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HISTORICAL DEVELOPMENT OF ATOMIC THEORY AND THE STRUCTURE OF THE ATOM Judy Vondruska PAN Workshop July 18, 2010 Democritus (460-370 BC) Greek philosopher First to propose existence of smallest particle “Atomos” used to describe particle Aristotle (384-322 BC) Greek philosopher Believed in the four elements of air, earth, water and fire Felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter. this view held for 2000 years primarily because Aristotle was the tutor of Alexander the Great. Model of Atom Discoveries/theories without understanding the nature of the atom… Johann Becher (1635-1682) and Georg Stahl (1660-1734) Developed the Phlogiston theory which dominated chemistry between 1670 and 1790. allflammable materials contain phlogiston, a substance without color, odor, taste, or mass that is liberated in burning. Once burned, the "dephlogisticated" substance was held to be in its "true" form, the calx. problem with the theory was that burning of metals resulted in an increase in the mass. This problem was solved by assigning negative mass to phlogiston. Antoine Lavoisier (1743-1794) known as the “Father of Modern Chemistry” Proposed the Law of Conversation of Mass which represents the beginning of modern chemistry Sidebar: was associated with a tax-collecting firm and was married to the daughter of the one of the firm's executives. This relationship with the tax firm led to Lavoisier's beheading at the guillotine in 1794. Joseph Proust (1754-1826) Proposed the Law of Constant Composition in 1799 the composition of a substance is always the same, regardless of how the substance was made or where the substance is found are always 2 atoms of there hydrogen and 1 atom of oxygen in a molecule of water. John Dalton (1776-1844) Developed the concept of the mole and proposed a system of symbols to represent atoms of different elements Proposed the Law of Multiple Proportions. when elements combine, they do so in the ratio of small whole numbers. For example carbon and oxygen react to form CO or CO2, but not CO1.8 Joseph Gay-Lussac ( 1778-1850) Announced Law of Combining Volumes in 1808. the relative volumes of gases in a chemical reaction are present in the ratio of small integers (assuming all gases are at the same temperature and pressure). showed that at the same temperature and pressure, two volumes of hydrogen gas reacted with one volume of oxygen gas to produce two volumes of water (as a gas). Amadeo Avogadro (1776-1856) Proposed what is now known as Avogadro's Hypothesis in 1811 at the same temperature and pressure, equal volumes of gases contain the same number of molecules or atoms when combined with Gay-Lussac's Law of Combining Volumes, the only possible formulas for hydrogen, oxygen and water are H2, O2 and H2O, respectively hypothesis not widely accepted for another 50 yrs Dimitri Mendeleev (1834-1907) Proposed the periodic law and developed the first periodic table in 1869. Medeleev's table was arranged according to increasing atomic weight and left holes for elements that were yet to be discovered The changing view of the atom… J. J. Thomson (1856-1940) Identified the negatively charged electron in the cathode ray tube in 1897 deduced that the electron was a component of all matter and calculated the charge to mass ratio for the electron. e/m = -1.76 x 108 coulombs/g Thomson and others also studied the positive rays in the cathode ray tube and discovered that the charge to mass ratio depended on filling gas in the tube. The largest charge to mass ratio (smallest mass) occurred when hydrogen was the filling gas. This particle was later identified as the proton. e/m = +9.58 x 104 coulombs/g Proposed the "plum pudding" model of the atom. the volume of the atom is composed primarily of the more massive (thus larger) positive portion (the plum pudding). The smaller electrons (actually, raisins in the plum pudding ) are dispersed throughout the positive mass to maintain charge neutrality. New Model of Atom Robert Millikan (1868-1953) Determined the unit charge of the electron in 1909 with his oil drop experiment at the University of Chicago. Thus allowing for the calculation of the mass of the electron and the positively charged atoms. e = -1.60 x 10-19 coulombs Wilhelm Roentgen (1845-1923) Attention was drawn to a glowing fluorescent screen on a nearby table determined that the fluorescence was caused by invisible rays originating from the partially evacuated glass Hittorf-Crookes tube he was using to study cathode rays (i.e., electrons) these rays penetrated the opaque black paper wrapped around the tube. Roentgen had discovered X rays (1895) Antoine Henri Becquerel (1852-1908) Observed thast fluorescent materials like potassium uranyl sulfate, K2UO2(SO4)2 emitted mysteries rays like those described by Roentgen Found that uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity, the spontaneous emission of radiation by a material. Demonstrated that the radiation emitted by uranium shared certain characteristics with X rays but, unlike X rays, could be deflected by a Pierre Curie (1859-1906) Marie Curie (1867-1934) Marie coined the term radioactivity After chemical extraction of uranium from the ore, Marie noted the residual material to be more "active" than the pure uranium She concluded that the ore contained, in addition to uranium, new elements that were also radioactive. This led to their discoveries of the elements of polonium and radium Sidebar: After being killed crossing the street, Pierre's teaching position at the Sorbonne was given to Marie. Never before had a woman taught there in its 650 year history! Henry Moseley (1887-1915) Discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each successive element in the periodic table proposed that the relationship was a function of the positive charge on the nucleus this rearranged the periodic table by using the atomic number instead of atomic mass to represent the progression of the elements this new table left additional holes for elements that would soon be discovered Ernst Rutherford (1871-1937) Proposed the nuclear atom as the result of the gold-foil experiment in 1911 (Activity) proposed that all of the positive charge and all of the mass of the atom occupied a small volume at the center of the atom and that most of the volume of the atom was empty space occupied by the electrons Developed planetary orbit model of atom New Model of Atom Max Planck (1858-1947) Made observations of the radiation of heated materials Showed (from purely formal / mathematical foundations) that light must be emitted and absorbed in discrete amounts (quanta) if it was to correctly describe observed Prior to then light had been considered as a continuous electromagnetic wave, thus the discrete nature of light was completely unexpected the energy of each quantum is equal to the frequency of the radiation multiplied by the universal constant: E=f*h, where h is 6.63 x 10-34 Js. Niels Bohr (1913-1963) Proposed improvement to Rutherford atomic model this reason, the planetary model of the atom is for sometimes called the Rutherford-Bohr model. Bohr added the idea of fixed orbits, or energy levels for the electron traveling around the nucleus hismodel allowed for the idea that electrons can become "excited" and move to higher energy levels for brief periods of time Problem with Rutherford-Bohr Model The attraction of the electron to the nucleus would cause it to spiral into the nucleus According to the electromagnetic theory, if a charged particle were accelerated around another charged particle then there would be a continuous radiation of energy the loss of energy would slow down the speed of the electron and eventually the electron would fall into the nucleus but such a collapse does not occur. Rutherford's model was unable to explain why there was no collapse Francis Aston (1877-1945) Invented the mass spectrograph in 1920 First person to observe isotopes. observedthat there were three different kinds of hydrogen atoms. While most of the atoms had a mass number of 1, he also observed hydrogen atoms with mass numbers of 2 and 3. Modern atomic masses are based on mass spectral analysis His work led Rutherford to predict the existence of the neutron or Na-23 or Na-24 Isotope: one of two or more atoms having the same number of protons but different numbers of neutrons James Chadwick (1891-1974) Was a collaborator of Rutherford's Discovered the neutron in 1932 research was showing that the nucleus of an atom contains more than just protons. This was because the atomic mass of the atom of most elements was greater than the atomic number (number of protons). Electrons contributed nothing to the atomic weight chose to bombard boron with alpha particles and analyze the interaction of the neutral particles with nitrogen. These particular targets were chosen partly because the masses of boron and nitrogen were well known. conservation of energy was used to determine the mass of the neutron Scientists realized neutrons could be used to probe atomic nuclei lead to bombardment of Uranium with neutrons (Fermi) observation of nuclear fission New Model of Atom Erwin Schrödinger (1887 – 1961) Werner Heisenberg (1901-1976) Schrodinger combined some particle behavior with wave behavior as suggested by deBroglie and formulated mathematical model for hydrogen atom Derived equation which gives the probability of finding the electron at some point in a 3-dimensional space at any given instant; gives no information about the path the electron follows solutions from equation yield information about probability maps or shapes on different energy levels Develop the wave-particle duality of subatomic particles Electrons exist in different places at different points in time, but it is impossible to say where the electron will be at a given time New Model of Atom Electrons do not travel in definite paths around the nucleus The exact path or position of moving electron cannot be predicted or determined Rather, there are regions inside the atom were electrons are likely to be found. Electron clouds – Regions inside an atom where electrons are likely to be found.
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