Chemistry • Year 11 • Module 1 • Lesson 13
Atomic Models – Historical Development
Build HSC Band 5–6 extended-response technique: evaluate experimental evidence, design investigations, and analyse how scientific models are revised when new evidence emerges.
1. Data + scenario: What if Geiger and Marsden had used a different target? (Band 5–6)
8 marks Band 5–6
Scenario. In 1911, Geiger and Marsden fired alpha particles at a thin gold foil under Rutherford’s direction. Imagine instead they had used two alternative targets:
- Target A: A foil of lithium (Z = 3, atomic mass 7). Lithium has a much smaller, less positively charged nucleus than gold (Z = 79).
- Target B: A foil 100 times thicker than the original gold foil.
The table below shows the actual gold foil results alongside predicted results for Targets A and B, based on Rutherford’s nuclear model.
| Deflection outcome | Gold foil (actual) | Lithium foil — Target A (predicted) | Thick gold foil ×100 — Target B (predicted) |
|---|---|---|---|
| % passing through (<5°) | 98.5% | ~99.8% | ~60% |
| % deflected 5–90° | ~1.45% | ~0.19% | ~37% |
| % back-scattered (>90°) | ~0.05% | <0.01% | ~3% |
Predicted values derived from Coulomb scattering theory. Illustrative.
Q1. Analyse and evaluate the predicted data to assess what conclusions Rutherford could have drawn if these alternative targets had been used first. In your response you must:
- Explain, using the Target A data, how a lithium foil result would still support the nuclear model despite showing fewer deflections.
- Analyse how the thick gold foil (Target B) data might have complicated or delayed the development of the nuclear model, referencing specific values.
- Identify which data column most clearly demonstrates that the atom is mostly empty space, and justify your choice quantitatively.
- Assess how the nature of scientific models — as provisional explanations revised by evidence — is illustrated by comparing the predicted and actual results.
- State one way in which Rutherford’s nuclear model would still be validated even if only the lithium foil data were available.
2. Experimental design — testing whether a new atomic model explains new spectral data (Band 5–6)
7 marks Band 5–6
Research question. A Year 11 student claims: “Because hydrogen emits spectral lines in the visible range (Balmer series), Bohr’s model should also predict spectral lines in the ultraviolet and infrared regions. If these lines exist, it is further evidence that quantised energy levels are real.”
Constraints: You have access to a hydrogen discharge tube, a diffraction grating spectrometer capable of measuring ultraviolet (200–400 nm) and infrared (700–2500 nm) wavelengths, a high-voltage power supply, and a darkened laboratory. The investigation must be completed in a single two-hour practical session.
Q2. Design the investigation and present it in the format below.
- State a testable hypothesis that predicts specific non-visible spectral lines from Bohr’s model (use the concept of energy level transitions).
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least four numbered steps, including how you will distinguish real spectral lines from instrumental noise.
- Explain what result would falsify your hypothesis (i.e. what would constitute evidence against quantised energy levels in this context).
- State two limitations of your experimental design and one improvement to increase reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Target A (lithium foil) analysis: The lithium foil data show that ~99.8% of alpha particles pass through with minimal deflection, with only <0.01% back-scattered [1 — uses specific data]. Even though the back-scatter rate is far lower than for gold, the overall pattern — overwhelmingly straight-through paths with a tiny fraction deflected — would still support the nuclear model. The lower deflection rate is explained by Rutherford’s model itself: lithium has Z = 3, meaning its nucleus has much weaker electrostatic repulsion than gold (Z = 79). The Coulomb repulsive force is proportional to the nuclear charge, so fewer near-direct hits with a smaller charge produce fewer large deflections. The pattern still indicates a concentrated positive nucleus; only the scale changes [1 — links lower deflection to smaller Z using Coulomb reasoning].
Target B (thick gold foil) complications: The thick foil data show ~60% pass-through, ~37% moderate deflection, and ~3% back-scatter — a far more “smeared” distribution [1 — uses specific values]. With a thick foil, alpha particles undergo multiple scattering events from successive gold nuclei, accumulating deflections that are not the result of a single near-miss with one nucleus. This would have made it much harder to conclude that the deflections arose from a single concentrated charge; Rutherford might have attributed the broad distribution to multiple small deflections consistent with the plum pudding model, potentially delaying the development of the nuclear model [1 — explains ambiguity and consequence].
Column most clearly demonstrating mostly empty space: The gold foil (actual) or lithium (Target A) both show this, but the lithium data are most striking: 99.8% of particles pass through with <5° deflection [1 — identifies column and quantitative justification]. This near-complete pass-through is unambiguous evidence that the vast majority of the atom’s volume is empty, with charge concentrated in a region that most particles never approach closely enough to be deflected. The thick foil (Target B) obscures this pattern through multiple scattering.
Nature of scientific models: The comparison illustrates that models are provisional — Rutherford’s nuclear model explains the actual gold foil data, and its predictions for alternative targets (lower Z → fewer deflections; thicker foil → multiple scattering) follow logically from the model. If the lithium data had been collected first, a scientist might have reached the nuclear model conclusion slightly less confidently (due to very few back-scatter events), but it would still have indicated a concentrated nucleus. Science progresses by testing predictions of models under varied conditions; when predictions match observations across multiple experimental setups (gold foil, lithium foil, Coulomb scattering theory), confidence in the model increases [1 — explicitly links provisional nature of models to the data comparison].
Validation from lithium data alone: Even with <0.01% back-scatter from lithium, Rutherford’s model would still be validated because the existence of any back-scatter — however rare — is completely inexplicable by the plum pudding model (which predicts only small, uniform deflections). The nuclear model uniquely predicts that back-scatter, however infrequent, must occur whenever an alpha particle approaches the concentrated nucleus, which the lithium data support [1].
Marking criteria (8 marks): 1 = uses specific Target A data values to explain lower deflection; 1 = links lower deflection to smaller nuclear charge (Z = 3 vs 79) using Coulomb repulsion; 1 = uses specific Target B values to explain why thick foil complicates interpretation; 1 = explains how multiple scattering could have delayed nuclear model; 1 = identifies and quantitatively justifies the column most clearly showing mostly empty space; 1 = correctly explains the ambiguity of thick foil multiple scattering; 1 = uses at least two specific data values in the evaluative judgement; 1 = addresses the provisional nature of models through the evidence comparison.
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If Bohr’s model of quantised energy levels is correct, hydrogen will emit discrete spectral lines in the ultraviolet region (corresponding to electron transitions from higher levels to n = 1, the Lyman series, λ ≈ 121–365 nm) and in the infrared region (transitions from higher levels to n = 3, the Paschen series, λ ≈ 820–1876 nm). The independent variable is the wavelength range examined (UV vs visible vs IR). The dependent variable is the wavelength and intensity of discrete spectral lines observed. Controlled variables include the hydrogen discharge tube voltage (constant at the recommended operating voltage), the spectrometer grating order (first order), and the duration of each scan (60 seconds per range) [1 — hypothesis with IV, DV, and controlled variables].
Procedure: (1) Set up the hydrogen discharge tube in the darkened laboratory, connected to the high-voltage power supply at the manufacturer’s specified voltage. Allow the tube to warm up for 2 minutes to reach a stable plasma. (2) Calibrate the diffraction grating spectrometer using a known mercury lamp spectrum (known lines at 404.7, 546.1, 577.0 nm) to verify the wavelength scale is accurate. Record the calibration data. (3) Align the spectrometer inlet with the discharge tube. Scan across the UV range (200–400 nm), recording the intensity vs wavelength output at 1 nm intervals. Repeat the scan three times and average the results to distinguish genuine spectral peaks from random noise. (4) Without moving the tube, reconfigure the spectrometer to scan the infrared range (700–2500 nm) under the same conditions. Scan three times and average [1 — four clear procedural steps]. To distinguish real lines from noise: require that a genuine spectral line appears in at least 2 of 3 scans at the same wavelength and has an intensity at least 3× the background level [1 — explicit noise-rejection criterion].
Falsification: If the hydrogen spectrum in the UV and IR regions shows a continuous band of emitted light (all wavelengths with roughly equal intensity) rather than discrete, isolated peaks, this would falsify the hypothesis. A continuous spectrum would indicate that electrons can transition between energy levels of any value, inconsistent with quantisation, and would constitute evidence against Bohr’s model of discrete energy levels [1].
Limitations: (1) Atmospheric absorption: water vapour and oxygen in air strongly absorb UV wavelengths below ~240 nm, potentially masking the shortest Lyman series lines. The measurement is done in air, not vacuum, limiting the UV range reliably accessible. (2) The diffraction grating spectrometer may have lower sensitivity in the UV range, meaning faint lines (corresponding to rare transitions) may fall below detection limits even if they exist [1 per limitation].
Improvement: Repeat the entire scan on three separate days to confirm reproducibility and eliminate any day-to-day instrumental drift. Alternatively, use a vacuum UV spectrometer to access the complete Lyman series without atmospheric absorption [1].
Marking criteria (7 marks): 1 = testable hypothesis naming IV, DV, and two controlled variables, and predicting specific UV/IR series (Lyman/Paschen); 1 = four numbered procedural steps; 1 = explicit method for distinguishing spectral lines from noise; 1 = states what would falsify the hypothesis (continuous spectrum); 1 = first valid limitation; 1 = second valid limitation; 1 = one specific improvement to reliability.