Taking the example of an element of atomic number $16$, explain how the electronic configuration of the atom of an element relates to its position in the modern periodic table and how valency of an element is calculated on the basis of its atomic number.

(N/A) The electronic configuration of an element with atomic number $16$ is $2, 8, 6$.
$1$. Period Number: The number of shells occupied by electrons determines the period number. Since this element has $3$ shells $(K, L, M)$, it belongs to the $3^{rd}$ period.
$2$. Group Number: For elements with more than $2$ valence electrons, the group number is calculated as $10 + \text{valence electrons}$. Here, the number of valence electrons is $6$, so the group number is $10 + 6 = 16$.
$3$. Valency: Valency is determined by the number of electrons required to complete the outermost shell (octet rule). For elements with $5, 6,$ or $7$ valence electrons, the valency is $8 - \text{valence electrons}$. Thus, the valency is $8 - 6 = 2$.