History of the Atom

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CognitoHistory of the atom — development of atomic models

Key facts & methods

Dalton (1803): atoms are tiny solid spheres. All atoms of an element are identical. Different elements have different atoms.
Thomson (1897): discovered the electron. Proposed the "plum pudding model" — a sphere of positive charge with electrons embedded throughout.
Rutherford (1909–11): gold foil experiment. Fired alpha particles at gold foil. Most passed through; a few bounced back. Conclusion: atoms are mostly empty space with a tiny, dense, positively charged nucleus.
Bohr (1913): electrons orbit the nucleus in fixed shells (energy levels) at specific distances. Explained why electrons don't spiral into the nucleus.
Chadwick (1932): discovered the neutron inside the nucleus. Explained atomic mass (protons alone couldn't account for it).

Alpha particles fired at a very thin sheet of gold foil.
Expected (based on plum pudding): all particles pass through with slight deflections.
Observed: most went straight through (atom is mostly empty space). Some deflected (nucleus repels positive alpha particles). A few bounced straight back (nucleus is small and dense).
This led to the nuclear model: tiny positive nucleus at centre, electrons orbiting outside.
Key point: the results could NOT be explained by the plum pudding model.

Scientific models change when new experimental evidence contradicts the existing model.
The atomic model evolved through: observations → hypothesis → experiment → new evidence → revised model.
Each scientist built on the work of the previous — this is how science progresses.
Dalton → Thomson: discovery of electron showed atoms are divisible.
Thomson → Rutherford: gold foil experiment showed mass concentrated in nucleus.
Rutherford → Bohr: observation of atomic spectra showed electrons in fixed energy levels.

Exam questions — 4 questions · 10 marks · AQA 8462 style

1 markPlum pudding modelAQA 8462 style

Who proposed the "plum pudding" model of the atom and what discovery prompted it?

A Dalton — discovery of the proton

B Thomson — discovery of the electron

C Rutherford — gold foil experiment

D Bohr — observation of atomic spectra

4 marksGold foil experimentAQA 8462 P1 style

Describe the results of Rutherford's gold foil experiment and explain what each observation tells us about atomic structure. (4 marks)

Hint: Three observations → three deductions. For each: state what was observed AND what it tells us about the atom.

+40 XP

1 markChadwick contributionAQA 8462 style

What did Chadwick's discovery of the neutron explain?

A Why electrons orbit the nucleus in fixed shells

B Why atoms are mostly empty space

C Why atomic mass is greater than can be accounted for by protons alone

D Why atoms are electrically neutral overall

1 markModel progressionAQA 8462 style

Which sequence correctly shows how the atomic model developed?

A Bohr → Rutherford → Thomson → Dalton

B Dalton (solid sphere) → Thomson (plum pudding) → Rutherford (nuclear) → Bohr (shells)

C Dalton → Bohr → Rutherford → Thomson

D Thomson → Dalton → Bohr → Rutherford