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Reactions and Compounds of Nitrogen Like carbon, nitrogen has four valence orbitals one 2s and three 2p , so it can participate in at most four electron-pair bonds by using sp3 hybrid orbitals. Unlike carbon, however, nitrogen does not form long chains because of repulsive interactions between lone pairs of electrons on adjacent atoms. These interactions become important at the shorter internuclear distances encountered with the smaller, second-period elements of groups 15, 16, and 17. For more information on internuclear distance, see Chapter 7 "The Periodic Table and Periodic Trends", Section 7.2 "Sizes of Atoms and Ions" and Chapter 8 "Ionic versus Covalent Bonding", Section 8.2 "Ionic Bonding". Stable compounds with NN bonds are limited to chains of no more than three N atoms, such as the azide ion N 3 . Nitrogen is the only pnicogen that normally forms multiple bonds with itself and other second-period elements, using overlap of adjacent np orbitals. Thus the stable form of elemental nitrogen is N 2, whose NN bond is so strong DNN = 942 kJ/mol compared with the NN and N=N bonds DNN = 167 kJ/mol; DN=N = 418 kJ/mol that all compounds containing NN and N=N bonds are thermodynamically unstable with respect to the formation of N2. In fact, the formation of the NN bond is so thermodynamically favored that virtually all compounds containing NN bonds are potentially explosive. Again in contrast to carbon, nitrogen undergoes only two important chemical reactions at room temperature: it reacts with metallic lithium to form lithium nitride, and it is reduced to ammonia by certain microorganisms. For more information lithium, seeChapter 21 "Periodic Trends and the ". At higher temperatures, however, N2 reacts with more electropositive elements, such as those in group 13, to give binary nitrides, which range from covalent to ionic in character. Like the corresponding compounds of carbon, binary compounds of nitrogen with oxygen, hydrogen, or other nonmetals are usually covalent molecular substances. Few binary molecular compounds of nitrogen are formed by direct reaction of the elements. At elevated temperatures, N2 reacts with H2 to form ammonia, with O2 to form a mixture of NO and NO2, and with carbon to form cyanogen NCCN ; elemental nitrogen does not react with the halogens or the other chalcogens. Nonetheless, all the binary nitrogen halides NX3 are known. Except for NF3, all are toxic,.