Oxygen

Oxygen is a chemical element that almost every living thing on Earth needs to stay alive. It is a gas at room temperature. It has no color and no smell. Oxygen makes up about 21 percent of the air you breathe. Nitrogen makes up most of the rest.

Oxygen is element number 8 on the periodic table. Its chemical symbol is O. An oxygen atom has 8 protons in its center. In the air, oxygen atoms usually travel in pairs. Two atoms stuck together form a molecule called O2. When three atoms stick together, they form ozone, O3. Ozone high in the atmosphere shields Earth from harmful rays from the Sun.

Animals use oxygen to get energy from food. When you breathe in, oxygen moves from your lungs into your blood. Your blood carries it to every cell in your body. Inside each cell, oxygen helps break apart sugar from the food you ate. This process releases energy and makes carbon dioxide as waste. You breathe the carbon dioxide back out.

Plants do the opposite during the day. They pull carbon dioxide from the air and use sunlight to make sugar. Oxygen is left over, so plants release it back into the air. This process is called photosynthesis. Almost every bit of oxygen on Earth was made this way. Much of it comes from tiny algae floating in the oceans, not just from trees.

Earth did not always have oxygen in its air. For the planet's first two billion years, the air had almost none. Then tiny bacteria called cyanobacteria learned to do photosynthesis. Slowly, over hundreds of millions of years, they filled the air with oxygen. Scientists call this the Great Oxidation Event. It poisoned many early life forms, but it also made animals possible later on.

Oxygen is very reactive, which means it likes to join with other elements. When iron meets oxygen and water, it forms rust. When wood burns, it is joining quickly with oxygen and giving off heat and light. Fire cannot burn without oxygen. That is why a cup placed over a candle puts the flame out.

Humans use oxygen for many jobs. Doctors give it to patients who have trouble breathing. Rockets carry tanks of liquid oxygen to help their fuel burn in space, where there is no air. Divers and mountain climbers carry oxygen tanks when they go where the air is thin or missing.