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What is Carbon Dioxide?

Carbon Dioxide
A model of carbon dioxide

Carbon dioxide is one of the simplest and commonest molecules in the universe. It has only three Atoms - one carbon and two oxygen atoms.

It's easy for the carbon atoms to join up with oxygen atoms because the outer shell (the valance shell) of the carbon atom has only four electrons in it, leaving room for four more before it is filled up. In the same way, the outer shell of an oxygen atom has only six electrons in it, leaving room for two more to make eight. When two oxygen atoms share their electrons with one carbon atom, all three of the atoms can fill up their shells - the carbon atom has four electrons of its own, plus four more that it shares with the oxygen atoms, and each oxygen atom has six electrons of its own, plus two more than it shares with the carbon atom. We call this a covalent bond.

There are carbon dioxide molecules out in space, where they formed in nebulae after the explosion of supernovas. All the carbon dioxide in space is very cold, so it is in the form of ice. When planets formed out of these nebulae, some of the carbon dioxide ice became part of the planets. There is carbon dioxide on Mars and the other planets, as well as on Earth.

Because Earth's surface is much warmer than space, most of the carbon dioxide on earth melted and became a gas - the molecules of carbon dioxide float in the air. Less than one percent of our air is carbon dioxide, but it is very important for all living things on Earth.

Forest fire
A forest fire puts carbon smoke into the air as the trees burn

Plants make their cells mostly out of carbon. The way plants get carbon is by breathing in carbon dioxide and breaking off the oxygen, which they then breathe out again. So the carbon in carbon dioxide is what all plants are made of, and the oxygen becomes the oxygen we breathe. When a plant dies and decays, or burns, the carbon in it returns to the air, where it mixes with oxygen to become carbon dioxide again.

In the last hundred years or so, carbon dioxide has become a big problem for all people on Earth. We have been burning so many hydrocarbons as gasoline for our cars, heating oil for our houses, or coal for our factories, that a lot more carbon than usual has gotten into the air, where it is making a lot more carbon dioxide than usual. This carbon dioxide is good for plants, but it also acts like a warm blanket around the Earth, trapping heat on the Earth instead of letting the heat go off into space. This is the main cause of global warming.

Learn by doing - Dry Ice

Bibliography and further reading about carbon dioxide:

Photosynthesis
Molecules
Electricity
Chemistry
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Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

Professor Carr's PSU page

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