How the Atomic Model Changed Over Time (with Experiments)

1.John Dalton (1803) – Solid Sphere Model

• Idea: Atoms are tiny, solid balls. Each element has its own kind of atom.

• Experiment: Dalton didn't do one single experiment for this model, but he based his ideas on chemical reactions and the laws of conservation of mass and definite proportions.

2.J.J. Thomson (1897) – Plum Pudding Model

• Discovery: The electron, a tiny negatively charged particle.

• Experiment: Cathode Ray Tube Experiment

  1. He passed electricity through gases in a sealed tube.

  2. A beam was attracted to a positive plate, showing the beam was negative (electrons).

• Model: A ball of positive charge with negative electrons spread through it like plums in a pudding

3.Ernest Rutherford (1909) – Nuclear Model

• Discovery: The atom has a tiny, dense, positive nucleus.

• Experiment: Rutherford Scattering Experiment

  • Alpha particles were fired at thin gold foil.

  • Most went straight through, but some were deflected or bounced back.

  • Showed that most of the atom is empty space, with mass concentrated in the nucleus.

4. Niels Bohr (1913) – Shell Model

• Idea: Electrons move in fixed orbits (shells) around the nucleus.

• Experiment: Bohr studied the light spectrum (especially of hydrogen).

  • He found that atoms give off light in specific patterns.

  • This matched the idea of electrons jumping between energy levels and releasing energy

5. James Chadwick (1932) – Discovery of the Neutron

• Discovery: The neutron, a particle in the nucleus with no charge.

• Experiment: Chadwick used radiation experiments and found that some particles had mass but no charge.

  • Helped explain why atoms had more mass than just protons could account for.

• Importance: Completed the picture of the atom’s nucleus (protons + neutrons).