Oxygen
[CAS# 7782-44-7]

Identification

• Classification: API >> Nervous system medication >> Central stimulant
• Name: Oxygen
• Synonyms: molecular oxygen
• Molecular Formula: O₂
• Molecular Weight: 32.00
• CAS Registry Number: 7782-44-7
• EC Number: 231-956-9
• SMILES: O=O

Properties

• Density: 1.1$+/-$0.1 g/cm³ Calc.^*
• Melting point: -218 $degree$C (Expl.)
• Boiling point: -183.0$+/-$9.0 $degree$C 760 mmHg (Calc.)^*
• Index of refraction: 1.123 (Calc.)^*

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS03;GHS04 Danger
• Risk Statements: H270
• Safety Statements: P220-P244-P370+P376-P403

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Oxidising gases	Ox. Gas	1	H270
Gases under pressure (compressed)	Press. Gas (Comp.)		H280
Gases under pressure (refrigerated liquid)	Press. Gas (Ref. Liq.)		H281
Gases under pressure (liquid)	Press. Gas (Liq.)		H280

• Transport Information: UN 1014; UN 1072

Discovery and Applications

Oxygen is a chemical element with the symbol O and atomic number 8. It is a colorless, odorless, and tasteless diatomic gas (O₂) under standard conditions, and it is essential for the survival of aerobic organisms. Oxygen constitutes approximately 21% of the Earth's atmosphere by volume and is the most abundant element in the Earth’s crust and biosphere, occurring in water, minerals, and organic compounds. It was discovered independently by Carl Wilhelm Scheele and Joseph Priestley in the late 18th century, and its role in combustion and respiration was later elucidated.

Molecular oxygen consists of two oxygen atoms joined by a double bond, forming a diatomic molecule with a bond length of 121 pm. It has paramagnetic properties due to two unpaired electrons in its π* antibonding orbitals, which explains its attraction to magnetic fields. Oxygen exists in several allotropes, with O₂ being the most common and ozone (O₃) forming in the upper atmosphere, where it absorbs ultraviolet radiation and protects living organisms from harmful UV light.

Oxygen is essential in respiration, where it serves as the final electron acceptor in the electron transport chain of mitochondria, enabling aerobic metabolism and the production of adenosine triphosphate (ATP). It also supports combustion, oxidizing fuels to release energy in industrial, domestic, and transportation applications.

In industry, oxygen is used in steel manufacturing, chemical production, and wastewater treatment. It supports processes such as oxidation reactions, oxy-fuel welding and cutting, and the production of chemicals like nitric acid and hydrogen peroxide. Liquid oxygen, obtained by cryogenic distillation of air, is widely used in rocket propulsion and as a medical oxygen supply for respiratory support.

Oxygen’s reactive nature allows it to form oxides with most elements. While it is highly reactive with metals and non-metals to form stable oxides, its gaseous form remains relatively safe under normal conditions. Its environmental role is critical, participating in the carbon and water cycles and maintaining life-supporting conditions on Earth.

Overall, oxygen is a chemically reactive, life-essential diatomic gas with broad applications in biology, industry, and environmental processes. Its diatomic form, reactivity, and biological importance make it one of the most critical elements for life and technological development.

References

2025. Influence of bulk-phase acidity, organic fraction, and dissolved oxygen on the photosensitized renoxification of nitrate in NaNO3/humic acid mixtures. Journal of environmental sciences (China).
DOI: 10.1016/j.jes.2024.08.023

2025. Reconstruction of the three-dimensional dissolved oxygen and its spatio-temporal variations in the Mediterranean Sea using machine learning. Journal of environmental sciences (China).
DOI: 10.1016/j.jes.2025.01.010

2025. Functional cardiac patch with “gas and ion” dual-effect intervention for reconstructing blood microcirculation in myocardial infarction repair. Biomaterials.
DOI: 10.1016/j.biomaterials.2025.123300