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Atomic symbol O
Atomic number 8
Classification Non-metal
Atomic mass 15.994 amu
Other Information
Date of discovery 1774
Name of discoverer Joseph Priestley
Name origin From the Greek words oxus (acid) and gennan (generate)
Uses Supports life
Obtained from From liquid air

Oxygen is the third most abundant element in the universe by mass and is essential to life on Earth.

Chemical Properties

At normal atmospheric pressure oxygen melts at 54.36 Kelvin (K) and boils at 90.2 K; on Earth it essentially exists only as a gas. It readily dissolves in water, which makes aquatic life possible. Under normal conditions oxygen forms molecules consisting of two oxygen atoms each sharing two electrons in the 2p subshell; this form of oxygen is known as O2. O2 is a colorless, odourless gas. In liquid form (below -193 degrees Celsius) it is pale blue.


There are three stable and 14 radioactive isotopes of oxygen. The stable isotopes are 16O, 17O and 18O, with 16O being by far the most common. Radioisotopes range from 10O to 26O, with the two most stable being 15O and 14O. This gives oxygen an atomic weight of slightly below 16.[1]


Oxygen is highly reactive. It reacts readily with most metals and many other elements, either through combustion or oxidation. As a result, many naturally occurring compounds contain oxygen.

Occurrence in Nature

Oxygen is the third most abundant element in the universe by mass after hydrogen and helium; approximately 5.9% of the mass of our solar system consists of oxygen. However, due to the large disparity in atomic weight between all isotopes of oxygen compared with 1H and 4He, less than 0.5% of the atoms in the solar system are oxygen atoms.[Citation Needed]

Due to its reactivity free oxygen is extremely rare in nature and would not exist on Earth without the process of photosynthesis. Geologists and paleontologists believe that free oxygen first appeared during the "Great Oxygenation Event" around 2.4 billion years ago. Young-Earth creationists reject this timescale.