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Democritus
Democritus had the idea that if you kept cutting an object, you would eventually end with an uncuttable piece which he named an atom. He also believed that everything was made up of small atoms moving in a void. -
John Dalton
Dalton believed that all matter is composed of atoms, which are small particles that cannot be created, divided, or destroyed. He also proved that atoms with the same element are identical and different elements have unlike types of atoms. Also, he explained how atoms join together to make new substances. -
George Johnstone Stoney
Stoney proposed that electricity was made of negatively charged particles in atoms called electrons. -
Joseph John Thomson
Thomson showed a mistake in Dalton's theory. There were actually smaller particles in atoms! Thomson conducted the Cathode-Ray Tube experiment and discovered that the smaller particles in atoms were negatively charged. He had discovered electrons. This also led him to believe that there were positive and negative forces in the atom. -
Max Planck
Planck developed the quantum theory by studying radiation. He also hypothesized that matter was discrete, not continuous. -
Plum-Pudding Model
Thomson presented his model of the atom (also known as the Chocolate Chip Ice-Cream model). It had electrons throughtout it. -
Ernest Rutherford
Rutherford tested Thomson's theory by conducting the Gold Foil Experiment. He discovered that atoms are mostly empty space with a dense, positive nucleus. He learned that a particle headed straight for the nucleus would be pushed back in the same direction it came, because like charges repel. HIs model of the atom shows that the center of the atom is the nucleus and has electrons surrounding it. -
Henry Moseley
Moseley was the first to justify the atomic number, which gives the number of protons in a nucleus, by studying the X-ray spectra of each element. His work was even used to rearrange the periodic table based upon atomic number instead of atomic mass! -
James Chadwick
Chadwick contributed to the atomic theory by proving the existence of uncharged particles in atoms, neutrons. He discovered that neutrons help reduce the repulsion between protons and stabilize the atom's nucleus. His discovery also played a key role in nuclear fission. His atomic model shows protons and neutrons in the nucleus with electrons outside. -
Niels Bohr
Bohr, a Danish scientist who worked with Rutherford, studied how atoms reacted to light. He proposed that electrons are located in levels at the certain distances from the nucleus.In his Planetary Model in 1913 the nucles is surrounded by electrons at different energy levels. There are no paths beteen the levels in his model. The electrons can jump form one path to another. -
Robert Millikan
Millikan determined the charge and mass of an elcetron by conducting the Oil Drop Experiment. He proved Thomson's theory that the mass of an electron is at least 1000 times smaller than the smallest atom. -
Works Cited
http://en.wikipedia.org/wiki/Democritus
http://en.wikipedia.org/wiki/Ernest_Rutherford
http://en.wikipedia.org/wiki/George_Johnstone_Stoney
http://en.wikipedia.org/wiki/Henry_Moseley
http://en.wikipedia.org/wiki/James_Chadwick
http://en.wikipedia.org/wiki/John_Dalton
http://en.wikipedia.org/wiki/Jj_thomson
http://en.wikipedia.org/wiki/Max_Planck
http://en.wikipedia.org/wiki/Niels_Bohr
http://en.wikipedia.org/wiki/Robert_Andrews_Millikan
