Which of the following is/are not correct with respect to energy of atomic orbitals of hydrogen atom? (A) 1s < 2p < 3d…

🧠 The "n-Only" Universe Hydrogen is a unique case in the periodic table because it has only one electron. Without other electrons to cause shielding or internal repulsion, the energies of subshells ($s, p, d$) do not "split." For Hydrogen, the energy depends only on the principal quantum number $n$.

🗺️ The Audit for H-Atom

• (A) $1s < 2p < 3d < 4s$: Correct. Energy follows $n=1 < 2 < 3 < 4$.
• (B) $1s < 2s=2p < 3s=3p$: Correct. In Hydrogen, $2s$ and $2p$ are degenerate (same energy).
• (C) $1s < 2s < 2p \dots$: INCORRECT. This implies $2p$ is higher than $2s$, which is only true for multi-electron atoms.
• (D) $1s < 2s < 4s < 3d$: INCORRECT. This says $4s$ is lower than $3d$. In Hydrogen, energy follows $n$ strictly: $n=3$ ($3d$) must be lower than $n=4$ ($4s$).

⚡ The "Ignore l" Rule When you see Hydrogen atom, look exclusively at $n$. $2s = 2p$. $3s = 3p = 3d$. If $n$ values are equal, energy is equal. If $n_1 < n_2$, then $E_1 < E_2$. Any statement breaking this $n$-strict order is false.

⚠️ Aufbau Trap The "standard" $4s < 3d$ order (Aufbau) only applies to atoms with MANY electrons where shielding effects are present. Do not use it for a solitary Hydrogen atom!

$\boxed{\text{Answer: (c)}}$