H2Moleküls

H2 molecules are diatomic hydrogen, the simplest and most abundant molecule in the universe. Each molecule consists of two hydrogen atoms linked by a covalent bond, sharing a single pair of electrons.

The H–H bond is a single covalent bond with a bond length of about 74 picometers and a bond energy of around 436 kJ/mol. H2 has no permanent electric dipole moment, so its pure rotational-vibrational spectrum in the infrared is very weak; its transitions are mainly quadrupole-allowed and Raman active, while ultraviolet electronic transitions occur in the Lyman and Werner bands.

At standard conditions hydrogen is a colorless, odorless diatomic gas. It condenses to a liquid at 20.28 K and becomes a solid around 14 K at 1 atm. It has a high energy density by mass and is highly flammable, reacting with oxygen to form water.

In interstellar space, H2 forms primarily on the surfaces of dust grains, via recombination of atomic hydrogen; at very high densities it can form efficiently in the gas phase as well. It is destroyed by ultraviolet photodissociation and reactions with radicals.

H2 is the most abundant molecule in the universe and the principal coolant in metal-poor star-forming regions. Giant molecular clouds, rich in H2, are the birthplaces of stars and play a key role in the thermal balance of galaxies.

Isotopes and variations: Common isotopologues include HD and D2. Under ambient conditions H2 exists as para- and ortho-hydrogen spin isomers with different rotational state populations, which affect heat capacity and spectra.

Industrial and safety aspects: H2 is used in petroleum refining, ammonia synthesis, and hydrogenation, and is explored as a clean energy carrier. It is highly flammable and can form explosive mixtures with air; storage and handling require precautions.