Dinitrogen

Dinitrogen is paradoxically both ubiquitous and inaccessible. It makes up 78 vol % of Earth's atmosphere — roughly 4 million gigatonnes in total, an inexhaustible reservoir at the human scale — but its N≡N triple bond (945 kJ/mol, one of the strongest known covalent bonds) makes it chemically near-inert at ambient temperature and pressure. All the "nitrogen fixation" that underpins the living biosphere (amino acids, nucleotides, chlorophyll) must break this energetic lock, which is possible only at very high temperature, with metallic catalyst, or via bacterial nitrogenases — MoFeS-cluster enzymes that achieve fixation at room temperature but at an energetic cost of 16 ATP per N₂ molecule reduced.

Industrially, dinitrogen is extracted from air by cryogenic distillation: compression at 6 bar, cooling to -190 °C, separation of constituents by boiling point (N₂ at -196 °C, O₂ at -183 °C, Ar at -186 °C). This "air separation unit" (ASU) produces ~150 Mt of N₂ worldwide per year, most of it consumed by Haber-Bosch (NH₃ synthesis, ~80 % of the market). The rest splits between inert-atmosphere metallurgy and electronics, food transport under nitrogen (anti-oxidation), biological-material storage in liquid nitrogen (-196 °C, cell, sperm and embryo conservation), and aircraft or Formula 1 tyre inflation (stable inert pressure gas).

A physical curiosity: dinitrogen condensed at very high pressure and low temperature transforms into a singular molecular solid called polymeric nitrogen, where each atom is bonded to three neighbours by single N-N bonds. First synthesised in 2004 (Eremets, Bayreuth), it is among the highest energy-density storable materials known (~2.5 times dynamite by mass), because its decomposition releases the original triple bond. Its applications remain purely speculative — its metastable stability at atmospheric pressure has never been demonstrated on a macroscopic scale.