Which of the following subshells will fill first in the third energy level?

D

F

P

S

Answer: S is the subshell.

Explanation: 'n' is the principal quantum which explains the energy level. Electrons are filled in the order of their penetrating power and their closeness to the nucleus. Thus thr order of filling electrons according to Aufbau Principle is

s > p > d > f

Thus when electrons will be filled in the third energy level that is when the value of n = 3, then the first subshell that wold receive the electron will be S.

For eg the electronic configuration of Sodium is 1s2 2s2 2p6 3s1

S will fill first in the third energy level