Which of the following is/are not correct with respect to energy of atomic orbitals of hydrogen atom?
(A) 1s < 2p < 3d < 4s
(B) 1s < 2s = 2p < 3s = 3p
(C) 1s < 2s < 2p < 3s < 3p
(D) 1s < 2s < 4s < 3d
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Hydrogen has only one electron, so its orbital energy depends on n alone and l does not matter.

1. One electron rule. Hydrogen has a single electron, so no electron shields another. Orbital energy is Eₙ = -13.6/n² eV, which depends only on n. So all orbitals with the same n have the same energy: 2s = 2p, and 3s = 3p = 3d.

   Statement	What it says	Verdict for hydrogen
   (A) 1s < 2p < 3d < 4s	n goes 1 < 2 < 3 < 4	correct
   (B) 1s < 2s = 2p < 3s = 3p	orbitals with the same n are equal	correct
   (C) 1s < 2s < 2p < 3s < 3p	puts 2s < 2p and 3s < 3p	not correct, the same n must give equal energy
   (D) 1s < 2s < 4s < 3d	puts n = 4 below n = 3	not correct, 3d < 4s in hydrogen

2. Source of the confusion. Orders like 2s < 2p and 4s < 3d come from the (n + l) rule. That rule is for multi-electron atoms, where shielding splits orbitals of the same shell. It does not apply to hydrogen.
3. So. The incorrect statements are (C) and (D), which is option (d).

The idea to keep. For hydrogen and other one-electron species like He⁺ and Li²⁺, energy depends only on n. Use the (n + l) rule only when there is more than one electron.

Answer from the NTA final answer key. Solution by PrepWiser.