Hydrogen
[CAS# 1333-74-0]

Identification

• Classification: Inorganic chemical industry >> Industrial gases such as hydrogen, nitrogen and oxygen
• Name: Hydrogen
• Molecular Formula: H₂
• Molecular Weight: 2.02
• CAS Registry Number: 1333-74-0
• EC Number: 215-605-7
• SMILES: [HH]

Properties

• Density: 0.089 g/mL (Expl.)
• Melting point: -259.2 $degree$C (Expl.)
• Boiling point: -252.8 $degree$C (Expl.)
• Flash point: -150 $degree$C (Expl.)
• Solubility: 0.00017 g/100 mL (Expl.)

Safety Data

• Hazard Symbols: GHS02;GHS04 DangerGHS02
• Risk Statements: H220
• Safety Statements: P203-P210-P222-P280-P377-P381-P403

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Flammable gases	Flam. Gas	1	H220
Gases under pressure (compressed)	Press. Gas (Comp.)		H280
Gases under pressure (refrigerated liquid)	Press. Gas (Ref. Liq.)		H281
Oxidising gases	Ox. Gas	1	H270
Respiratory sensitization	Resp. Sens.	1	H334
Gases under pressure (liquid)	Press. Gas (Liq.)		H280
Carcinogenicity	Carc.	1A	H350
Germ cell mutagenicity	Muta.	1B	H340

• Transport Information: UN 1049; UN 1966; UN 2034; UN 2600

Discovery and Applications

Hydrogen is the simplest and most abundant chemical element in the universe, represented by the symbol H and atomic number 1. It consists of a single proton in its nucleus, often accompanied by one electron in its neutral state, and has no neutrons in its most common isotope. Hydrogen was first recognized as a distinct element in the late eighteenth century by Henry Cavendish, who identified it as a combustible gas produced from the reaction of acids with metals. Its name, derived from the Greek words “hydro” (water) and “genes” (creator), reflects its ability to form water when burned in oxygen.

Hydrogen exists as a diatomic molecule, H₂, under standard conditions. The H–H bond is a nonpolar covalent bond with high bond energy, which contributes to the molecule’s stability under normal temperatures and pressures. Hydrogen gas is colorless, odorless, tasteless, and highly flammable. Its low molecular weight gives it the highest diffusivity and the lowest density of all gases, properties that have influenced both scientific study and practical applications.

In nature, hydrogen is a key component of water, hydrocarbons, and organic compounds. It participates in fundamental chemical reactions such as acid-base chemistry, reduction, and hydrogenation. Hydrogen is central to energy processes in living organisms, including photosynthesis in plants and respiration in animals, where it is involved in the transfer of electrons and the formation of chemical bonds.

Industrial production of hydrogen has historically been achieved through methods such as the reaction of steam with carbon (steam reforming of natural gas), electrolysis of water, and gasification of biomass or coal. These processes provide hydrogen for a wide range of chemical and industrial uses. Hydrogen is a primary feedstock in the production of ammonia for fertilizers via the Haber-Bosch process, in refining petroleum products, and in the hydrogenation of oils and fats in the food industry.

Hydrogen is also a key energy carrier. In recent decades, it has been explored for use in fuel cells to produce electricity, with water as the only byproduct, making it an environmentally attractive alternative to fossil fuels. Compressed hydrogen gas or liquefied hydrogen can be stored and transported, though its low density requires specialized high-pressure or cryogenic containers. Hydrogen-powered vehicles and fuel cells are being developed for transportation, portable energy, and stationary power generation.

In scientific research, hydrogen serves as a standard in nuclear magnetic resonance spectroscopy, as a reducing agent in chemical reactions, and as a component in high-energy experiments. Its isotopes, deuterium and tritium, are used in nuclear fusion research, labeling studies in chemistry, and specialized applications in medicine and physics.

Hydrogen’s properties—high reactivity, low molecular weight, and abundance—have made it a critical element in both natural processes and technological applications. Its role as a chemical building block, energy carrier, and industrial reagent underscores its fundamental importance across chemistry, energy, and materials science.

References

2025. Hydrogen activates ACOD1-itaconate pathway to ameliorate steroid-associated osteonecrosis. Biomaterials.
DOI: 10.1016/j.biomaterials.2025.123428

2025. Fresh apple slice preservation driven by molecular hydrogen. Food Chemistry.
DOI: 10.1016/j.foodchem.2025.144886

2025. The role of nano hydrogen water on fruit quality and antioxidant system of dragon fruit during postharvest storage. Food Chemistry.
DOI: 10.1016/j.foodchem.2025.144915