Most Tested Facts (from NEET 2015–2024)

1. Bohr Energy — Most tested calculation: $E_2(He^+) = -\frac{13.6 \times 4}{4} = -13.6 \text{ eV} \quad \text{(same as ground state H)}$ E_3($Li^{2+}$) = -\frac{13.6 \times 9}{9} = -13.6 \text{ eV} \quad \text{(also same as ground state H)} Pattern: E_n(Z) = −13.6 eV when $Z^{2}$ = $n^{2}$ (i.e., Z = n).

2. Spectral Lines — Recurring formula: From n=5: 10 lines. From n=4: 6 lines. From n=3: 3 lines. NEET trap answer is always $n^{2}$ (25, 16, 9) — avoid it.

3. Quantum Number Validity — NEET standard: Invalid sets tested: (n=2, l=2), (n=3, l=3), (l=n), (|mₗ| > l) Rule: l ∈ [0, n−1]; mₗ ∈ [−l, +l]

4. Anomalous Configuration — NEET staple: Cr (Z=24) = [Ar] 3$d^{5}$ 4$s^{1}$; Cu (Z=29) = [Ar] 3$d^{10}$ 4$s^{1}$ Both are half/full d stability anomalies. Both lose one 4s electron.

5. Photoelectric Effect — Conceptual trap: Intensity → count of electrons; Frequency → KE and whether emission occurs. If ν < ν_{0}: NO electrons emitted (even with laser-level intensity).

6. de Broglie — Number of waves in Bohr orbit: Number of complete waves in nth Bohr orbit = n (from nλ = 2πr). 4th orbit has 4 complete standing waves.

7. Series Regions — direct memory: Lyman=UV, Balmer=Visible (ONLY), Paschen/Brackett/Pfund=IR. Mnemonic: "Only Balmer is Visible/Bright."

8. Nodes — Quick calculator: 3p: radial=1, angular=1. 3d: radial=0, angular=2. 3s: radial=2, angular=0.