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Key Idea: The concept of the atom was first proposed by Democritus and Leucippus.

Theory: Atoms are indivisible, tiny particles that make up everything.

Importance: This was the earliest concept of atomic structure, even though it was purely philosophical.

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Key Idea: Dalton’s Atomic Theory

Theory: Atoms are indivisible, and each element is made of identical atoms. (Later found out that atoms arent indivisible). Atoms combine in fixed ratios to form compounds.

Importance: Laid the groundwork for modern atomic theory with empirical evidence.

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Discovery: The electron using the cathode ray tube experiment.

Theory: Proposed the Plum Pudding Model of the atom, where electrons are embedded in a positively charged called it something like "soup."

Importance: This was the first evidence that atoms are divisible into smaller particles.

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Experiment: Gold Foil Experiment

Discovery: The nucleus – a small, dense, positively charged core within the atom.

Theory: Nuclear Model – electrons orbit the nucleus, most of the atom is empty space.

Importance: Revolutionized the understanding of atomic structure, disproving Thomson’s model.

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Theory: Bohr Model of the Atom

Key Idea: Electrons orbit the nucleus in fixed energy levels (quantized orbits).

Importance: Explained atomic emission spectra and the stability of atoms.

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Discovery: The neutron

Importance: Showed that atoms are made of protons, neutrons, and electrons, completing the picture of atomic structure.

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Dalton’s indivisible atoms were challenged by Thomson’s discovery of the electron.

Rutherford’s nuclear model built on Thomson’s work but placed electrons outside the nucleus.

Bohr refined Rutherford’s model by quantizing electron orbits.