History of the Atom. Democritus (400 BC) Proposed that matter was composed of tiny, invisible particles. Gr. atomos.

History of the Atom

Democritus (400 BC)

• Proposed that matter was composed of tiny, invisible particles.

• Gr. atomos

Aristotle

• Said that all substances are composed of four elements: Earth, Wind, Fire, and Water.

John Dalton (1803 AD)• the first coherent atomic theory

• Matter is composed of small particles called atoms.

• All atoms of an element are identical.

• During chemical reactions, atoms are neither created nor destroyed, but are simply rearranged.

J.J. Thompson (1897 AD)

• Used cathode rays to calculate the atomic mass of atoms.

Cathode Ray Tube Experiment

Ernest Rutherford (1910 AD)

• alpha particles (He nuclei) were shot at a thin piece of gold foil.

• most of the particles passed through the foil with little or no deflection, some were deflected to a great degree.

Gold Foil Experiment

Isotopes

Radioactive Decay

C14 Decay

Carbon Dating

Carbon Dating VideoClick on the picture above to view.