The Evolution Of The Atomic Theory :: essays research papers

The Evolution of the Atomic Theory     The five atomic theorys of the past 2 centuries represent the suddenadvancement of science in modern times. Begining with a basic theory on thebehavior of atoms to the current model, some changes have been do, andsome ideas are still the aforementioned(prenominal). Ancient Greek philosophers belived thateverything was made up of invisible particles called atmos. Since then thetheory of atoms did not progress untill 1803.     John Dalton was the first scientist to compose a theory of matter basedon atoms. Daltons atomic theory is based on four concepts. He stated     "1. All elements are composed of atoms, which are indivisable andindestructable particles.     2. All atoms of the same element are hardly alike in particular, theyhave the same mass.     3. Atoms of different elements are different in particular, they havedifferent masses.&nb sp    4. Compounds are formed by the joining of atoms of two or moreelements." 1     All of Daltons ideas account for the laws of definate and multipleproportions and the law of conservation of mass. Some of Daltons points arestill thought to be true, but everywhere time this origional theory has been modifyed. The first of these modifications came in 1897 when J.J. Thomson discoveredthe electron. Based on the work of William Crookes and his "Crookes tube"(Cathode-ray tube), Thomson discovered a negative charged particle was thecause of the light produced by the cathode-ray tube. He also discovered thatthese particles are present in all elements. These cathode-ray particles arenow known as electrons. Soon after the discovery of electrons the protonwas discovered. This led Thomson to conclude that ther were an equalnumber of both particles present in the atom.     Twelve years later Lord Ernest Rutherford was exper imenting withalpha particles. He shot a stream of them at a peice of gold foil surroundedby zinc-sulfide. When an alpha particle strikes ZnS it produces a flash oflight. The particles broadly stayed in a constant stream through the foil, but afew were deflected. This led Rutherford to belive that there must be a small,dense ball of protons in the middle of the atoms to deflect the smallnumber of particles.      Neils Bohr was the next physicist to advance the atomic theory. Heexplained what Rutherford could not about how the electron could stay in go around around the nucleus. When the electron has little energy it is closer to thenucleus, when it absorbs more energy it travels farther from the nucleus. There is a definate number of electrons that can be in the same orbit. Whenthe orbits closest to the nucleus are filled the atom is at a ground state.