nitrogen

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Nitrogen is a nonmetal, with an electronegativity of 3.*4. It has five electrons in its outer shell and is, therefore, trivalent in most compounds. The triple bond in molecular nitrogen (N2) is one of the strongest. The resulting difficulty of converting N2 into other compounds, and the ease (and associated high energy release) of converting nitrogen compounds into elemental N2, have dominated the role of nitrogen in both nature and human economic activities.

At atmospheric pressure molecular nitrogen condenses (liquefies) at *7 K (−**5.*9 °C) and freezes at *3 K (−**0.*1 °C)[1] into the beta hexagonal close-packed crystal allotropic form. Below *5.4 K (−**7.6 °C) nitrogen assumes the cubic crystal allotropic form (called the alpha-phase). Liquid nitrogen, a fluid resembling water in appearance, but with *0.8% of the density (the density of liquid nitrogen at its boiling point is 0.**8 g/mL), is a common cryogen.

Unstable allotropes of nitrogen consisting of more than two nitrogen atoms have been produced in the laboratory, like N3 and N4.[7] Under extremely high pressures (1.1 million atm) and high temperatures (***0 K), as produced using a diamond anvil cell, nitrogen polymerizes into the single-bonded cubic gauche crystal structure. This structure is similar to that of diamond, and both have extremely strong covalent bonds. N4 is nicknamed "nitrogen diamond."[8]

Other (as yet unsynthesized) allotropes include hexazine (N6, a benzene analog)[9] and octaazacubane (N8, a cubane analog).[*0] The former is predicted to be highly unstable, while the latter is predicted to be kinetically stable, for reasons of orbital symmetry.[*1]