Atomic Theory The Experiments
John Dalton • John Dalton 1766– 1844) was an English chemist, meteorologist,physicist, and school-teacher. He is best known for his pioneering work in the development of modern atomic theory, and his research into color blindness (sometimes referred to as Daltonism, in his honor).
Early Models of the Atom • J.J. Thomson – 1897 • Robert Millikan – 1909 • Lord Kelvin (a.k.a. William Thomson) - 1910 • Earnest Rutherford – 1911 • Niels Bohr - 1922 • James Chadwick - 1932
J.J. ThomsonThe Cathode Ray Tube • JJ Thomson is credited with the discovery of the electron.
J.J. Thomson’s experimentDiscovery of the Electron • Thomson used a gas discharge tube to study cathode rays. • He was able to show that cathode rays are actually made up of negatively charged particles called electrons.
Millikan’s ExperimentDiscovery of the mass of the electron • A fine mist of oil drops are sprayed into the chamber. • Oil drops fall through a small hole in the top plate are ionized by X-rays and are then caused to levitate between the plates by applying an electric charge.
Bohr Modelof the Atom • Electrons orbit the nucleus in orbits that have a set size and energy. • The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit. • Radiation is absorbed or emitted when an electron moves from one orbit to another.
Problems with the Bohr Model • The model is only valid for the hydrogen atom (with one electron). • The first assumption concerning the electrons in fixed circular orbits violates the laws of classical mechanical physics. • The ANGULAR MOMENTUM assumption violates the Heisenberg uncertainty principle. • Heisenberg Uncertainty Principle - It is impossible to determinethe position (x) and the momentum (mv) of a particle simultaneously with certainty. • In the Bohr model, it is assumed that there are fixed angular momentums for eachquantized orbital.
Bothe-Becker ExperimentDiscovery of the Neutron • In 1928, a German physicist, Walter Bothe, and his student, Herbert Becker, took the initial step in the search. They bombarded beryllium with alpha particles emitted from polonium and found that it gave off a penetrating, electrically neutral radiation, which they interpreted to be high-energy gamma photons.
Discovery of the Neutron • However, further investigations into the properties of the radiation revealed contradictory results. • In 1932, Chadwick proposed that this particle was Rutherford's neutron. Through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost exactly the same as that of a proton.
Atomic Structure A closer look at the Atom
Atom • a basic unit of matter that consists of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons.
Example • Atomic Number = protons • Protons = electrons • Mass Number = protons + neutrons • So, neutrons = mass number – atomic number