Contribution: First proposed that all matter is made of tiny, indivisible particles called atomos

Beliefs: Different materials (iron, clay, cheese) were made of unique types of atoms with specific properties

Significance: Conceptual beginning of atomic theory, though purely philosophical and not based on experiments.

Contribution: Formulated the Law of Conservation of Mass.

Key Idea: Matter is neither created nor destroyed in chemical reactions.

Significance: Laid a foundational principle for understanding chemical processes.

Contribution: Revived atomic theory with scientific evidence.

Key Idea: Elements are made of atoms; all atoms of an element are identical, and different elements have different atoms.

Significance: Developed the first modern scientific atomic model.

Contribution: Scientists began using discharge tubes which are gas-filled tubes with electrodes to study matter.

Discovery: Electric currents revealed particles with negative charge, setting the stage for identifying subatomic particles.

Contribution: Discovered positive rays canal rays in discharge tubes.

Key Idea: Matter contains positive charges, balancing the previously discovered negative charges.

Significance: Identified the presence of positive components in atoms.

Contribution: Discovered the electron.

Method: Measured properties of cathode rays (mass, response to magnets).

Model: Plum Pudding Model electrons embedded in a positively charged matrix.

Significance: First model to incorporate subatomic particles.

Experiment: Gold Foil Experiment – bombarded thin gold foil with alpha particles.

Findings: Most particles passed through; some deflected sharply.

Conclusion: Atom is mostly empty space with a dense, positively charged nucleus.

Later Discovery: Identified protons as the positively charged particles.

Significance: Replaced Thomson's model with the nuclear model of the atom.

Contribution: Applied quantum theory to Rutherford’s model.

Model: Planetary Model electrons orbit the nucleus in fixed energy levels.

Significance: Explained electron behavior using physics; still familiar in simplified atomic diagrams.

Contribution: Developed the Uncertainty Principle.

Key Idea: Cannot know both the position and momentum of an electron simultaneously.

Impact: Led to the development of Quantum Theory and the Cloud Model of the atom.

Key Concept: Electrons exist in orbitals areas where they're likely to be found, not fixed paths.

Representation: Electron clouds show probability densities rather than specific locations.

Significance: Most accurate model of atomic structure to date, based on Heisenberg and others' work.