Development of the Atomic Model (History)

With Years and Chronological Timeline

The Ancient Greeks

(490-460 BCE)

• First recorded ideas about organization of matter —> came from ancient Greeks

• Believed that all substances were made from four fundamental elements: water, fire, air, earth.

• Greek Philosopher “Democritus” —> believed matter composed of tiny, indivisible particles called “atomos,” which is Greek for “indivisible.”

John Dalton

J. J. Thomson

(1893)

• End of 19th century —> JJ Thomson awarded Nobel Peace Prize for forming a new idea about the atom

• Used cathode ray tube —> discovered presence of particles within the atom

• His model states: atom = positively charged sphere w/ negatively charged particles embedded w/in it

• later named electrons

• Model = analogous to chocolate chip cookie in which the:

  • Chips = negatively charged particles

  • Cookie dough = positively charged particles

Ernest Rutherford

Niels Bohr

1911

James Chadwick

1932

• James Chadwick provided evidence for the neutron:

  • A dense, neutral (chargeless) particle located in the nucleus.

• This completed the basic subatomic model of the atom:

  • Protons (positive charge)

  • Neutrons (no charge)

  • Electrons (negative charge, orbiting the nucleus)

• Chadwick’s nuclear model set the stage for learning how electrons are organized in energy levels around the nucleus.