Continuing with the chemistry series:
Nitrogen is fourth on the list of the most abundant chemically active elements in the solar system. Helium and Neon are more abundant, but for all practical purposes, neither of these form compounds. Neon is light enough that most of it has escaped the earth's atmoshere, but significant amounts are found in the sun and the atmospheres of gas giants.
One of the more common forms of nitrogen is the diatomic N2 gas, which is unusually stable, and which combines with other elements only with some difficulty. It's also present, though only to the amount of a few percent, in the atmospheres of Venus and Mars, and it's the predominant gas in the atmosphere of Titan, one of Saturn's moons. I'll discuss the compounds later.