duminică, 28 aprilie 2013



FIRST hypotheses about the structure of matter

      Since ancient times, the issue deeper knowledge of objects and natural phenomena.
  At the dawn of scientific knowledge, the matter is considered to be a whole
and continuously.
  This idea was supported by Xenofanes and school to eleatica in fifth-century BC
   A current averse to this view appeared in the next century, when Leukippos and then Democritus introduced the hypothesis of discontinuity of matter, which is understood as being composed of very small particles that can not be fragmented and the very reason they named atoms (atoms in Greek means something that can not be cut).
     Then seek to identify elements that make up the substance to be brought proof discontinuity them.
      The concept of Empedocles in the fifth century BCE as the constituents of matter as earth, water, air and fire was approved a century later Aristotle, who tried to argue because these "essential elements" correspond perfectly qualities of various objects: dry, wet, cold, hot .

      All these concepts - though remarkable for that time - were still unfounded, rather far from a correct and deep understanding of how implicit knowledge consists matter and its properties.


     Problem elucidate the atomic structure of matter was left in a "sleep" for centuries.
    Some alchemists have tried to suggest that the matter should be composed of simple substances (in the early seventeenth century - century, for example van Helmont claimed that all such substances derived from water).
    Only towards the end Seol - XVIII - century, J. Dalton issued a revolutionary hypothesis in scientific knowledge: the atomic hypothesis.
   Namely, he said that the Greeks have resumed ideas on composition of particulate matter that can not be divided, thus defining the structural unit atom of any substance [the smallest particle of the substance which still preserves its properties (physical and chemical)].

Fundamental laws of chemistry based on the assumption atomistic

           Underlying atomistic hypothesis formulation "mass conservation law" (independently) by Lomonosov and Lavoisier in the late eighteenth - century) that "the masses are not affected atoms in chemical reactions in which they participate, in other words in any chemical reaction, the mass of the reaction products is equal to the mass of reactants. "
       All atomistic hypothesis underlies Enunciation "law defined proportions" (by Proust, in the last year of the eighteenth - century): "The formation of chemical combinations, pestreaza components each report thoroughly and determined mass."

         At the beginning of the century - nineteenth - century even Dalton, based on his own assumptions atomist, elaborates "the law of multiple proportions": "If two elements can form more chemical combinations, the quantity of an item that combine with one and the same amount of the other is in a ratio of integers and low value. "

            For example, a quantity of nitrogen of the mass m can be combined with various amounts of oxygen.

   The formation consists of nitrogen and oxygen combine, for example, N2O3, the number of nitrogen atoms remains the same, while the number oxygen atoms is obviously another, and is in a relationship Integer and lower values.
 So check multiple proportions law developed by J. Dalton formulated based on the idea that developed atomist.

Chemical   Plant
N ( % )

O ( % )
Table nitrogen
Table oxygen
 The ratio between the mass of oxygen that is combined with the same weight of nitrogen
(Carbon monoxide, dinitrogen / nitrous oxide)





(Dinitrogen trioxide / nitrogen dioxide)





0,57/ 1,71 = 1/3

(Dinitrogen pentoxide / nitrogen dioxide)





0,57/ 2,85 = 1 /5

  (Carbon monoxide, nitrogen oxide / nitrogen)                       





0,57/ 1,14 = 1/2
(Dinitrogen tetroxide, nitrogen dioxide dimer)





0,57 / 2,28 =1/ 4

        Also at the beginning of the century - nineteenth - century Avogadro made ​​another fundamental law of chemistry that bears his name and which states the following: "Equal volumes in different agze, under the same conditions of temperature and pressure, have the same number of molecules. "
     Although there crystallized the concept of "molecule", as a system made ​​up of many atoms.

       All these ideas have established the correct understanding of the structure of matter, in the sense that it is not continuous, but is composed of structural entities (atoms, molecules) to be studied in a new light, given that the correct understanding of the properties of any substance is subject to a clear understanding of its structure.

                        Identification of constituents atoms

         During the entire nineteenth century, atoms continued to be considered elementary particles, indivisible, nepunandu the problem of investigating their possible internal structure.
         Only at the end of the century (1897), J.J. Thomsom managed to emphasize electron constituent of the atom, as a negatively charged particle. The value of the electric charge of the electron was known, moreover, "the elementary electric charge" (it was determined for the first time indirect electrolysis experiments): e = e = 1,6 * 10-19  C.
       In other words, the electric charge of the electron is:

                           qe =  - e 

                      qe = 1,6 * 10-19 C

           Direct information on the electron experiments were obtained by passing electric current through a rarefied gas, given that they, under normal circumstances, have a low electrical conductivity. If you apply a voltage between two electrodes large enough at the ends of a tube containing dilute gas, arises an electric discharges (accompanied by luminous phenomena).
       Downloading the look up of pressure in the tube.
       At low pressures (below 10-2 torr) luminous phenomena disappear inside the tube: instead the opposite wall of the cathode luminescence occurs due to radiation from the surface of leaves, called cathode radiation.
       J.J.Thompson showed, by diverting these radiation with various external fields, as recorded deviations are independent of the nature of the gas found in the tube, and the nature of the material they are made of electrodes, which led to the conclusion that the particles consist of universal identical negative, so the electrons.
     All J.J. Thompson managed to perform and interpret an interesting experiment deviated cathode radiation (using an electric field and a magnetic field parallel to each other), which was calculated based on the specific task of cathode radiation, so the electron specific task (relationship between electric charge and its mass:
     (Q / m) rad.catodica = (que / me) = - 1.76 * 108 C / g

      Noted with an electron mass me, which is therefore the value:

                       me = 1,6 * 10-19 C  /  - 1,76 *108 C / g              ,
                    me = 9,1 * 10-28   g = 9,1 * 10-31   kg

       So it is proved that the atom is not indivisible (presumption that caught his name again proved to be false).
    The electron is a particle smaller than atom electrically charged negative that goes into it.
   But since it is electrically neutral, it was necessary to identify and another constituent or positively charged.

PROTON - Component atomic TASK   positive electrical

        Another constituent of the atom, was also highlighted on
thunder experiments in dilute gases.
     Canal rays, the antithesis of the cathode consist of positive particles shows different values ​​for specific tasks, depending on the nature of the gas in which they arise.
      They have been highlighted for the first time by Goldstein, also at the end of the century - nineteenth - century.
       They are obtained in a discharge tube similar to that used to produce razelo cathode consisting of perforated cathode and change its layout near the middle of the tube, so that at one end of the tube there is a luminescence dating of a light beam of rays from the right holes in the cathode.
     Hydrogen canal ray products were identified protons, particles which E. Rutherford, their discoverer (1911) found them to be positive charge equal in magnitude to the electron's charge and mass equal to the mass m practically atom hydrogen:

                   q1,6 * 10-19 

                   mp =  1,67 * 10 -24  g = 1,67 * 10-27 kg

He defends the idea that because they see - as shown - proton mass is much greater than that of the electron (1837 times) masses of electrons in an atom might negligible compared with those of protons entering its composition, in other words, to say mass every atom should be equal to an integer multiple of the mass of a proton.


       Mass of an atom proved indeed to be an integer multiple of the mass of a proton.
       However, for other atoms except hydrogen, the coefficient of proportionality of the atomic mass (ma) and the proton (mp) is greater than the number of electrons in the atom Z respectively. So

ma  =  A * mp

A = Z for  Z = 1

A > Z for Z > 1 

         This would mean that the atom would have been made ​​only
electrons and protons, the number of protons were higher than
of electrons for any atom except hydrogen, which
unable cancellation would amount to the total electric charge
every atom Z> 1! However, this is in obvious contradiction with
experimental observations which show that all atoms are neutral
the electrical point of view.
    Therefore if an atom is composed of electrons Z, he must also have Z protons, to ensure neutrality power.
      So if A is the atom's mass times the mass proton (A ≥ Z), it 
dovesete than that along the Z protons, the atom still contains (A - 
 Z) particles of mass equal to the mass of the proton, but neutral in terms electric (only hydrogen does not contain such particles).
       Because of this, these particles discovered J.Chadwick
(1932), received the name of neutrons. Noting so with qn and mn
mass neutron electron charge respectively obtain:

q= 0

mn = 1,67 * 10 -24  g = 1,67 * 10-27 kg 

     Thus it became clear that the atom has internal structure, highlighting three types of elementary particles that come clear in its composition: electrons, protons, neutrons.