The Earth's atmosphere is a relatively thin blanket of gasses surrounding the planet's surface, averaging a thickness of only seven miles. It is divided into four layers: the troposphere, stratosphere, mesosphere and thermosphere. These layers contain a number of gasses, two in abundance and several others in minuscule amounts.
Nitrogen makes up 78% of our atmosphere. It is an inert gas and essentially just fills up the space not used by more active gasses.
Oxygen makes up another 20 to 21 percent of our atmosphere. It's essential for life on Earth, and interestingly enough, it is poisonous when highly concentrated. Our concentration of 20 to 21 percent seems just right.
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The other 1 to 2 percent of our atmosphere is made up of various trace gasses, including the following:
Argon - 0.93 percent Carbon Dioxide - 0.036 percent Neon - 0.00182 percent Helium - 0.000524 percent Methane - 0.00015 percent Krypton - 0.000114 percent Hydrogen - 0.00005 percent