- Oxygen (O) is a nonmetal with atomic number 8, positioned in Group 16 of the periodic table, known as the chalcogens.
- It has six valence electrons, enabling it to readily form covalent and ionic bonds, and it commonly exhibits oxidation states of –2, –1, 0, and +2.
- Its atomic structure consists of eight protons, typically eight neutrons, and eight electrons arranged in two shells (two in the inner shell and six in the outer shell).
- Oxygen occurs naturally as three stable isotopes: oxygen-16 (¹⁶O), which makes up about 99.76% of natural oxygen; oxygen-17 (¹⁷O), comprising 0.04%; and oxygen-18 (¹⁸O), about 0.20%.
- Under standard conditions, oxygen exists primarily as a diatomic gas (O₂), a colorless, odorless, tasteless substance essential for aerobic life.
- Another allotrope, ozone (O₃), is a pale blue gas with strong oxidizing properties and a crucial role in absorbing harmful ultraviolet radiation in the stratosphere.
- Oxygen is the third most abundant element in the universe by mass, after hydrogen and helium, produced in large quantities within massive stars through stellar nucleosynthesis via the triple-alpha process and subsequent fusion reactions.
- On Earth, oxygen is the most abundant element in the crust and oceans, making up nearly half of the crust’s mass and one-third of the planet’s mass overall, mostly in the form of oxides, silicates, and water. Free molecular oxygen (O₂) became a significant component of the atmosphere about 2.4 billion years ago during the Great Oxidation Event, largely due to photosynthetic microorganisms such as cyanobacteria. Today, oxygen constitutes about 20.95% of Earth’s atmosphere by volume.
- Oxygen’s identification as a distinct element in the late 18th century marked a turning point in chemistry. Swedish chemist Carl Wilhelm Scheele is believed to have produced oxygen as early as 1772, calling it “fire air,” but his results were published later. English chemist Joseph Priestley independently discovered it in 1774 by heating mercuric oxide, referring to it as “dephlogisticated air.” French chemist Antoine Lavoisier recognized it as a key element in combustion and respiration, naming it “oxygène” (from Greek oxys, acid, and genes, producer), reflecting the then-mistaken belief that it was essential to all acids.
- Oxygen is fundamental to life and many geological and chemical processes. In biology, it is the final electron acceptor in aerobic respiration, enabling organisms to extract energy from organic molecules efficiently. Plants, algae, and cyanobacteria produce oxygen as a byproduct of photosynthesis, sustaining the atmospheric O₂ balance. Industrially, oxygen is produced on a large scale by fractional distillation of liquefied air or by electrolysis of water. It is used in steelmaking, chemical manufacturing, wastewater treatment, and as an oxidizer in rocket propulsion. In medicine, oxygen therapy is vital for patients with respiratory conditions, while hyperbaric oxygen treatment is used for decompression sickness and certain wounds.
- Ozone (O₃), an allotrope of oxygen, plays a dual role: in the stratosphere, it protects life from ultraviolet radiation; at ground level, it is a pollutant that can damage respiratory health and vegetation. Oxygen’s strong oxidizing nature makes it highly reactive, supporting combustion and corrosion. While essential for life, excessive oxygen exposure at high pressures or concentrations can cause oxygen toxicity, affecting the central nervous system and lungs. Similarly, reactive oxygen species (ROS) generated in cells can damage DNA, proteins, and lipids, linking oxygen metabolism to aging and certain diseases.
