Which of the following arrangements represents the correct order of electronic energy level?
1s 2p 2s 3p 3s 3d 4s
1s 2s 2p 3s 3p 3d 4s
1s 2s 2p 3s 3p 4s 3d
1s 2s 3s 2p 3p 4s 3d
Correct answer is C
The correct order of electronic energy level is OPTION C according to the Aufbau principle.