Question

Krypton (atomic number 36) has the electronic configuration $$[Ar]4s^{2}3d^{10}4p^6$$ the 37th electron will go into which one of the following sublevels:-
(A) 4f
(B) 4d
(C) 3p
(D) 5s

Answer 1

Hint: We will explain this question from “Aufbau principle”. According to the Aufbau rule, the 37th electron will go into a higher energy level after '4p' sublevel.

Complete answer:
The atomic number of the given atom kr is 36; its electronic configuration is $$[Ar]4s^{2}3d^{10}4p^6$$.
Where Ar is a Noble gas whose atomic number is 18.
From the electronic configuration, we can understand that all sub-level up to 4p orbitals are fulfilled by thirty six electrons.
According to Aufbau rule in the ground state for a multi-electron system electrons first occupy lowest available energy levels completely before occupying higher levels.
So the increasing order of sub-level based on energy will be-
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d.
Where the numbers 1,2,3,4,5 are principal quantum numbers indicated as n and s,p,d,f etc are sublevels under principal quantum number.
Now, the answer will be- when the 37th electron will go into the 'kr' atom, it will occupy '5s' sublevel as after '4p' ,'5s' this is the next higher energy level.

So the correct answer is (D).

Note: Almost all elements do follow Aufbau principle especially elements of lower atomic number.
2. Ruthenium $$(Rh)$$, Rhodium $$(Rh)$$, Silver$$(Ag)$$, and platinum (Pt) of higher atomic number elements as well as elements of lower atomic number copper (29) and chromium (24) do not follow the Aufbau rule.