The atmosphere is predominately made of nitrogen (N₂) that makes up ~78% of the atmosphere. The other three dominant gasses are oxygen (O₂), which makes up ~21% of the atmosphere, argon (Ar) at ~0.9%, and carbon dioxide (CO₂) at ~0.04% of the atmosphere. There are other gasses as well that make up a really small portion of the atmosphere (<<1%), and those areneon (Ne), helium (He), methane (CH₄), hydrogen (H₂), and krypton (Kr). Water vapor is also present in small amounts being ~0.4% on average and slightly higher over our oceans at ~1%.

All atmospheres are made out of layers of gas -- they divide into layers that have different pressures and composition.

On Earth, the lowest layer is where weather patterns occur. The air we breathe is almost 80% nitrogen and only 20% oxygen! Higher in the atmosphere the composition is different and there is the famous ozone layer which is very rich in oxygen. Overall, our atmosphere is made of nitrogen, oxygen and a very small percentage of other gases like argon, water vapour, carbon dioxide and helium.

Earth's atmosphere contains 78% nitrogen, 21% oxygen, 0.9% argon, and 0.03% carbon dioxide with very small percentages of other elements. Water vapor is also contained in the atmosphere.

The Earth's atmosphere is made of:
About 80% nitrogen
About 20% oxygen
About 1% water vapor (depends on where in the world you are)
About 0.04% carbon dioxide
Trace amounts of other gasses

Depending on where you are in the atmosphere, other gasses may also be important. For example ozone is important near the stratopause (the boundary between the stratosphere and mesosphere), but not on the ground.