# Which is the bigger ion, F- or O-?

Well, according to the proton-electron ratio $\ce{O-}$ should be bigger than $\ce{F-}$

What about the charge/electron density in $\ce{F}$? Will it not affect the size of the atom of $\ce{F-}$?

• I think $\ce{O-}$ is the bigger guy. Since $\ce{F-}$ have a complete octet and also have more protons than $\ce{O-}$. – ashu Sep 7 '14 at 15:35
• @Siddharth Yadav, What about the charge/electron density in O? – RBW Sep 7 '14 at 18:46

## 1 Answer

definitely $\ce{O-}$

In $\ce{O-}$ 7 outer electrons are tied to nuclei with 6 positive charges (number 8 minus 2 electons of inner shell), while in $\ce{F-}$ 8 outer electrons are tied to nuclei with 7 charges. The difference, however, should be quite small.