# Question #0442e

$N a$ has 11 electrons and 11 protons thus it has the less effect on the outermost electron cloud.
$M g$ has 12 electrons and 12 protons, here there is an increase in proton number in the nucleus without any increase in the orbitals. the nucleus of $M g$ exerts more attractive force than $N a$ and thus the size of $M g$ is smaller than $N a$.