Give reasons for the following:
$(a)$ Lithium atom is smaller than Sodium atom.
$(b)$ Chlorine (Atomic Number $17$) is more electronegative than Sulphur (Atomic Number $16$).

(N/A) The electronic configuration of $Li$ $(Z=3)$ is $2, 1$,while that of $Na$ $(Z=11)$ is $2, 8, 1$. Lithium has $2$ shells,whereas Sodium has $3$ shells. As the number of shells increases down the group,the atomic size increases. Therefore,the Lithium atom is smaller than the Sodium atom.
$(b)$ The electronic configuration of $Cl$ $(Z=17)$ is $2, 8, 7$ and that of $S$ $(Z=16)$ is $2, 8, 6$. Electronegativity depends on the effective nuclear charge and atomic size. Chlorine has a smaller atomic size and a higher effective nuclear charge compared to Sulphur. Consequently,Chlorine has a greater tendency to attract shared electrons,making it more electronegative than Sulphur.