2. structure of atom — Chemistry STD 11 Science — Question

According to aufbau principle, the correct order of energy of $4 p$ and $5 s$-orbitals is $4 p \,<\,5 s$

Aufbau principle:

In the ground state of the atoms, the orbitals are filled with electrons in order of increasing energy.

The $4 p$ sub-energy level is at a lower energy than the $5$ sub-energy level

For $4 p , n =4$ and $1=1$. Hence, $( n +1)=4+1=5$

For $5 s , n =5$ and $1=0$. Hence, $( n +1)=5+0=5$

As the value of $( n +1)$ for $4 p$ orbital is same as that of $5 s$ orbital, $4 p$ orbital is filled before $5$ orbital as $4 p$ orbital has lower value of $n$ than $5 s$ orbital.