Atomic Models, Historical Development

Three subatomic particles: proton (relative mass 1, charge +1, in nucleus); neutron (relative mass 1, charge 0, in nucleus); electron (relative mass ≈0, charge −1, in shells). Atomic number Z = protons; mass number A = protons + neutrons; electrons = Z for neutral atoms. Nuclide notation: ᴬ_Z X (e.g. ⁵⁶₂₆Fe).

Pause, copy the highlighted subatomic particle table into your book before moving on.

We just saw the properties of subatomic particles. That raises a question: how did scientists actually discover the internal structure of the atom, and why did models keep changing? This card answers it → each major experiment (cathode rays, gold foil, emission spectra) revealed something that the current model could not explain, forcing a revision.

Atomic models evolved with new evidence: Dalton (1803), indivisible sphere; Thomson (1904), plum pudding (electrons in positive sphere); Rutherford (1911), nuclear model (tiny dense nucleus, mostly empty space); Bohr (1913), fixed energy-level orbits. Each model was revised when experimental results contradicted it.

Add the highlighted model timeline to your notes before the check below.

This is the most important single experiment in the history of atomic theory and is frequently examined.

We just saw how models evolved from Dalton to Bohr. That raises a question: what exactly did Rutherford observe in the gold foil experiment, and why did each observation disprove the plum pudding model? This card answers it → each of the three key observations maps to a specific conclusion about nuclear structure.

In Rutherford's gold foil experiment (the alpha-particle scattering was carried out by Geiger and Marsden under his direction): most alpha particles passed straight through (atom is mostly empty space); a small fraction deflected at large angles (>90°) (concentrated positive nucleus exists); very rare back-scatter (~1 in 20,000) (nucleus is extremely small and very dense). These observations disproved Thomson's plum pudding model, which predicted only small uniform deflections.

Pause, write the highlighted gold foil observations into your book.

When atoms are excited (by heat or electrical energy), electrons jump to higher energy levels. When they fall back to lower levels, they release photons of light. The energy of the photon matches the energy difference between the two levels:

This is why a sodium street lamp emits a characteristic yellow-orange colour, and why hydrogen emits a specific set of red, blue-green, blue, and violet lines (Balmer series). Each element has a unique spectral fingerprint, used in spectroscopy to identify elements.

We just saw how Rutherford's gold foil experiment revealed the nuclear atom. That raises a question: Rutherford's model still had a problem, classical physics predicted orbiting electrons would spiral into the nucleus. How did Bohr resolve this? This card answers it → Bohr proposed quantised, stable orbits; atoms only emit light when electrons jump between these discrete levels.

Excited electrons jump to higher energy levels; when falling back they emit photons of specific energy (E = hf). Only discrete energy jumps are allowed (quantised levels) → only specific frequencies emitted → discrete spectral lines → each element has a unique spectral fingerprint. Bohr's key insight: energy levels are quantised, electrons cannot exist between allowed levels.

Add the highlighted emission spectra explanation to your notes before the check below.

We just saw how excited electrons emit photons of specific energy when they fall back to lower levels, producing a discrete emission spectrum. That raises a question: how can you actually observe this in a school laboratory, using real metal ion solutions rather than a spectrometer? This card answers it → the flame test, a simple qualitative practical for collecting primary data from a flame test using different ionic solutions of metals (CH11-10).

The flame test is a practical technique for identifying metal ions by the colour they produce when heated in a flame. A clean nichrome or platinum wire (chosen because they are unreactive and do not themselves colour the flame) is dipped into a metal ion solution and then held in a hot, non-luminous Bunsen flame. Heat energy excites the metal ion's electrons to higher energy levels; as they fall back down, they emit photons of specific, characteristic wavelengths, exactly the same electron-transition process you have just studied, but now visible directly as flame colour.

Procedure, collecting primary data

1. Clean a nichrome or platinum wire by dipping it in dilute hydrochloric acid, then holding it in the hot part of a blue Bunsen flame until it produces no colour.
2. Dip the clean wire into the metal ion solution (or a paste of the solid salt with a drop of concentrated HCl).
3. Hold the wire in the hot, non-luminous part of the flame and record the colour observed.
4. Clean the wire again between each test to avoid contaminating the next result.
5. Repeat with each unknown solution and compare the observed colours against known reference colours to identify the metal ion present.

Flame test: dip a clean nichrome/platinum wire in a metal ion solution, hold in a hot Bunsen flame, record the colour. Common colours: Li⁺ crimson, Na⁺ yellow-orange, K⁺ lilac, Ca²⁺ orange-red, Sr²⁺ red, Ba²⁺ pale green, Cu²⁺ blue-green. This is primary data (observed first-hand). The colour arises from the same process as atomic emission spectra, excited electrons emit photons of specific energy as they fall back to lower levels. Limitation: similar colours can be confused, and a strong colour (e.g. Na⁺) can mask a weaker one.

Pause, copy the highlighted flame colour table and the primary-data explanation into your book before moving on.

We just saw how the flame test provides primary data on metal ions by observing their characteristic colours. That raises a question: how do you structure exam answers on the history of atomic models and flame test evidence? This card answers it → for each model, state the evidence that supported it and the evidence that forced the next revision; for flame tests, name the ion, then explain the colour using electron transitions.

For exam answers on atomic models: always link each model to the experiment that created or destroyed it. For "significance of gold foil" answers: quote three observations and one conclusion each. For emission spectra and flame test answers: explain quantised energy levels → discrete frequencies → unique spectral fingerprint, or observed colour. Scientific models are tentative and are revised when new evidence emerges.

Pause, copy the highlighted model-change framework into your book before moving on.