What Is Oxygen?

Oxygen is a chemical element with the symbol O and atomic number 8, classified as a nonmetal. It is the third most abundant element in the universe by mass (after hydrogen and helium) and by far the most abundant element in Earth's crust, where it makes up nearly half of all mass when bound up in rocks, minerals, and water.

At room temperature, oxygen exists as a colorless, odorless diatomic gas (O₂) that makes up about 21% of the air we breathe — a concentration that's remarkably stable thanks to the ongoing balance between photosynthesis (which produces oxygen) and respiration and combustion (which consume it). Without this continuous biological replenishment, atmospheric oxygen would have been consumed long ago.

Oxygen sits in Period 2, Group 16 of the periodic table — the chalcogen group — alongside sulfur, selenium, and tellurium. Its electron configuration of [He] 2s² 2p⁴ leaves it two electrons short of a stable full outer shell, which is exactly why oxygen reacts so readily with most other elements, forming oxides.

Physical & Chemical Properties

Oxygen has an atomic mass of 15.999 u and an extremely low density of 0.001308 g/cm³ in its gaseous state. It has a melting point of -218.8°C and a boiling point of -183°C, meaning it only becomes liquid or solid under cryogenic conditions far colder than anything occurring naturally at Earth's surface.

Oxygen is one of the most reactive nonmetals on the periodic table, readily forming bonds with nearly every other element to create oxides. This reactivity is the chemical basis of combustion — fire is, at its core, a rapid oxidation reaction releasing energy as heat and light. Oxygen also exists in a second important form called ozone (O₃), a three-atom molecule that behaves very differently from ordinary O₂ and forms a protective layer in Earth's upper atmosphere that blocks harmful ultraviolet radiation.

Electron Configuration Explained

Oxygen's electron configuration is [He] 2s² 2p⁴, meaning it has the same stable two-electron core as helium, plus six additional electrons distributed across the 2s and 2p subshells of its second shell.

With four electrons in the 2p subshell (which can hold up to six), oxygen needs exactly two more electrons to achieve a full, stable outer shell matching the noble gas neon. This "two electrons short" configuration explains oxygen's strong tendency to either gain two electrons outright (forming the O²⁻ ion seen in compounds like metal oxides) or to share electrons through two covalent bonds, as it does in water and carbon dioxide.

This electron-hungry behavior is also why oxygen molecules naturally pair up as O₂ rather than existing as single atoms — two oxygen atoms can share electrons with each other through a double covalent bond, each satisfying the other's need for additional electrons.

History & Discovery

Oxygen was independently discovered by several scientists in the 1770s. English chemist Joseph Priestley is generally credited with isolating it first in 1774 by heating mercury oxide, though Swedish chemist Carl Wilhelm Scheele had produced it slightly earlier without immediately publishing his results. French chemist Antoine Lavoisier later conducted systematic experiments that correctly identified oxygen's role in combustion and respiration, and gave the element its name — derived from Greek words meaning "acid-former," based on the mistaken belief at the time that all acids contained oxygen.

Lavoisier's work on oxygen was foundational to the entire field of modern chemistry, helping overturn the previously dominant (and incorrect) "phlogiston theory" of combustion, which held that burning materials released a fire-like substance rather than reacting with a gas in the surrounding air.

What Is Oxygen Used For?

Oxygen's reactivity and biological necessity make it essential across medicine, industry, and everyday life:

  • Respiration: Virtually all complex life on Earth depends on oxygen to extract energy from food through cellular respiration — humans breathe roughly 11,000 liters of air, and therefore around 2,300 liters of oxygen, every single day.
  • Medical oxygen therapy: Supplemental oxygen is administered to patients with breathing difficulties, during surgery, and in emergency medicine, typically delivered from pressurized tanks or concentrators.
  • Steel manufacturing: Pure oxygen is blasted into molten iron in the basic oxygen process to remove impurities like carbon, a critical step in modern steel production.
  • Rocket propulsion: Liquid oxygen serves as the oxidizer in most major rocket engines, combined with fuels like liquid hydrogen or kerosene to produce thrust.
  • Water treatment: Oxygen is used in various water purification processes, including helping break down organic pollutants in wastewater treatment.

Common Oxygen Compounds

Oxygen forms more compounds than almost any other element, given its reactivity with nearly everything:

  • Water (H₂O): Earth's most essential compound, with oxygen bonded to two hydrogen atoms, covering about 71% of the planet's surface.
  • Carbon dioxide (CO₂): Produced by respiration and combustion, and absorbed by plants during photosynthesis — a gas central to both the carbon cycle and modern climate science.
  • Metal oxides: Compounds like iron oxide (rust, Fe₂O₃) and aluminum oxide form when metals react with atmospheric oxygen, a process responsible for both corrosion and several important industrial materials.
  • Ozone (O₃): A reactive three-oxygen molecule that forms a protective UV-blocking layer in the stratosphere, though it's also a harmful pollutant at ground level.

Fun Facts About Oxygen

  • Oxygen makes up about 65% of the mass of the human body, almost entirely as part of water molecules within cells and tissues.
  • Liquid oxygen is pale blue in color, a fact rarely seen since oxygen is normally encountered only as a colorless gas.
  • Photosynthesis by plants, algae, and cyanobacteria produces the vast majority of atmospheric oxygen, with an estimated 50-80% coming from ocean-dwelling marine phytoplankton rather than land plants.
  • Earth's atmosphere wasn't always oxygen-rich — early in the planet's history, oxygen levels were extremely low until photosynthetic organisms gradually transformed the atmosphere over roughly two billion years.

Frequently Asked Questions

Why is oxygen essential for life?
Most complex organisms use oxygen in cellular respiration, a process that efficiently extracts energy from food. Without oxygen, cells must rely on far less efficient anaerobic processes, which most animal life cannot sustain for long.

Is oxygen flammable?
Oxygen itself doesn't burn, but it's essential for combustion to occur — it's the oxidizer that allows fuels to burn. Higher oxygen concentrations make fires burn hotter and faster, which is why pure oxygen environments are handled with extreme caution.

What is oxygen's atomic number?
Oxygen has atomic number 8, meaning each atom contains 8 protons in its nucleus.

What is the difference between oxygen and ozone?
Ordinary oxygen gas (O₂) consists of two bonded oxygen atoms, while ozone (O₃) consists of three. Ozone is far more reactive and, at ground level, is considered a pollutant, while in the upper atmosphere it serves a protective function against UV radiation.