Science · Class 12
Atoms and Nuclei
Learn Atoms and Nuclei with notes, examples, and practice questions.
3 sections15–25 min read6 MCQs · 2 examples
Chapter Overview
Atomic structure (Bohr model, hydrogen spectrum) and nuclear physics (binding energy, radioactivity, fission/fusion) — high-weight board topics.
Bohr Model
Radius
r_n ∝ n²/Z
Energy
E_n = −13.6 Z²/n² eV
Spectral line
1/λ = R(1/n₁² − 1/n₂²)
Nucleus
Mass defect
Δm = [Zm_p + Nm_n] − M_nucleus
Binding energy
BE = Δm·c²
Activity
A = λN = A₀e^(−λt)
Half-life
T₁/₂ = 0.693/λ
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Bohr energy
Ground state H atom.
- 1E₁=−13.6 eV
Answer
−13.6 eV
Half-life
λ=0.693 yr⁻¹.
- 1T₁/₂=1 yr
Answer
1 year
Key Points to Remember
- ✓ Lyman, Balmer, Paschen series
- ✓ α, β, γ radiation penetration
- ✓ Mass-energy equivalence
Exam Tips
- • Energy in eV for atoms
- • Use log for decay problems
- • Draw energy level diagram
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Formula Cheat Sheet
Quick reference — NCERT board exam formulas
Bohr Model
- Radius:r_n ∝ n²/Z
- Energy:E_n = −13.6 Z²/n² eV
- Spectral line:1/λ = R(1/n₁² − 1/n₂²)
Nuclear Physics
- Mass defect:Δm = [Zm_p + Nm_n] − M_nucleus
- Binding energy:BE = Δm·c²
- Activity:A = λN = A₀e^(−λt)
- Half-life:T₁/₂ = 0.693/λ
Practice MCQs — Atoms and Nuclei
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
Question 1 of 6Score: 0/0
Bohr model applies best to: