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Chemistry Timeline:

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460 BC

Greek philospher Democritus describes the “Atomos”

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  • Democritus was an ancient Greek philosopher who lived between 460-375 BC. He is best known for his development of his concept of the “Atomos”. Democritus described the idea of taking a loaf of bread and cutting it into half and then cutting each half into half and so on until one reached an uncuttable unit of matter. He termed this unit of matter the “Atomos”.
  • According to Democritus, many different types of atoms exist. Democritus thought that iron atoms had small hooks on them giving iron it’s strength. He thought that clay had ball and socket joints which is why clay could be molded. Furthermore, he thought salt atoms were spiked due to salt’s “sharp” taste.
  • Democritus’ ideas fell out of favor due to the ideas of another popular philosopher who lived at the same time with contrasting ideas. Aristotle proposed that matter was made up of wind, fire, air, and water. This view became widely accepted and overcame many if Democritus’ ideas for the next 2000+ years.

1808 AD

John Dalton Describes His Postulates.

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  • John Dalton was an English school teacher and scientist best known for his experiments with gasses.
  • Dalton’s work had influence on atomic theory at a time that the entire idea had no scientific credibility. It could be said that through Dalton atomic theory gained it’s first step towards being considered a real science.
  • Dalton described four postulates describing atoms. This was signifiant because atoms had not been considered in the Western world since Democritus.

  • 1) All matter is made of atoms. Atoms are indivisible and indestructible.

  • 2) All atoms of a given element are identical in mass and properties

  • 3) Compounds are formed by a combination of two or more different kinds of atoms.

  • 4) A chemical reaction is a rearrangement of atoms.
1904 AD

J. J. Thomson Discovers The Electron

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  • English physicist (1856-1940) who worked with cathode rays. Discovered the electron (1904).
  • Noticed that one can change the direction of a beam in a CRT tube with a magnet.
  • Discovered that these were negatively charged electrons.
  • Disproved one of Dalton’s postulates.
  • Thomson stated that the atom must be made up of a positive matrix with negative electrons spaced throughout. This resembled the English dish “Plum Pudding”.
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1911 AD

Ernest Rutherford Discovers the Nucleus

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  • New Zealand born British physicist (1871-1937).
  • Had several grad students set up an experiment that disproved the “plum pudding model” (1911).
  • Rutherford shot alpha particles from a radioactive source at a piece of gold foil.
  • To his surprise some of the particles bounced back indicating that there was something in the center of each atom.
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1913 AD

Niels Bohr discovers electron shells

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  • Niels Bohr (1885-1962) Danish physicist who described electron shells (1913).
  • Stated that electrons are not randomly placed throughout the atom.
  • Electrons can jump from shell to shell if given enough energy.
  • When the electron goes back to the ground state it emits a photon of light.
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