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Warnings: The True Story of How Science Tamed the Weather by Mike Smith tells the story of our storm warning system. See our online store book collection.
Shown here are four representations chemists use for molecular oxygen. In colored molecular models, oxygen is traditionally shown in red.
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Windows to the Universe original artwork by Randy Russell.


Oxygen is a chemical element with an atomic number of 8 (it has eight protons in its nucleus). Oxygen forms a chemical compound (O2) of two atoms which is a colorless gas at normal temperatures and pressures.

Oxygen is very reactive, so oxygen atoms are incorporated into many common chemical compounds, such as water (H2O), carbon dioxide (CO2), carbon monoxide (CO), sulfur oxides (SO2 and SO3), and nitrogen oxides (NO and NO2).

About 20% of Earth's atmosphere is oxygen. This hasn't always been the case, though. Early in our planet's history, the atmosphere had almost no oxygen. Microbes that produce their food via photosynthesis generate oxygen as a byproduct. Oxygen from photosynthetic microbes eventually built up in the atmosphere, drastically changing our planet's environment and the history of life in the process.

Oxygen plays a critical role in respiration, the energy-producing chemistry that drives the metabolisms of most living things. We humans, along with many other creatures, need oxygen in the air we breath to stay alive. Oxygen is generated during photosynthesis by plants and many types of microbes.

Oxygen can also form a molecule of three atoms, which is known as ozone (O3). Ozone in Earth's stratosphere plays a helpful role by blocking most of the harmful UV radiation from the Sun, while ozone in the troposphere is a hazardous pollutant.

Last modified February 8, 2006 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA