Oxygen

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

Chemical Properties[edit]

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.

Isotopes[edit]

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]

Compounds[edit]

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[edit]

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.

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