...contains roughly 78%
nitrogen, (normally
inert except upon
electrolysis by
lightning[1] and in certain biochemical processes of
nitrogen fixation), 21%
oxygen, 0.93%
argon, 0.04%
carbon dioxide, and trace amounts of
other gases, in addition to about 3%
water vapor.
http://en.wikipedia.org/wiki/Earth's_atmosphere
It is a valid question MYK.