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Transcript
History of the Atom • 440 BC – Democritus developed the original atomic concept – There exist indivisible particles called atoms (“a” meaning not; “tomos” meaning cut • There exist empty space between atoms • Atoms are completely solid • Atoms are homogeneous with no internal structure • Atoms differ in size, shape and weight 1789 Antoine Lavoisier • Law of conservation of matter – matter can be neither created no destroyed, merely changed from one form to another 1800’s Joseph Proust • Law of Definite Proportions • Observed that specific substances always have the same ration by mass 1 1808 John Dalton – Atomic theory explains and verifies the Law of Conservatio of Matter • Theory of atoms – All elements are made up of tiny particles called atoms – The atoms of a given element are identical, but are different from any other element – Atoms of different elements combine to form compounds – A given compound always has the same relative number and types of atoms – In a chemical reaction atoms are reorganized, resulting in changes in the way they are bound together; the atoms themselves are not changed 1810 John Dalton • Law of Multiple Proportions – Elements in a compound combine in whole number ratios by mass • Thus he says atoms react as whole units 2 1859 – Julius Plucker • Sealed gases in tubes with electrodes on each end – One end – the anode – was positively charged, thus attracted negative ions – The other end – the cathode – was negatively charged, thus attracte positive ions 1897 JJ Thomson • Used a cathode ray tube with a beam that glowed between the electrodes when they were connected to an electrical charge – traveling from the cathode to the anode • The beam was attracted to a positive charge, leading Thomson to propose that there were small negatively charged particles that he called electrons 1907 JJ Thomson • Develops the “plum pudding” model of the atom – A sphere of positive electricity (the pudding_ with electrons (the plum bits) scattered in it 3 1906 Ernst Rutherford • Announced that alpha particles can be scattered by air • Discovers the presence of the nucleus 1913 Robert Millikan Determined the charge on the electron 1913 Neils Bohr Working with Rutherford proposed the planetary model of the atom 4 Late 1910’s Based on earlier work by E. Goldstein (1886) Millikan, Thomson and coworkers proposed the presence of a positively charged particle called the proton Goldstein observed what he called canal rays while using a cathode ray tube with the rays traveling in the opposite direction of Thomson’s 1920 Rutherford Proposes the presence of a third particle, found in the nucleus when the total mass of the atom cannot be attributed to the proton and electron 1924 Wolfgang Pauli States the quantum exclusion principle No two particles can occupy the same position at the same time in an atom 5 1926 Werner Heisenberg Proposes a theory of the movement of electrons in the atom – matrix mechanics States it is impossible to accurately predict both the position and the momentum of any object (including an electron) at the same time – the Heisenberg uncertainty principle 1926 Erwin Schrodinger Developed the wave equation to explain electron activity Considered the electron’s behavior as like that of a wave rather than a particle 1926 Max Born Suggested that the wave function of the Schrodinger equation was a measure of the probability of finding an electron at a given location 6 Quantum Mechanical Model of the Atom Based on Shrodinger’s wave equation, describes the electrons as a cloud surrounding the nucleus The higher the energy of the electron, the further it can travel from the nucleus 1932 James Chadwick Finds high energy particles with essentially the same mass as a proton and no charge – the neutron 7