Chemistry - history of the atom

John Dalton

Proposed that all matter is composed of tiny, indivisible particles called atoms, which combine in simple, whole-number ratios to form compounds based on experimental evidence and observations of chemical reactions (1803).

Count Amadeo Avogadro

Observed that equal volumes of gases at the same temperature and pressure contain the same number of molecules, leading to the concept of the mole and Avogadro's constant (1811).

Dmitri Mendeleev

Organized elements by increasing atomic mass and properties, leaving gaps for undiscovered elements and predicting their properties (1869).

Sir Joseph Thomson

Discovered the electron by observing negatively charged particles (electrons) in experiments with cathode ray tubes (1897).

Niels Bohr

Proposed the Bohr Model of the Atom where electrons orbit the nucleus in specific energy levels or shells, explaining the line spectrum of hydrogen using quantized energy levels (1913).

James Chadwick

Discovered the neutron by observing a new, uncharged particle with mass similar to a proton in experiments bombarding beryllium with alpha particles (1932).