Oxygen (Z=8): The Element at the Heart of Life

An Element Essential to Life and Combustion

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).