dihydrogen

Dihydrogen, commonly called molecular hydrogen or H2, is the diatomic molecule consisting of two hydrogen atoms. It is the simplest and most abundant molecule in the universe, occurring mainly in interstellar space and on gas giant planets. On Earth, hydrogen is mostly bound in water and organic compounds, but free dihydrogen gas can be produced, stored, and used industrially.

In the gas phase at standard conditions, dihydrogen is colorless, odorless, and highly flammable. It is the lightest of all gases and has a very low density. The molecule consists of a single nonpolar H–H covalent bond, with a bond length of about 74 picometers and a bond dissociation energy near 436 kJ/mol. Its thermodynamic properties give it extremely low boiling and melting points: approximately 20.3 kelvin for the boiling point and around 14 kelvin for the melting point.

Natural hydrogen includes several isotopes, with protium (the most common) as the dominant form, and small amounts of deuterium and tritium. The gas may also contain heteronuclear species such as HD.

Production and applications: Industrial production is dominated by steam methane reforming and, increasingly, electrolysis of water. Dihydrogen is a key feedstock for hydrogenation reactions in chemical synthesis and petroleum refining. It is also explored as a clean energy carrier for fuel cells and as a fuel for rockets (liquid hydrogen) due to its high energy content per unit mass.

Safety and environment: Dihydrogen is highly flammable and can form explosive mixtures with air. It disperses rapidly and leaks can be hard to detect. While hydrogen itself is non-toxic, handling requires appropriate safety measures to prevent fires and material embrittlement. When produced from renewable energy, it is considered a potential clean energy vector.