# Question #f86bf

The carbide ion, ${\left(C \equiv C\right)}^{-} 2$, has a formal charge of $- 2$. The oxidation number of each $C$ is $- I$.
The sum of the oxidation numbers must add to the charge of the ion. Since the $C$'s have a $- I$ oxidation state, and the charge of acetylide is $- 2$, this condition is satisfied. Of course, $C a$ is an alkaline earth metal, and has an oxidation number of $+ I I$.
To expand on the question, calcium carbide is clearly the derivative of acetylene gas, $H - C \equiv C - H$. Oxidation numbers may be assigned when we break the bonds, with the charge going to the most electronegative atoms (here $C$). Can you give the oxidation numbers of $C$ for methane, ethane, and ethylene??