₂O₃, N₂O₄, and N₂O₅ all decompose rapidly at room temperature. NO₃, N₄O, and N(NO₂)₃ are very reactive.

N₂O is stable and rather unreactive at room temperature, while NO and NO₂ are quite reactive but nevertheless quite stable when isolated.

• 
  Nitric oxide, NO
• 
  Nitrogen dioxide, NO₂
• 
  Nitrous oxide, N₂O
• 
  Dinitrogen trioxide, N₂O₃
• 
  Dinitrogen tetroxide, N₂O₄
• 
  Dinitrogen pentoxide, N₂O₅
• 
  Trinitramide, N(NO₂)₃

NO_x

NO_x (often written NOx) refers to NO and NO₂. They are produced during combustion, especially at high temperature. These two chemicals are important trace species in Earth's atmosphere. In the troposphere, during daylight, NO reacts with partly oxidised organic species (or the peroxy radical) to form NO₂, which is then photolysed by sunlight to reform NO:

NO + CH₃O₂ → NO₂ + CH₃O

NO₂ + sunlight → NO + O

The oxygen atom formed in the second reaction then goes on to form ozone; this series of reactions is the main source of tropospheric ozone. CH₃O₂ is just one example of many partly oxidised organic molecules that can react with NO to form NO₂.

These reactions are rather fast, so NO and NO₂ tend to cycle, but the sum of their concentration ([NO] + [NO₂]) tends to remain fairly constant. Because of this cycling, it is convenient to think of the two chemicals as a group; hence the term NO_x.

In addition to acting as a main precursor for tropospheric ozone, NO_x is also harmful to human health in its own right.

NO_x may react with water to make nitric acid, which may end up in the soil, where it makes nitrate, which is of use to growing plants.

See also

• Nitrogen oxide sensor
• Sulfur nitrides, which are valence isoelectronic with nitrogen oxides

References