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  • The famous scientist Democritus named the smallest piece of matter. According to Democritus’ atomic theory, the universe and all matter obey the following principles:1. Everything is composed of “atoms”, which are physically, but not geometrically, indivisible 2. Between atoms, there lies empty space. 3. Atoms are indestructible 4. Atoms have always been, and always will be, in motion 5.There are an infinite number of atoms, and kinds of atoms, which differ in shape, and size.
  • Dalton’s atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass. Dalton’s atomic theory also stated that all compounds were composed of combinations of these atoms in defined ratios.Dalton also postulated that chemical reactions resulted in the rearrangement of the reacting atoms.
  • The British physicist J. J. Thomson performed experiments studying cathode rays and discovered that they were unique particles, later named electrons. Rutherford proved that the hydrogen nucleus is present in other nuclei. In 1932, James Chadwick showed that there were uncharged particles in the radiation he was using. These particles, later called neutrons, had a similar mass of the protons but did not have the same characteristics as protons.
  • Ernest Rutherford, Hans Geiger and Ernest Marsden carried out their Gold Foil Experiment to observe the effect of alpha particles on matter. Rutherford devised a way to record the location of the alpha particles by surrounding the bombarded object with a sheet coated in ZnS, which would emit of flash of light when hit with an alpha particle.He then hypothisized that these particles would penetrate a thin metal foil, although they may scatter slightly because of the charge in the metal atom's subatomic particles. After shooting alpha particles through the thin sheet of gold, Geiger, Marsden and Rutherford discovered that a small porportion of the molecules were scattered at larger than 90° angles. This caused them to conclude that there was a small fraction of the total volume of the atom that held most of the mass of the atom.
  • The discoveries of the electron and radioactivity at the end of the 19th century led to different models for the structure of the atom. In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. Bohr's theory could explain why atoms emitted light in fixed wavelengths.
  • In 1913, Neils Bohr, a student of Rutherford's, developed a new model of the atom. He proposed that electrons are arranged in concentric circular orbits around the nucleus. This model is patterned on the solar system and is known as the planetary model. The Bohr model can be summarized by the following four principles: 1. Electrons occupy only certain orbits around the nucleus. Those orbits are stable and are called "stationary" orbits. 2. Each orbit has an energy associated with it. The orbit nearest the nucleus has an energy of E1, the next orbit E2, etc.3. Energy is absorbed when an electron jumps from a lower orbit to a higher one and energy is emitted when an electron falls from a higher orbit to a lower orbit. 4. The energy and frequency of light emitted or absorbed can be calculated by using the difference between the two orbital energies.
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