Retrieval Practice
Energy Levels & Wave-Particle Duality — AQA A-Level Physics
Q1. Define excitation.
When an electron in an atom absorbs energy and moves to a higher energy level, without leaving the atom.
Q2. Define ionisation.
The removal of an electron from an atom, leaving a positive ion.
Q3. What is the ionisation energy of hydrogen?
13.6 eV — the energy needed to remove an electron from the ground state (n = 1) to n = ∞.
Q4. Why must the energy transferred in excitation exactly match an energy gap?
- Energy levels are discrete.
- An electron can only exist at specific levels, so it can only absorb the exact amount of energy needed to jump between two levels.
Q5. Write the equation linking photon energy to an electron transition.
ΔE = hf = hc/λ, where ΔE is the energy difference between the two levels.
Q6. What is an emission spectrum?
A set of bright coloured lines on a dark background, produced when excited atoms de-excite and emit photons at specific frequencies.
Q7. What is an absorption spectrum?
Dark lines on a continuous coloured background, produced when a cool gas absorbs photons at frequencies matching its energy level gaps.
Q8. What evidence do line spectra provide about atomic energy levels?
Line spectra show that photons are emitted/absorbed only at specific frequencies, proving that energy levels in atoms are discrete (not continuous).
Q9. State the de Broglie equation.
λ = h/mv = h/p, where λ is the de Broglie wavelength, h is Planck's constant, m is mass, v is speed, and p is momentum.
Q10. What happens to the de Broglie wavelength if the speed of a particle increases?
- The wavelength decreases (λ ∝ 1/v).
- Faster particles have shorter wavelengths.
Q11. State two pieces of evidence for wave-particle duality of electrons.
- Wave: electron diffraction through thin graphite crystals (circular ring pattern).
- Particle: deflection by electric and magnetic fields.
Q12. Why does electron diffraction work but diffraction of a tennis ball does not?
- Diffraction requires the wavelength to be comparable to the gap size.
- Electrons at ~100 V have λ ≈ 10⁻¹⁰ m (comparable to atomic spacings).
- A tennis ball has λ ≈ 10⁻³⁴ m — far too small for any observable diffraction.
Q13. How does increasing the accelerating p.d. affect the electron diffraction pattern?
- The diffraction rings get smaller.
- Higher p.d. means faster electrons, shorter de Broglie wavelength, and less diffraction.