Key Points: PYQ-Focused Revision

Most Tested Facts (NEET 2018–2024):

1. Balmer = Visible — Asked almost every other year. Lyman = UV, Balmer = Visible, Paschen = IR. Mnemonic: "LBP = UV-V-IR."

2. Spectral Lines Count — N = n(n−1)/2. NEET 2023: "An excited H atom emits 10 spectral lines. Initial level?" Answer: n=5 (since 5×4/2=10). Common wrong answers use $n^{2}$=10 or 2n=10.

3. Half-life Fractions — NEET 2021, 2024: "What fraction remains after 3 half-lives?" Answer: 1/8. "How many half-lives for 1/32 to remain?" Answer: 5 half-lives.

4. KE and PE Signs — NEET 2019, 2023: "KE and PE of ground state electron of H are respectively:" Correct: +13.6 eV and −27.2 eV. Wrong trap: (d) both negative.

5. Alpha Decay Equation Balancing — NEET 2021: "^{226}{88}Ra → ? + ^{4}{2}He." Answer: ^{222}_{86}Rn (A−4=222, Z−2=86=Rn).

6. Nuclear Density — "Which is true about nuclear density?" Correct: constant for all nuclei. Wrong: increases with A.

7. Bohr Radius Ratios — NEET 2020, 2022: "Ratio of radii of 3rd orbit H to 1st orbit $He^{+}$?" r_{3}(H)/r_{1}($He^{+}$) = (0.529×9/1)/(0.529×1/2) = 9 ÷ 0.5 = 18.

8. BE/A Curve — "Why does fission release energy?" Because fission products have HIGHER BE/A (closer to Fe-56 peak) than U-235. Products are more stable = more binding energy released.

9. Mean Life > Half-life — τ = 1.443 t_{1}/{2}. If t{1}/_{2} = 5 y, τ = 7.22 y (not 5 y, not 3.47 y).

10. Distance of Closest Approach — d ∝ Z/KE. If KE doubles, d halves. If Z doubles, d doubles.