An element $X$ has a mass number of $35$ and the number of neutrons is $18$. Determine the atomic number and electronic configuration of $X$. Also, identify the group number, period number, and valency of $X$.

(N/A) The atomic number of an element is calculated as: Atomic number = Mass number - Number of neutrons.
Atomic number of $X = 35 - 18 = 17$.
The element with atomic number $17$ is Chlorine $(Cl)$.
The electronic configuration is determined by distributing $17$ electrons in shells: $2, 8, 7$.
Since the valence shell has $7$ electrons, the group number is $10 + 7 = 17$.
The number of occupied shells is $3$, so the period number is $3$.
The valency is calculated as $8 - (\text{valence electrons}) = 8 - 7 = 1$.