History of the Atom 440 BC – Democritus and Leucippus 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 nor destroyed, merely changed from one form to another

1800’s Joseph Proust Law of Definite Proportions Observed that specific substances always have the same ratio by mass Law of Definite Proportions Observed that specific substances always have the same ratio by mass

1808 John Dalton – Atomic theory explains and verifies the Law of Conservation 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 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 –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 Law of Multiple Proportions –Elements in a compound combine in whole number ratios by mass Thus he says atoms react as whole units

1859 – Julius Plucker Sealed gases in tubes with electrodes on each end to form the cathode ray tube –One end – the anode – was positively charged, thus attracted negative ions –The other end – the cathode – was negatively charged, thus attracted positive ions Sealed gases in tubes with electrodes on each end to form the cathode ray tube –One end – the anode – was positively charged, thus attracted negative ions –The other end – the cathode – was negatively charged, thus attracted 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 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 Develops the “plum pudding” model of the atom –A sphere of positive electricity (the pudding) with electrons (the plum bits) scattered in it

1906 Ernst Rutherford Announced that alpha particles can be scattered by air Discovers the presence of the nucleus Announced that alpha particles can be scattered by air Discovers the presence of the nucleus

1913 Robert Millikan Determined the charge to mass ratio of the electron

1913 Neils Bohr Working with Rutherford proposed the planetary model of the atom

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 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

1932 James Chadwick Finds high energy particles with essentially the same mass as a proton and no charge – the neutron

1924 Wolfgang Pauli States the quantum exclusion principle No two particles can occupy the same position at the same time in an atom States the quantum exclusion principle No two particles can occupy the same position at the same time in an atom

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 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 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

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 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