In which periods are $Na$ and $F$ located? What is the order of their atomic radii and why?

(A) $F$ is in the $2^{nd}$ period and $Na$ is in the $3^{rd}$ period. The order of atomic radii is $Na > F$. This is because $Na$ has electrons in the $3^{rd}$ shell $(n=3)$,whereas $F$ has electrons only up to the $2^{nd}$ shell $(n=2)$. As the number of shells increases,the atomic radius increases.