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400 BCE
Democritus Theory
Democritus theory explained that matter cannot be continuously divided. The smallest indivisible unit of matter is an atom (atomos). -
John Dalton's Atomic Theory
John Dalton discovered atoms are real in 1803. Combining his ideas with Democritus, he developed a four part theory.
Atomic Theory:
1. Matter is composed of atoms which cannot be created or destroyed.
2. Atoms are identical in mass and chemical and physical properties.
3. Atoms come together to for compounds.
4. Chemical Reaction is the rearrangement of atoms.
Dalton thought of an atom as a small hard dense ball. This theory is still used today. -
Robert Millikan
Using Dalton's theory, he used the Oil Drop Experiment to discover electrons. This experiment sprayed oil into a chamber where the droplets would be observed through a microscope. Next an x-ray as used to remove electrons from the air.
"Robert Millikan - Purdue University." N.p., n.d. Web. 21 Oct. 2016. -
Ernest Rutherford's Discovery
Rutherford discovered the nucleus and protons. He figured that Millikan's discovery of electrons had to have protons. He took positively charged particles and shot them through a thin sheet of gold foil. The positively charged particles deflected and other passed through. Concluded that atoms are mostly empty space and there is a nucleus and electrons that it deflected off of so it must be positively charged particles in an atom also. -
Niels Bohr
Bohr came up with the model that placed electrons in energy levels. These electrons are in a electric cloud. Low energy electrons are in the center and high energy electrons are toward the outside of the cloud. -
Werner Heisenburg
in 1925 Heisenburg made the Quantum Theory with Erwin Schrodinger. It explained that energy is made up of small seperate units of energy. Heisenburg also came up with Uncertainty Principle which explained that values of certain pairs of variables can't both be known. For example to determine how fast electrons are moving the electron cloud cannot be determined if you know the location.
"Werner Heisenberg - Important Scientists - The Physics of ..." N.p., n.d. Web. 21 Oct. 2016. -
Erwin Schrodinger
Schrodinger came up with the idea that you could find electrons in a certain position. Schrodinger and Heisenburg came up with the Quantum Theory which stated that energy is made up of small separate energy units. He also introduced sub-energy levels where the cloud is most dense is where you find electrons are the greatest. "Erwin Schrodinger - Biography, Facts and Pictures." N.p., n.d. Web. 21 Oct. 2016. -
James Chadwick
Chadwick is known for the founding of neutrons. In his experiment he bombarded elements with alpha particles. Beryllium released radiation then released protons from the nucleus. Chadwick interpreted that radiation composed of particles of mass that were equal to protons without a charge. These became known as neutrons. "James Chadwick | Biography and Facts | Britannica.com." N.p., n.d. Web. 21 Oct. 2016.
