Which of the two elements given below would be chemically more reactive: $X$ of atomic number $18$ or element $Z$ of atomic number $16$? And why?

(B) Element $Z$,having atomic number $16$,is chemically more reactive than element $X$ of atomic number $18$.
$1$. The electronic configuration of element $X$ (atomic number $18$) is $2, 8, 8$. Its outermost shell is completely filled,meaning it has a stable octet configuration. Therefore,it is chemically inert (a noble gas).
$2$. The electronic configuration of element $Z$ (atomic number $16$) is $2, 8, 6$. Its outermost shell contains $6$ electrons. To achieve a stable octet,it needs to gain $2$ electrons. Because its valence shell is incomplete,it is chemically reactive.