atomic theory

Term: John Dalton — 1803

Solid Sphere Model Definition: Matter is made of tiny, indivisible spheres called atoms. Atoms of the same element are identical. Atoms cannot be created or destroyed

Term: Why it mattered (John Dalton)
Definition: First scientific atomic theory.

Term: J.J. Thomson — 1897 — Plum Pudding Model
Definition: Discovered the electron using cathode rays. Atom is a positive 'pudding' with negative electrons embedded.

Term: Why it mattered (J.J. Thomson)
Definition: First model to include subatomic particles.

Term: Ernest Rutherford — 1909 — Nuclear Model
Definition: Gold foil experiment showed: Most alpha particles passed through → atom is mostly empty space. Some deflected → small, dense, positive nucleus.

Term: Why it mattered (Ernest Rutherford)
Definition: Proved the existence of the nucleus.

Term: Niels Bohr — 1913 — Planetary Shell Model
Definition: Electrons orbit the nucleus in fixed energy levels (shells). Electrons absorb/emit energy when jumping between shells. Explained emission spectra.

Term: Why it mattered (Niels Bohr)
Definition: Introduced quantised energy levels.

9Term: Erwin Schrödinger — 1926 — Quantum Mechanical Model
Definition: Electrons exist in orbitals, not fixed paths. Orbitals are probability clouds. Wave

mechanical description of electrons.

10Term: Why it mattered (Erwin Schrödinger)
Definition: This is the modern model used today.