Abstract:
Nitrogen is the most abundant element in the earths atmosphere constituting 78.09 % by volume. It is a colorless, odorless, tasteless and inert diatomic gas at standard conditions. Despite its abundance only few hundred nitrides are known compared to thousands of extensively studies and well characterized oxides. One of the reasons for this is the extremely high bond energy of N2 (941 kJ/mol) which is a possible decomposition product of nitrides explaining their low thermal stability. For the same reason N2 does not act as nitriding agent at temperatures below ~ 1300 oC and ambient pressure. On the other hand, bond energy of O2 is comparatively small (499 kJ/mol). Hence, removal of N2 from Si3N4 takes place at atmospheric pressure at about 1900 C whereas SiO2 is thermally stable over 2000 C without any loss of O2. Moreover, the relatively very high energy of formation of the N3- anion from atomic N (more than three times that of O2- from O) is responsible for the reluctance of nitrogen to form ionic bonds except with the most electropositive of elements. These thermodynamic factors, therefore, explain both the rarity of nitrides and also their tendency to form unusual and often unique structure types.
