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Atomic History

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BC: The End.

FC: The History of an Atom (: By: Anna and Michael

1: Pages Used: http://www.nobelprize.org

2: Once upon a time, there were 5 great scientists. You see, these were no ordinary scientists. They would one day be known around the globe.

4: Our first scientist, John Dalton, came with his Atomic theory because of his research into gases. He discovered that certain gases only could be combined in certain proportions even if two different compounds shared the same common element or group of elements. He tested out this theory, which caused him to make an interesting discovery. Elements combine at the atomic level in fixed ratios. This ratio would be different in compounds due to the weights of the elements being combined. Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were. | His theory shows a reason for the arrangements of atoms for influencing properties. Dalton's models eventually turned into the understanding of 3-dimensional qualities of molecules and the affect of structure on properties. Of course, Dalton's theory did have some incorrect data, but it also provided the foundation for future generations of scientists.

6: Our next scientist, J.J Thomson, built a cathode ray tube ending in a pair of metal cylinders with a slit in them. These cylinders were connected to an electrometer, a device for catching and measuring electrical charge. Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays. He found that when the rays entered the slit in the cylinders, the electrometer measured a large amount of negative charge, but not much electric charge if the rays were bent so they would not enter the slit. As Thomson saw it, the negative charge and the cathode rays must somehow be stuck together: you cannot separate the charge from the rays. For his model, he considered that the structure of an atom is something like a raisin bread. He envisioned matter made of atoms that were spheres of positive charge spiked with electrons throughout. Electrons were chunks of plum distributed through a positively charged sphere of pudding.

7: He is credited for the discovery of the electron and isotopes. Without electrons, we wouldn't know that charges cancel out, which in turn would take us back to the original model.

8: Our next scientist, Ernest Rutherford, worked under J.J. Thomson in college. However, Rutherford's 1911 analysis claimed that the "plum pudding model" of the atom was incorrect. His experiment involved the firing of radioactive particles through minutely thin metal foils (mostly gold) and detecting them using screens coated with zinc sulfide (a scintillator). Rutherford found that although most of particles passed straight through the foil approximately 1 in 8000 were deflected. Leading him to believe that most of the atom was made up of 'empty space."

9: Rutherford's 1911 analysis stated that the "plum pudding model" of J. J. Thomson was incorrect. Rutherford's new model for the atom, based on the experimental results, had a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom. This core, contained the bulk of the atomic mass. After Rutherford's discovery, scientists started to realize that the atom is not a single particle, but is made up of teeny subatomic particles. They eventually discovered that atoms have a positively-charged nucleus in the center. Since electrons were found to be even smaller, this meant that the atom consists of mostly empty space. (So he was right!!!)

10: The next scientist we are going to talk about, studied under our last scientist, Lord Rutherford. His name is James Chadwick. His teacher assumed that there were neutral, massive particles in the nucleus of atoms. James Chadwick was assigned the task of tracking down evidence of Rutherford's neutron. After a decade-long struggle to track it down, he performed tests on a new type of radiation which had been baffling physicists for years, and which had previously been mistaken for “gamma rays”. A sample of Beryllium was bombarded with alpha particles, which causes it to emit this mysterious radiation. It was then discovered by Irene Joliot-Curie that this radiation, after striking a proton-rich surface, would discharge some of the protons, which could then be detected using a Geiger counter (a device that measures radiation). From there, Chadwick just had to put all the pieces of the puzzle together. In the end, Chadwick finally solved the puzzle and officially discovered the neutron in 1932.

11: With the discovery of the neutron, the atomic model seemed more complete than ever. The overall charges remained the same, and now there no longer seemed to be a discrepancy between the atomic mass and the atomic number. Neutrons would also play an important role in the processes involved in creating nuclear explosions and nuclear energy.

12: Our last scientist studied under JJ Thomson, thats right, its Neils Bohr! While at Manchester University, Bohr had merged Rutherford's nuclear structure to Max Planck's quantum theory and ended up getting a model of atomic structure which remains valid to this day. He also introduced the theory of electrons traveling in orbits around the atom's nucleus, Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one emitting a photon. This became a basis for quantum theory. Without his help, we wouldn't know that that electrons travel in orbits around the atom's nucleus.

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