Here are four different ways chemists use to show a molecule of oxygen. In colored molecule models like these, oxygen is usually red. Click on image for full size (20 Kb GIF) 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!