Which of the following oxides of nitrogen is unstable in air?

The oxide of nitrogen that is unstable in air is NO (nitric oxide). Nitric oxide is a highly reactive gas and readily reacts with oxygen in the air to form nitrogen dioxide (NO2). Nitrogen dioxide is a brownish gas that is responsible for the brown haze seen over many cities. NO2 is relatively stable in air and is not as reactive as NO. On the other hand, NO2 can further react with oxygen to form nitrogen trioxide (N2O3) and nitrogen pentoxide (N2O5). Both of these oxides are relatively stable in air and are not as reactive as NO. Therefore, the unstable oxide of nitrogen in air is NO.