What is the electronic configuration of elements having atomic number $17$?

Hint:Atomic number is the number of protons present in the nucleus and it is denoted by $Z$. It is also known as nuclear charge.

Complete step by step answer:
The electronic configuration of an element can be written by making use of following quantum numbers:
A.Principal quantum number ($n$) $ \to $ shell
It is given by Bohr. It represents the name, size, and energy of the shell to which the electron belongs.
The value of $n$ lies between $1$ to $\infty$ .
Greater the value of $n$, greater is the distance from the nucleus.
Greater the value of $n$, the greater is the energy of the shell.

B.Azimuthal quantum number ($l$) $ \to $ given by Sommerfeld
Represents the value of subshell, orbital and orbital angular momentum.
Value of $l$ lies between $0$to $(n - 1) $. Example: if $n$ = $1$ then $l$ = $0$ which means $1s$.
If the value of $n$ is the same, then the order of energy will be $s < p < d < f$.

C.Magnetic quantum number ($m$) $ \to $ given by linde.
Represents the shape of different orbitals and orientation of electron clouds(orbital).
Value of $m$ lies from $ - l$ to $ + l$.

D.Spin quantum number ($s$) $ \to $ given by Goudsmit and Uhlenback.
Represents the direction of electron spin around its own axis
For clockwise spin or spin up electron $ \pm \dfrac{1}{2}$
For anti- clockwise or spin down electron $ \mp \dfrac{1}{2}$
Thus, the above quantum number helps us in writing down the electronic configuration of an element. The element having atomic number is Chlorine so its electronic configuration will be $1{s^2}2{s^2}2{p^6}3{s^2}3{p^5} $.

Note:
The electronic configuration of an element is always written in increasing order of energy.
For neutral atom: the number of proton is equal to number of electron
For charged atom: the number of proton = $Z$$ - $ (charge on atom)
where $Z$ is the number of protons only.