Key Historical Figures and Concepts in Atomic and Quantum Physics

Democritus

Greek philosopher (400 BC) who proposed that all matter is made of tiny, invisible particles called atoms.

Robert Boyle

First modern chemist; defined an element as a substance that cannot be broken down into simpler substances by chemical means.

Joseph Priestley

Discovered oxygen in 1794.

Antoine Lavoisier

Father of Modern Chemistry; proposed the Law of Conservation of Mass.

Joseph Proust

Proposed the Law of Definite Proportions: a compound always contains the same elements in the same proportions by mass.

John Dalton

Proposed the Atomic Theory of Matter in 1803.

Benjamin Franklin

Studied electricity and concluded there are two types of charges: positive (+) and negative (−).

Michael Faraday

Suggested that atomic structure is related to electricity.

J.J. Thomson

Discovered the electron; proposed the plum pudding model and found the electron's charge-to-mass ratio.

Robert Millikan

Oil drop experiment; determined the charge of a single electron (1.60 × 10⁻¹⁹ C).

Henri Becquerel

Discovered radioactivity in 1896.

Ernest Rutherford

Gold foil experiment; discovered the nucleus and that atoms are mostly empty space.

Niels Bohr

Proposed electrons travel in fixed energy levels and emit photons when changing levels.

Henry Moseley

Determined atomic number equals the number of protons and defines the element.

James Chadwick

Discovered the neutron in 1932.

Max Planck

Proposed energy is emitted in packets called quanta; E = hv.

Albert Einstein

Proposed light has both wave and particle properties; called light particles photons.

Louis de Broglie

Suggested electrons have wave properties and exist at specific frequencies.

Werner Heisenberg

Uncertainty Principle: cannot know exact position and momentum at the same time.

Erwin Schrödinger

Developed the wave equation for electrons; led to orbitals and the quantum mechanical model.

Quantum Theory

Describes the wave behavior of electrons and the probability of finding them in certain regions.