duminică, 28 aprilie 2013



By "atomic model" means a simplified representation of a schematic structure of the atom, aiming to explain its properties.
       The introduction of such a concept was imperative, because without a base could not reach intuitive understanding of atomic structure (ie the structure of matter that surrounds us), which as noted earlier in this paper, is the very foundation of scientific knowledge itself.

            The first atomic model was developed by JJ Thompson after he managed to emphasize the electron as a particle constituent atom prompting by drift experiments in electric and magnetic fields, the specific tasks.
           Thompson's model suggestively called "raisin cake", since the atom was regarded as a compact sphere within which were randomly distributed electric charges both positive and negative electrical ones who ensure the neutral atom orcarei view.
                 But as occur naturally, the idea that atoms exist, besides electrons (negative charge particles), particles with a positive electrical charge, which stated immediately that are the size of the task as electrons, since every atom is neutral, regardless of the number of its electrons (denoted by Z). As mentioned above, were positively charged particles called "protons" by E. Rutherford in 1911, when he conducted a famous experiment that bears his name, which has managed to prove on the one hand the mass of the proton is much greater than the mass of the electron, on the other hand, the model developed by Thompson is unrealistic, and requires a radical change of the internal structure of atom optics.
               Rutherford's experiment consists of sending ᶑ particles (helium nuclei) at a thin gold foil so that they can consider that consists of a single "layer" of atoms.
         It was found that the overwhelming majority of particles passed ᶑ basically turning neither in metallic foil, while a small part suffered drastic trajectory deviations.
             Rutherford concluded that atoms (in particular) gold atoms) are by no means some compact spheres (as stated by Thompson), in which case almost all of the incident beam particles ᶑ altered trajectory as more or less significant .
         Therefore found the first experimental evidence of the fact that all electric charge of atom is concentrated basically in its center, in a very small spatial region in relation to the size of the atom itself.
This spacious central atom that contain all of its positive charge, was called "atomic nucleus".
        The conclusion (which soon proved to be incomplete) the atomic nucleus as composed of Z protons. This statement is based on fact found experimentally that the mass of the nucleus is an integer multiple of the mass of a proton. As discussed above, inconsistency was done quickly but feel that the proportionality factor between atomic mass (actually mass of the nucleus, electrons from the masses could neglect) and was equal to the Z mass than with hydrogen to all other elements is greater than Z. This number denoted by A, is therefore the number of particles in the nucleus equal to the mass of the proton mass, and that only Z of them are electrically charged (protons) to understand here that the remaining ( A - Z) neutral, which earned them the name of neutrons.
         Thus became clear that the atom has an internal structure different from that proposed by Thompson, ie it consists of electronic shell and core which in turn is made up of protons and neutrons (collectively "nucleons").
      Rutherford himself to propose a model in which the atom is regarded as a whole consists of a small central core loaded with a positive electric charge Z times larger than a proton with a mass of A times a nucleon (proton and nucleons with the same mass), and the electrons revolving around it gets its great planets around the Sun, neutralizing the positive charge equal to the negative charge Z and with negligible mass compared to the nucleons. The atomic model was called the "planetary model".