Oxygen (symbol O, atomic number 8) is a non-metallic chemical element from the chalcogen family. In the form of dioxygen (O₂), it makes up about 21% of Earth's atmosphere and plays a fundamental role in cellular respiration and combustion reactions.
Contribution of oxygen to vital processes
Molecular oxygen (O₂) is essential for aerobic cellular respiration in most eukaryotic organisms. It acts as the final electron acceptor in the mitochondrial electron transport chain, enabling efficient ATP production. In its absence, cells rely on fermentation, which yields much less energy. Consequence of a deficit: Hypoxia, metabolic distress, and rapid cell death. Oxygen is thus critical for short-term survival.
History of Discovery
1771: Carl Wilhelm Scheele's Experiments The Swedish chemist Scheele produces oxygen by heating various metal oxides but publishes his results late.
1774: Joseph Priestley Isolates the Gas The English chemist obtains pure oxygen from heated mercury oxide (HgO). He speaks of "dephlogisticated air."
1785: Antoine Lavoisier Defines Its Nature Lavoisier rejects the phlogiston theory and demonstrates that oxygen is an element involved in respiration and combustion. He introduces the name "oxygen," from the Greek oxys (acid) and genes (engender), mistakenly believing it was necessary for the formation of acids.
Atomic Structure
Electron configuration: 1s² 2s² 2p⁴ Valence electrons: 6 (p⁴ configuration), tendency to complete its octet through two covalent bonds.
Isotopes:
16O: 99.76%, stable, main natural isotope.
17O: 0.04%, stable, used in NMR and geochemistry.
18O: 0.20%, stable, isotopic tracer in climatology.
Physical Properties
Diatomic gas (O₂), colorless, odorless.
Molar mass: ≈ 31.998 g/mol
Melting point: 54.36 K (−218.79 °C)
Boiling point: 90.20 K (−182.96 °C)
Density (gas at 0 °C, 1 atm): ~1.429 g/L
Paramagnetic: two unpaired electrons in O₂
Chemical Reactivity
Strong oxidizing power: reacts with most elements (forming oxides).
Combustion agent in exothermic reactions.
Forms ozone (O₃) through UV action: important role in the stratosphere.
Reacts with organic compounds (biological oxidation, cellular respiration).
Industrial and Technological Applications
Used in respiratory systems (hospitals, diving).
Metal cutting and welding (oxy-fuel cutting).
Steel and other metal production (blast furnaces, converters).
Wastewater treatment (active oxygenation).
Combustion agent in cryogenic engines (with H₂).
Cosmological and Astrophysical Role
Produced during stellar nucleosynthesis (helium fusion in massive stars).
Major constituent of terrestrial planets (silicate rocks, oxides).
Present in planetary nebulae, supernovae, evolved stars.
O I, O II, O III spectral lines: diagnostic tools in astrophysical spectroscopy.
Fundamental Physical Issues
O₂ molecule: paradigmatic example of covalent bonding with molecular orbitals.
Magnetic properties (paramagnetism) described by MO theory.
Used in the study of stable isotopes to understand past climates (paleoclimatology).
Role in fundamental redox equilibria (high standard reduction potential).
