Here are four different ways chemists use to show a molecule of oxygen. In colored molecule models like these, oxygen is usually red.
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Windows to the Universe original artwork by Randy Russell.


Oxygen (O2) is a kind of gas. A lot of the air you breathe is oxygen. That's a good thing, since we need oxygen to stay alive!

About 4/5ths of the air in Earth's atmosphere is nitrogen (N2). Almost all of the rest of air is oxygen.

Normal oxygen molecules have two oxygen atoms in them. There is another, less common kind of oxygen. It is called ozone. Ozone molecules (O3) have three oxygen atoms in them.

Many kinds of molecules have oxygen atoms in them. Some of the most important ones are water (H2O), carbon dioxide (CO2), carbon monoxide (CO), sulfur oxides (SO2 and SO3), and nitrogen oxides (NO and NO2).

Most living creatures need oxygen to get energy from food. This is called respiration. That's why we need to breathe oxygen! Plants and some kinds of microbes make oxygen. They use sunlight to make food. That is called photosynthesis. They give off oxygen during photosynthesis.

When Earth was young, its atmosphere didn't have much oxygen. Then microbes and plants started adding oxygen to the air. Now we have quite a bit of oxygen in the air. Lucky for us!

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