The value of the screening constant is estimated to be $S=4.15$.
Work Step by Step
*Slater's rules state that - Electrons for which $n$ is larger than $n$ of the electrons of interest contribute 0 to $S$. - Electrons with the same value of $n$ as the electrons of interest contribute 0.35 to $S$. - Electrons with value $n-1$ contribute 0.85 to $S$. - Electrons with even smaller values of $n$ contribute 1.00 to $S$. Both $F^-$ and $Na^+$ have the same electron configuration $1s^22s^22p^6$. That means with an electron with $n=2$, - 2 electrons in $n=1$ each contributes 0.85 to $S$. - 8 electrons in $n=2$, but only 7 each contributes 0.35 to $S$. Therefore, the value of the screening constant is $$S=2\times0.85+7\times0.35=4.15$$