Examples of gases include oxygen, nitrogen, helium, hydrogen, argon, fluorine, krypton, neon, radon, xenon and chlorine. These gases are single elements, though many gases are compounds or mixtures. Such gases include carbon dioxide, carbon monoxide, methane, air, ammonia, formaldehyde, ozone, nitrous oxide and natural gas.
Gases can be both crucial and harmful to life. There would be very little life on Earth without air, a gas made of nitrogen, oxygen, carbon dioxide, argon and other gases present in trace amounts. Pain-relieving gases, such as nitrous oxide, made modern medicine possible by allowing patients to undergo long and complex operations. Natural gas, which is a mixture of methane, alkanes and other gases, is used for heating and making fertilizer. Natural gas is also beginning to replace gasoline and diesel as fuels used in transportation.
The noble gases, consisting of helium, radon, argon, krypton, neon and xenon, don't form compounds easily because their outer shells contain all the electrons they can possibly have. However, they can glow in vivid colors, making them useful for neon signs. They are also used for less frivolous reasons, including anesthesia and radiation therapy.
One of these noble gases, radon, is radioactive and dangerous to breathe. Some other dangerous gases are chlorine, formaldehyde, fluorine and carbon monoxide.