[george_smith70]

I know that the CN molecule exists in comets where pressure can be as low as 10^-10 mm Hg. I'm just wondering why this species can be in a comet but is unstable under normal terrestrial conditions?  Any explanations would greatly help. Thanks

[AWK]

CN is a free radical.

http://en.wikipedia.org/wiki/Radical_(chemistry)

[IITian]

In chemistry, radicals (often referred to as free radicals) are atoms, molecules or ions with unpaired electrons on an otherwise open shell configuration. These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions. Radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes, including human physiology. For example, superoxide and nitric oxide regulate many biological processes, such as controlling vascular tone. "Radical" and "free radical" are frequently used interchangeably, however a radical may be trapped within a solvent cage or be otherwise bound. The first organic free radical identified was triphenylmethyl radical, by Moses Gomberg in 1900 at the University of Michigan.

copied from WIKI