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Democritus

*proposed that atoms are "uncuttable"
*named them after the Greek word "atomos" which means "indivisible"
*said that atoms are small, hard particles made of a single materials
*said that atoms are always moving
*Greek philosopher around 440 BC

John Dalton

*All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed.
*Atoms of the same element are exactly alike, and atoms of different elements are different
*Atoms join with other atoms to make new substances
*Performed experiments and found that elements combine in specific proportions because they are made of individual atoms
*British school teacher and scientist in late 1700's

J. J. Thomson

*Discovered that there are small particles inside the atom and therefore atoms can be divided into smaller parts
*Experimented with a cathode-ray tube
* Plum-pudding model of the atom
*Said that the atom is mostly positiviely charged material
*electrons are small, negatively charged particles located throughout the positive material
*British scientist in 1897

Ernest Rutherford

*Gold foil experiment
*proposed that the atom has a small, dense, positively charged nucleus
*the atom is mostly empty space through which electrons travel
*electrons travel around the nucleus like planets around the sun, but their arrangement could not be described
*1909, student of Thomson

Niels Bohr

*worked with Rutherford
*suggested that electrons travel around the nucleus in definite paths
*electrons can jump from a path at one level to a path in another level
*1913 Danish scientist

Erwin Schrodinger and Werner Heisenberg

*developed Modern theory of the atom
*Austrian and German physicists
*electrons do not travel in definite paths
*there are regions called electron clouds where the electrons are likely to be found
*20th century scientists

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