The nitrogen group

Group 15 of the periodic table — nitrogen, phosphorus, arsenic, antimony, and bismuth — shows the same top-to-bottom shift from nonmetallic to metallic character seen across many groups, but with an unusually dramatic range of chemical behaviour along the way. Nitrogen at the top is a strongly electronegative, gaseous nonmetal; phosphorus and arsenic are nonmetals or metalloids that exist as solids with several structural forms; antimony and bismuth at the bottom show increasingly metallic properties, including metallic lustre and electrical conductivity.

Nitrogen's chemistry deserves particular attention because it is central to both industrial chemistry and biology, and because it displays an unusually wide span of oxidation states — essentially every integer from -3 to +5. In ammonia (NH3), nitrogen is at oxidation state -3, having gained electron density from three hydrogen atoms in each covalent bond; in nitric acid (HNO3), nitrogen is at +5, its electron density pulled heavily toward three oxygen atoms. Between these extremes, nitrogen forms compounds at essentially every intermediate oxidation state, including nitrous oxide (N2O, +1), nitric oxide (NO, +2), and nitrogen dioxide (NO2, +4), among others — a range matched by very few other elements and central to the nitrogen cycle in the environment, industrial fertiliser production, and atmospheric chemistry.

Elemental nitrogen gas, N2, is a striking exception to nitrogen's general reactivity: despite nitrogen atoms individually being highly electronegative with five valence electrons eager to form bonds, the two nitrogen atoms in N2 are joined by an exceptionally strong triple bond, among the strongest single chemical bonds known. Breaking this bond requires a large input of energy, which is why nitrogen gas, despite making up roughly 78% of the atmosphere and being thermodynamically capable of many exothermic reactions, remains kinetically almost inert at ordinary temperatures — industrial nitrogen fixation (the Haber process, converting N2 and H2 into ammonia) requires high temperature, high pressure, and a catalyst specifically to overcome this kinetic barrier.