# Why is that, in terms of electronegativity, a C-O bond in CO_2 is more polar than the F-F bond in F_2?

Because all isotopic fluorines of the same kind (e.g. $\text{^19 "F}$) are identical. The electronegativity of fluorine is equal to itself, therefore the electronegativity difference is $0$.
The electronegativity of carbon is about $2.5$, and that of oxygen is about $3.5$, and naturally, $\left(3.5 - 2.5 = 1.0\right) > 0$. So, the electronegativity difference is greater for a $\text{C"-"O}$ single bond than a $\text{F"-"F}$ single bond.