Key Discoveries and Models in Atomic Theory for Chemistry Students

John Dalton

Solid Sphere Model (Billiard Ball Model) pictured the atom as a tiny, solid, hard, and indivisible sphere.

J.J. Thomson

Plum Pudding Model & Discovery of the Electron pictured negative electrons embedded uniformly inside a positively charged sphere.

Ernest Rutherford

Nuclear Model (Planetary Model) & Discovery of the Nucleus proposed an atom composed of a central, extremely dense positive nucleus surrounded by orbiting negative electrons in empty space.

Max Planck

Discovery of Energy Quantization (E = hν) discovered that energy transfer is not continuous, but occurs in fixed discrete units called quanta.

Albert Einstein

Discovery of Photons & Photoelectric Effect discovered that light consists of quantized energy packets (photons) capable of ejecting electrons from metal surfaces.

Niels Bohr

Bohr Model (Quantized Orbit Model) pictured electrons occupying restricted, 2D circular orbits (stationary states) around a central nucleus.

Louis de Broglie

Discovery of Matter Waves (Wave-Particle Duality) discovered that matter possesses wave properties with characteristic wavelengths (λ = h/p).

Werner Heisenberg

Discovery of the Uncertainty Principle discovered that defined electron trajectories (orbits) cannot exist because observation alters an electron's state.

Erwin Schrödinger

Quantum Mechanical Model (Electron Cloud / Orbital Model) replaced 2D orbits with 3D mathematical wave functions (psi²) representing electron probability density clouds.

Wolfgang Pauli

Discovery of the Spin Quantum Number (ms) & Exclusion Principle discovered that an atomic orbital can hold a maximum of two electrons, which must have opposite spins.

Friedrich Hund

Discovery of Hund's Rule of Maximum Multiplicity discovered that single electrons fill degenerate orbitals with parallel spins to minimize electron-electron repulsion.