Question

Krypton (Atomic number $36$) has the electronic configuration $\left[Ar\right]4s^{2}3d^{10}4p^6$. The $37$th electron will go into which one of the following sub-levels?

• A. $4f$
• B. $4d$
• C. $3p$
• D. $5s$

Answer 1

Answer: (D)

$5s$

The 37th electron will go into 5s sub-level.
According to aufbau principle, the correct order of energy of 4p and 5s-orbitals is $4p<5s$
Aufbau principle:
In the ground state of the atoms, the orbitals are filled with electrons in order of increasing energy.
The 4p sub-energy level is at a lower energy than the 5s sub-energy level
For 4p, $n=4$ and $l=1$. Hence, $(n+l)=4+1=5$
For 5s, $n=5$ and $l=0$. Hence, $(n+l)=5+0=5$
As the value of $(n+l)$ for 4p orbital is same as that of 5s orbital, 4p orbital is filled before 5s orbital as 4p orbital has lower value of n than 5s orbital.
Note: If two orbitals have same value of $(n+l)$, then the orbital with lower value of n will be filled first.