Chemistry - history of the atom

Scientist

Discovery

Method

Year

Diagram 

John Dalton

Atomic Theory

Dalton proposed that all matter is composed of tiny, indivisible particles called atoms, which combine in simple, whole-number ratios to form compounds. He based his theory on experimental evidence and observations of chemical reactions.

1803

Count Amadeo Avogadro

Avogadro's Law

Avogadro observed that equal volumes of gases at the same temperature and pressure contain the same number of molecules, regardless of their chemical nature. This led to the concept of the mole and Avogadro's constant.

1811

Dmitri Mendeleev

Periodic Table

Organization of elements by increasing atomic mass and properties. He left gaps for undiscovered elements and predicted their properties.

1869

Sir Joseph Thomson

Discovery of the Electron

Thomson conducted experiments with cathode ray tubes and observed that they produced a beam of negatively charged particles (electrons). 

1897

Niels Bohr

Bohr Model of the Atom

Bohr proposed a model of the atom where electrons orbit the nucleus in specific energy levels or shells. He explained the line spectrum of hydrogen using quantized energy levels.

1913

James Chadwick

Discovery of the Neutron

Chadwick conducted experiments bombarding beryllium with alpha particles and observed a new, uncharged particle with mass similar to that of a proton. This particle was identified as the neutron.

1932