# For an atom that becomes an ion, does it have a formal charge, or net charge? What is the difference between formal charge, oxidation state, and net charge?

Jul 21, 2016

For an atom that just became an ion, like ${\text{O}}^{2 -}$, that is not its formal charge, but it is its net charge.

Formal charge is the hypothetical charge for an atom when assuming that electrons are perfectly evenly shared in a chemical bond.

It's not the same as an oxidation state; it's pretty much the complete opposite.

• Strictly speaking, ${\text{O}}^{2 -}$ doesn't have a formal charge because it's not within the context of a molecule.
• In ${\text{CO}}_{2}$, we have $: \stackrel{. .}{\text{O"="C"=stackrel(..)"O}} :$. The formal charge on $\text{C}$ is $0$, and the formal charge on each $\text{O}$ is $0$. We can say that because the atoms are in the context of a molecule.

Oxidation state is the hypothetical charge for an atom when assuming that electrons would be completely transferred from one atom to another in a chemical bond.

• If you look at a lone atom, its charge is its oxidation state. The oxidation state of the ${\text{O}}^{2 -}$ anion is $\textcolor{b l u e}{- 2}$, the same as its charge, $2 -$.
• The oxidation state of $\text{O}$ in ${\text{CO}}_{2}$ would be $- 2$, so the oxidation state of $\text{C}$ in ${\text{CO}}_{2}$ would be $+ 4$. We write that as $\stackrel{+ 4}{{\text{C")stackrel(-2)("O}}_{2}}$. It does not mean that carbon has given away all of its $n = 2$ valence electrons.

Net charge is simply the total charge on an atom or molecule or compound.

• For ${\text{O}}^{2 -}$, it's only one atom, so its net charge is its actual charge, $\textcolor{b l u e}{2 -}$.
• For $: \stackrel{{\delta}^{-}}{\text{C")-=stackrel(delta^(+))("O}} :$, the net charge is $0$, because the formal charge on $\text{C}$ is $- 1$, and the formal charge on $\text{O}$ is $+ 1$, so $- 1 + 1 = 0$.