Oxygen

Oxygen is the chemical element with atomic number 8 and the symbol O. It is a colorless, odorless gas that makes up about 21 percent of the atmosphere by volume, and it is the most abundant element in the Earth’s crust when its compounds are counted. Most living things use oxygen to release energy from food, and most combustion needs it as well.

History of oxygen

As early as 1608 the Dutch inventor Cornelius Drebbel showed that heating saltpeter, or potassium nitrate, released a gas. That gas was oxygen, although it was not recognized as a distinct substance. Credit for the discovery is now shared among chemists in three countries.

Carl Wilhelm Scheele, a Swedish pharmacist, produced oxygen in June 1771, but his account was not published until 1777. Joseph Priestley, an English clergyman and chemist, made it in 1774 by focusing sunlight on mercuric oxide and published first. He found that a candle burned more brightly in the gas and that it made breathing easier, and he called it dephlogisticated air. Scheele called his gas fire air.

The French chemist Antoine Lavoisier also claimed the discovery and gave the element its name, from the Greek words oxys, meaning acid, and genes, meaning forming. He mistakenly believed that oxygen was a part of all acids. The name stuck even though that idea was later shown to be wrong.

Properties of oxygen

Oxygen has a standard atomic weight of about 15.999, given by IUPAC as an interval from 15.99903 to 15.99977 because natural samples vary slightly. Its electron configuration is [He] 2s2 2p4, so an oxygen atom needs two more electrons to fill its outer shell. This gap makes oxygen highly reactive, and it combines with most other elements.

At room temperature, oxygen is a gas with a density of about 0.0013 grams per cubic centimeter. It condenses to a pale blue liquid at minus 183.0 degrees Celsius and freezes at minus 218.8 degrees Celsius. It is fairly soluble in water, which allows fish and other aquatic life to breathe in rivers, lakes and oceans. Oxygen belongs to group 16, called the chalcogens, and it also exists as ozone, a molecule of three oxygen atoms.

Occurrence and production of oxygen

Oxygen makes up 21 percent of the atmosphere by volume. This is about halfway between 17 percent, below which breathing becomes difficult for people who are not acclimatized, and 25 percent, above which many organic materials become highly flammable. The element and its compounds make up 49.2 percent of the Earth’s crust by mass, and about two-thirds of the human body, mostly in water, which is roughly nine-tenths oxygen by weight.

The Royal Society of Chemistry notes that oxygen first built up in the atmosphere around 2 billion years ago, from the photosynthesis of blue-green algae. It is the third most abundant element in the Sun. The atmosphere of Mars contains only about 0.15 percent oxygen, according to a reference summary in PubChem.

There are two main ways to produce oxygen gas. The first is the distillation of liquid air. The second is to pass clean, dry air through a zeolite, a porous mineral that traps nitrogen and lets oxygen through. A newer method gives higher purity by passing air over a partly permeable ceramic membrane. In the laboratory, oxygen can be made by electrolysis of water, or by adding a manganese(IV) oxide catalyst to hydrogen peroxide solution.

PubChem records a first ionization energy of 13.618 electronvolts, a covalent radius of about 66 picometers and a van der Waals radius of 152 picometers for oxygen. In the ocean, oxygen is overwhelmingly present as part of water molecules, so its concentration in seawater, around 857,000 milligrams per liter in combined form, is far higher than that of any other element there. Oxygen in excited states also produces the bright red and yellow-green colors of the aurora borealis.

Uses of oxygen

The greatest commercial use of oxygen gas is in the steel industry, where streams of high-pressure oxygen burn away impurities in molten pig iron to produce steel. Large amounts also go into making chemicals, including nitric acid, hydrogen peroxide, ethylene oxide, which is used to make antifreeze and polyester, and chloroethene, the precursor of PVC.

Oxygen is combined with acetylene for oxy-acetylene welding and cutting of metals, because the flame is extremely hot. A growing use is in treating sewage and industrial effluent. Liquid oxygen combined with liquid hydrogen is an excellent rocket propellant. Hospitals use oxygen for patients with respiratory problems, and its use in medicine is directed by clinicians.

Liquid oxygen is a pale substance stored at very low temperatures in insulated tanks, and its low boiling point is why hospitals and industrial plants depend on regular deliveries or on-site separation plants. Because oxygen supports the burning of nearly anything flammable, equipment for oxygen must be kept clean of oil and grease.

Isotopes of oxygen

Oxygen has three stable isotopes: oxygen-16, oxygen-17 and oxygen-18. IUPAC data give their abundances as about 99.757 percent, 0.038 percent and 0.205 percent. Oxygen-16 dominates, which sets the element’s atomic weight close to 16. Scientists compare the ratio of oxygen-18 to oxygen-16 in water, ice and minerals to study past climates, because the ratio shifts with temperature.

Oxygen in life and the environment

Plants, algae and some bacteria release oxygen by photosynthesis, which uses sunlight to split water, while animals and plants take it in for respiration to get energy. Oxygen returns to the atmosphere in the form of carbon dioxide when this energy is released from food. Ozone, formed in the upper atmosphere, shields Earth’s surface from much of the Sun’s ultraviolet radiation.

Oxygen is not toxic at the levels in the air, but high-oxygen atmospheres increase fire risk, and pure oxygen supports vigorous burning. Industrial handling follows strict safety rules for that reason, and the information here is general background rather than advice.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue