Rohr.+atomichistory.+fall.+2010

__Ancient__ Leucippus was Democritus’ teacher. He is believed to have originated the idea of an atom. There is not much about him, besides being a Greek philosopher. His ideas went onto his student who was able to create an idea of what an atom is. []

In 400 BC Democritus believed that matter was made up of tiny particles that were invisible to the eye. The different interactions of the particles decided whether an object was smooth or rough. If they are smooth they formed liquids because they can move around freely. If they are rough then they stuck together to form a solid. They were able to connect with each other by tiny hooks. They did not really know the force between the tiny particles, so they believed that they just stuck together. These particles were called atoms. They said atoms were not only invisible but indivisible and unchangeable. Atoms were believed to have just be a solid structure that came in different shapes and sizes. However, other Greek philosopher’s like Aristotle dismissed his ideas. The church was a very important part of this time period, and science was against religious beliefs. [] [] Aristotle on 300BC decides that there are only four kinds of elements in existence. These elements are fire, water, air, and earth. Within these elements are four kinds of properties they can have; cold hot, wet and dry. This idea contradicts the beliefs of Democritus. He argued against what Democritus discovered. Since the church went with what Aristotle decided on, early beliefs about the atom were dismissed. [] []

__1800-1875__ Dalton Stated the atomic theory in 1803: “(1) all matter was composed of small indivisible particles termed atoms, (2) atoms of a given element possess unique characteristics and weight, and (3) three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules).” This became an important base for chemistry to come. During this time, chemistry was a big part of science. New discoveries were being made, starting with the atomic theory.

Avogadro’s law states “States that under equal conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules” he was from Italy, and during this time there were revolutionary ideas. He also found Avogadro’s number: 6.02X1023. [|http://www.rsc.org/chemsoc/timeline//pages/1811.html] []

Mendeleev in 1869 published his idea of the periodic table. He organized the table in terms of increasing mass. Since in his time the only thing known about an atom was its atomic weight, that’s how he was able to make a periodic table of the elements. There were only 60 known elements at the time, therefor there were gaps left in his table for future discoveries. [] []

__1875-1900__ Roetgen discovered x-ray in 1895. In Germany at the time they were called Roetgen rays. He made the discovery by finding out that cathode ray tubes in the dark with paper covered barium platinocyanide would light up on the paper. When he furthered experimenting with the x-rays, he figured out that it could take pictures of bones in a human body. His first was of his wife’s hand. []

Becquerel in 1896 made a major discovery concerning radiation. He set uranium salts near a photographic plate that was coved with opaque paper. For his experiment he placed these plates in the light (outside). Then he placed these plates into his desk draws. To his surprise on the plates appeared a fogged image. He found that uranium emits radiation that is common to the uranium atom. []

J.J. Thompson in 1897 made the discovery of the electron. He made the discovery through the use of a cathode ray tube. He noticed that the glowing light coming from the tube would bend towards a positively charged plate. He theorized that a small negative part of the atom caused this, and he was correct. He made the plum pudding model. Where there was a positive sphere, and within that there were electrons that’s were negatively charged.



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__1900-1915__ Marie Curie was one of the main women who have made discoveries in chemistry. Her discovery of radiation was huge. During this time period women taking on a dominance roll, like her, was looked down on. In 1903 she was discovered two elements, Polonium and Radium. Working with her husband, they were able to separate radium from radioactive residue. This helped the study the properties of radium. She promoted the use of radium throughout World War I to help soldiers. [] []

Millikan found the charge of an electron in 1910. He used his oil drop experiment to find the electrical charge of an electron. In his experiment he dropped a charged oil drop into an electrically charged field. This demonstrated the atomic electric structure. This was during World War I when he made this discovery. []

Rutherford in 1911 did the gold foil experiment. In this experiment he sent a radioactive ray into a magnetic field to a gold piece of foil. The particles reflected in three different directions. Some went to the positive plates, some went to the negative plates, and some went straight through. He named the three kinds of radioactivity as: alpha rays (positive), beta rays (negative), gamma rays (neutral). With this information Rutherford was able to create a new drawing of the atom. Where the new atom had mostly empty space except for a dense center and electrons moving around in random ways. []

Bohr continued along with the theory Rutherford suggested about electron placement in an atom. Instead of an electron moving randomly, he said that the electrons move around in set orbitals. The outer orbits hold more electrons then the inner orbits. This is an example of a planetary model; since the electrons orbit around the center. The electrons in the outer orbits also suggest what chemical properties the element has. When atoms get excited, they jump from one energy level to another then come back down. When this happens the atom emits light, which is the spectrum. []

__1915-1950__ Heisenberg in 1925 invented matrix mechanics. This was the first version of what is quantum mechanics. With Bohr and Einstein, he was able to create the uncertainty principle. This is an important part of how quantum mechanics works. During World War II he became a leader in Germany’s nuclear project. []

Schrodinger in 1926 was able to create his wave equation that became an important part of quantum theory. Because of Nazi threats in 1933 because of World War II, he won the Nobel prize award. He studied a lot about his wave equation and how it relates. The work he done helped explain what was missing in Heisenberg’s matrix mechanics. He also built upon the model of the atom. He believed that electrons are in a cloud like area around the nucleus where it might be found. This was based on the probability equation, of where something is most likely to be found. [] [] Chadwick in 1832 while investigating beryllium disintegration, he found the nucleus. He was able to discover that the mass of a nucleus is slightly bigger than a proton. He found this by the conservation of momentum and energy, since a nucleus is undetectable because it is neutral. He was also asked to help make the atomic bomb during World War II, as the leader of the British effort on the Manhattan project. []