What kind of carbon ions exist?

I think there are $\ce{C^{4-}}$ and $\ce{C^{2-}}$ existing. How can we find the charge of ions in the elements? Should we look at the group or valence?




Many different carbon ions exist in the gas phase. 


In [Electronic spectra of linear carbon anions](http://www.sciencedirect.com/science/article/pii/S0301010497003583), spectra of $\ce{C\_2^-}$, $\ce{C\_3^-}$... $\ce{C\_10^-}$ are discussed. 


Dianions such as $\ce{C\_7^{2-}}$ have also been observed.


In the solid phase, aluminum carbide and berylium carbide have more than 50% ionic character according to the Handbook of Refractory carbides and nitrides, and correspond to $\ce{C^{4-}}$. Calcium carbide, which could be considered a salt of acetylene, corresponds to $\ce{C2^{2-}}$ rather than $\ce{C^{2-}}$ 


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