Which atom has the correct ground state electronic configuration?
A. ${\rm{Cl}}:\left[ {{\rm{Ne}}} \right]3{s^1}3{p^6}$
B. ${\rm{Mo}}:\left[ {{\rm{Kr}}} \right]5{s^1}4{d^5}$
C. ${\rm{Cu}}:\left[ {{\rm{Ar}}} \right]4{s^2}3{d^6}$
D. ${\rm{As}}:\left[ {{\rm{Ar}}} \right]4{s^1}4{d^{10}}4{p^3}$

Hint: We know that electronic configuration describes the distribution of electrons in atomic orbitals. Electronic configuration is represented by using the energy level, type of atomic orbital and number of electrons in the particular orbital.

Complete step by step answer:
To determine the electronic configuration, we need the atomic number of elements. In ground state, the number of protons and electrons is equal. The full filled or half filled orbitals are more stable than other electronic configurations. Let’s find the correct option.
> Option A is chlorine. The atomic number of chlorine is 17. So, the electronic configuration is chlorine is \(1{s^2}2{s^2}2{p^6}3{s^2}3{p^5}\) or \({\rm{Ne}}\left[ {3{s^2}s{p^5}} \right]\) But configuration of chlorine is given as ${\rm{Cl}}:\left[ {{\rm{Ne}}} \right]3{s^1}3{p^6}$ which is incorrect.
> Option B is Molybdenum. Atomic number of Molybdenum is 42. So, the electronic configuration is expected as $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}3{d^{10}}4{p^6}5{s^2}4{d^4}$ but it is not correct as half filled d or s orbital is more stable than 4d orbital filled with four electrons and completely filled 5s orbitals . So, the actual configuration is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}3{d^{10}}4{p^6}5{s^1}4{d^5}$ or $\left[ {{\rm{Kr}}} \right]4{d^5}5{s^1}$.
> Option C is copper. Atomic number of copper is 29. So, the number of electrons in copper is 29. Now, we write the electronic configuration. The electronic configuration of copper is expected as $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}3{d^9}$ or $\left[ {{\rm{Ar}}} \right]4{s^2}3{d^9}$. Electronic configuration of copper is more stable when d sublevel is completed and only one electron is required for that. So, excitation of electrons from 4s to 3d orbital takes place. So, electronic configuration copper is $\left[ {{\rm{Ar}}} \right]4{s^1}3{d^{10}}$. But electronic configuration of copper is given as ${\rm{Cu}}:\left[ {{\rm{Ar}}} \right]4{s^2}3{d^6}$ which is incorrect.
> Option D is arsenic. Atomic number of arsenic is 33. So, the electronic configuration is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}3{d^{10}}4{p^3}$ or \(\left[ {{\rm{Ar}}} \right]3{d^{10}}4{s^2}4{p^3}\)But electronic configuration of arsenic is given as ${\rm{As}}:\left[ {{\rm{Ar}}} \right]4{s^1}4{d^{10}}4{p^3}$ which is not correct.

Hence, correct option is B, that is, ${\rm{Mo}}:\left[ {{\rm{Kr}}} \right]5{s^1}4{d^5}$.

Note: Aufbau principle is used to determine electronic configuration of elements. According to this principle, the lower energy orbitals are to be filled first and then the higher energy orbitals are to be filled.