Good question, Samuel.
The air around us is made of several different gases. Mostly
nitrogen (78%) and some oxygen (21%) and traces other gases like argon,
water, carbon dioxide, ozone, and more.
These gases weren't always in our atmosphere, and they came
from all sorts of different places. Scientists believe that most of the
nitrogen in the air was carried out from deep inside the earth by
The nitrogen molecule is heavier than most other molecules in
the atmosphere, so it tends to settle towards the bottom. Other lighter
gases, like hydrogen or helium, would tend to the top of the
atmosplere, where it is more likely to be lost to space. So the heavier
gases like nitrogen and oxygen tend to stick around, since they are at
the bottom of the atmosphere.
I hope this answers your question.
(republished on 08/01/06)