CHEMLEC2A Atomic Models

Democritus

(400–300 BC) Proposed that all matter is made of tiny, indivisible particles called "atomos."

John Dalton

(1803) Stated that matter is made of indivisible atoms, and atoms of one element are identical but different from other elements.

Atomic Theory

Atoms cannot be created, destroyed, or changed into other elements; compounds form in fixed ratios of atoms.

Solid Sphere Model

The model that shows atoms as solid, indivisible spheres that combine to form compounds.

J. J. Thomson

(1897) Discovered the electron using the Cathode Ray Tube experiment and measured its charge-to-mass ratio.

Cathode Ray Tube Experiment

Showed that electrons are negatively charged particles with a charge-to-mass ratio of 1.76 × 10⁸ C/g.

Plum Pudding Model

(1897) The atomic model that shows the atom as a sphere of positive charge with electrons scattered throughout.

Robert Millikan

(1909) Measured the electron’s charge (≈ 1.602 × 10⁻¹⁹ C) using the Oil Drop Experiment and calculated its mass.

Oil Drop Experiment

Determined the exact charge of an electron and allowed calculation of its mass (≈ 9.11 × 10⁻²⁸ g).

Ernest Rutherford

(1909) Discovered the nucleus through the Gold Foil Experiment, showing that atoms are mostly empty space.

Gold Foil Experiment

Most alpha particles passed through foil, but some deflected, proving the existence of a small, dense, positively charged nucleus.

Nuclear Model

Shows the atom is mostly empty space with a dense, positively charged nucleus and electrons around it.

James Chadwick

(1932) Discovered the neutron, a neutral particle with nearly the same mass as a proton.

Alpha Particle Bombardment Experiment

This experiment proved the existence of neutrons within the nucleus.