Dalton’s Atomic Theory Still accepted except Atoms are indivisible

J.J. Thomsom 1897 Discovered negatively charged subatomic particle- the electron By 1900- mass of electron 1 / 2000 mass of H atom

Cathode Rays Negatively charged particles moving at high speeds Same regardless of type of metal or type of gas used Therefore, electrons are part of all atoms

Robert Millikan Determined charge of electron Determined the charge to mass ratio of electron 1916- calculated mass of electron Electron → charge -1 → mass 1 / 1840 of H atom

Atom? Atoms are electrically neutral Electrical charges are carried by matter Electric charges are all in whole number multiples of a basic unit When positive charges = negative charges the result is neutral

E. Goldstein 1886-Found rays in cathode ray tube moving opposite the cathode rays Called them- canal rays Made of Positive particles Called Protons Mass 1840 times mass of electron

James Chadwick 1932 Found another fundamental particle No charge Mass about equal to proton Called- Neutron

Ernest Rutherford Explored atom with alpha particles (helium atoms with +2 charge- lost 2 electrons) Directed alpha particles into thin layer of gold foil Prediction: alpha particles would pass through foil with very little deflection

Results Most alpha particles passed straight through foil Few particles bounced back at large angles toward alpha particle source

Rutherford’s New Atom 1.The atom is mostly empty space This accounts for most of the alpha particles passing straight through the foil

2. Positive charges and most of the mass are concentrated in a small region. This accounted for the large angles of deflection.

Nucleus Named by Rutherford Positive, massive region in center of atom Made of protons & neutrons

Comparison If Lancer stadium was an atom: Then the nucleus would be the size of a marble at the 50 yard line!