Which of the following arrangements represents the correct order of electronic energy level?

Analyze the question: You are asked for the order of increasing energy of atomic sublevels (the Aufbau order).

Concept: Electrons fill sublevels from the lowest energy to the highest. The order is decided by the (n + l) rule (Madelung rule):

• Lower n + l means lower energy.
• If two sublevels have the same n + l, the one with the lower n comes first.

Here l = 0 for s, 1 for p, 2 for d.

Calculate n + l:

• 1s: 1 + 0 = 1
• 2s: 2 + 0 = 2
• 2p: 2 + 1 = 3
• 3s: 3 + 0 = 3 (same as 2p, but n is higher, so it comes after 2p)
• 3p: 3 + 1 = 4
• 4s: 4 + 0 = 4 (same as 3p, but n is higher, so it comes after 3p)
• 3d: 3 + 2 = 5

Correct order: 1s < 2s < 2p < 3s < 3p < 4s < 3d

Check the options:

• A: 2p comes before 2s, which is wrong.
• B: 3d is placed before 4s, which is wrong because 4s fills first.
• C: 1s 2s 2p 3s 3p 4s 3d, which matches the correct order.
• D: 3s comes before 2p, which is wrong.

Tip: Remember the diagonal-rule sequence: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d... The 4s sublevel always fills before 3d. That is why potassium (19) is [Ar]4s¹, not 3d¹.