The shape of the lesson
By the end of the lesson, learners can
- Jump between levels: Choose two hydrogen levels and watch the photon.
- Emission, absorption, and the gap: Three ideas carry the marks.
- Photon: E = hf = hc/λ, momentum p = E/c = h/λ.
- Photoelectric: hf = Φ + KE_max; threshold f₀ = Φ/h; intensity sets rate, frequency sets KE_max.
- de Broglie: λ = h/p; for an electron through voltage V, p = √(2m₋eV).
- Energy levels: hf = E₁ − E₂ gives the wavelength of each spectral line.
One picture every idea returns to
Electrons in an isolated atom occupy discrete energy levels (negative, because they are bound). A jump down emits a photon and a jump up absorbs one, with hf = E₁ − E₂. Because only fixed energy gaps are allowed, only certain photon energies appear, giving a line spectrum rather than a continuous one.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Atoms only emit certain colours.. An electron in an atom can sit only at fixed energy levels. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Jump between levels | Choose two hydrogen levels and watch the photon. Project tab 4 of the lab (Energy levels and spectra) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Emission, absorption, and the gap | Three ideas carry the marks. | Mix-Pair-Share |
| 30 to 43 min | Check and plenary | The quick-check questions from the student lesson, answered on mini whiteboards one at a time; re-teach the one most get wrong, then a random call on the last. | Individual, then whole class |
| 43 to 45 min | Exit and homework | Set the worksheet below (2 questions, 19 marks) from the topic practice set. Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Emission, absorption, and the gap, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Energy levels are negative, so always take the difference carefully: the gap is E₁ − E₂ with E₁ the higher (less negative) level. | Convert the gap from eV to joules before using hf or hc/λ. A line spectrum is itself the evidence that the levels are discrete. |
Energy levels and line spectra: topic worksheet
2 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced. These questions cover the whole topic, so sibling lessons share them.
Fig. 8.1 shows some of the electron energy levels of a hydrogen atom.
(a) Explain why the energy values are negative. [1]
(b) Calculate the energy of the photon emitted in the n = 3 to n = 2 transition, in eV and in joules. [2]
(c) Calculate the wavelength of this photon. [2]
(d) State the region of the spectrum in which this line appears. [1]
(e) Explain why the spectrum of hydrogen consists of discrete lines rather than a continuous band. [3]
A hydrogen atom emits a photon when its electron falls from the n = 4 level (−0.85 eV) to the n = 1 level (−13.6 eV). The photon then strikes a metal of work function 4.5 eV.
(a) Calculate the energy of the emitted photon, in eV. [1]
(b) Calculate the wavelength of the photon and state the region of the spectrum. [3]
(c) Calculate the maximum kinetic energy of the photoelectron released from the metal, in eV. [2]
(d) A photon from the n = 3 to n = 2 transition (1.89 eV) instead strikes the same metal. Explain why no photoelectron is emitted. [2]
(e) State what determines the number of photoelectrons emitted per second by the metal. [1]
Answer key and marking notes
- (a)Energy must be supplied to free the electron from the atom; the zero is taken at infinity, so bound states are negative ✓
- (b)ΔE = −1.51 − (−3.40) = 1.89 eV ✓ = 1.89 × 1.60 × 10⁻¹⁹ = 3.02 × 10⁻¹⁹ J ✓
- (c)λ = hc/ΔE = 1240/1.89 ✓ = 656 nm ✓
- (d)Visible (red) ✓
- (e)The energy levels are discrete ✓; only certain transition energies are possible ✓; each gives one fixed photon energy and wavelength, so the spectrum is a set of lines ✓
- (a)ΔE = −0.85 − (−13.6) = 12.75 eV ✓
- (b)λ = 1240/12.75 ✓ = 97 nm ✓; ultraviolet ✓
- (c)KE_max = 12.75 − 4.5 = 8.25 eV ✓✓
- (d)1.89 eV is less than the work function 4.5 eV ✓; a single photon cannot supply enough energy to free an electron, so none is emitted ✓
- (e)The intensity (number of photons arriving per second) ✓
The lesson's own checks as a quiz
The quick-check and exam-style questions from the student lesson, printable with the key.
The existence of a line spectrum is evidence that atomic energy levels are:
- Adiscrete (quantised)
- Bcontinuous
- Call equal
- Dalways positive
When an electron moves from a higher to a lower energy level, the atom:
- Aemits a photon
- Babsorbs a photon
- Cgains kinetic energy only
- Dloses an electron
The energy of the photon emitted in a transition between two levels is:
- Ahf = E₁ − E₂
- Bhf = E₁ + E₂
- Chf = E₁ × E₂
- Dhf = E₂ − E₁
An absorption line spectrum appears as:
- Adark lines on a continuous spectrum
- Bbright lines on a dark background
- Ca single continuous band
- Dno spectrum at all
Quiz key
Only fixed energy gaps are allowed, so only certain photon energies (wavelengths) appear. Sharp lines mean the levels are discrete.
A downward transition releases energy as a photon of energy equal to the gap between the levels.
The photon carries exactly the difference between the two levels: hf = E₁ − E₂, with E₁ the higher level.
Atoms absorb photons of exactly the transition energies from a continuous beam, removing those wavelengths and leaving dark lines at the same positions as the emission lines.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.