ACT SSC Physics — Unit 4
Quantum Energy Levels — Flashcards & Quiz
Quantum energy levels in atoms are discrete allowed states for bound electrons, and ACT SSC Physics Year 12 Unit 4 uses the hydrogen atom as the key example. Transitions between levels absorb or emit photons with energy equal to the level difference, producing the characteristic hydrogen spectrum. The Bohr model explains the spectrum and supports the quantised-energy picture.
Key Points
- Bohr model: electrons occupy discrete circular orbits with quantised angular momentum.
- Hydrogen energy levels: E_n = –13.6/n² eV. Ground state is n = 1 (–13.6 eV); ionisation is n = ∞ (0 eV).
- Transitions: ΔE = E_f – E_i = hf = hc/λ. Absorption promotes electron to higher n; emission drops it to lower n.
- Spectral series: Lyman (to n = 1, UV); Balmer (to n = 2, visible); Paschen (to n = 3, IR).
- Rydberg formula: 1/λ = R(1/n₁² – 1/n₂²), where n₁ < n₂.
- Emission spectra are discrete lines, not continuous — direct evidence that energy is quantised.
Common Mistakes to Avoid
- Forgetting the negative sign in E_n = –13.6/n² — bound electrons have negative energy (relative to free electron at 0).
- Using n₁ as the upper level in the Rydberg formula — n₁ is LOWER, n₂ is UPPER.
- Confusing absorption and emission — absorption goes up, emission goes down.
- Applying the Bohr formula to atoms beyond hydrogen — it's only exact for H.
- Not converting between eV and J when calculating photon wavelength.
Exam Strategy
BSSS Unit 4 quantum energy level questions ask you to calculate photon wavelengths for specific transitions. Method: (1) calculate E_n for initial and final levels, (2) find ΔE, (3) apply ΔE = hc/λ to find wavelength, (4) identify the spectral series based on the lower n. Include diagrams showing energy levels and transition arrows.
Revision Tip
Hydrogen transition calculations are drillable — build a Revizi deck with 10+ (n₁, n₂) pairs asking you to calculate the photon wavelength.
Last updated: March 2026