[ReVeR]

Hello.
I was wondering why a polyatomic ion Cn has a charge of negative 1.. i mean does it randomle get 1 extra electron or why?
Thx

[jdurg]

Yes, actually.  The CN ion grabs an electron from something like a metal or hydrogen to get that negative charge.  That's why CN doesn't really exist, but NaCN, LiCN, KCN, RbCN, CsCN, HCN, etc. do exist.  The metal/hydrogen donates its electron to the CN ion forming M+CN-.

[Alberto_Kravina]

That's why CN doesn't really exist

Correct! But there is a compound that has the formula (CN)₂, it is called "cyanogen"  

[Borek]

I posted this information not long ago. CN^- is isoelectronic with very stable particle - N₂.

[ReVeR]

what does the mean? the isoelectronic with N2?

[Borek]

It has identical orbitals and identical number of electrons. N₂ is very stable particle, which means that its structure (in terms of electrons/orbitals) has very low energy. If you remove one proton you have an ion CN^- - it is less stable, but 'inherits' some of its stability from the N₂.

Note that this stability is present also in nitrosonium ion (NO⁺) - here we add one proton to particle. Nitrosonium is stable enough to allow preparation of salts - like nitrosonium perchlorate. But that's rather exotic particle.