<p> Atomic/Periodic Table History Timeline Key</p><p>Democritus (~400 BC)  Matter is made of 2 things: Empty space and tiny particles called atomos Aristotle (~360 BC)  Matter is made of continuous substances he called hyle Antoine Lavoisier (1760’s)  Father of Modern Chemistry  Law of Conservation of Mass  Law of Definite Proportions  Compiled a list of 23 known elements Joseph Proust (1799)  Law of Constant Composition  Law of Definite Proportions John Dalton (1803)  1st Atomic Theory (Solid Sphere Model)  Law of Multiple Proportions Dmitri Mendeleev (1869)  1st Periodic Table E. Goldstein (1885)  Developed a modified cathode-ray o Discovered protons o Discovered that protons have a lot more mass than electrons ~2000 times more J. J. Thomson (1897)  Cathode- Ray tube (Glass tube with electrical particles going through it) o The cathode was attached to the negative terminal and the anode was charged to the positive terminal o Discovered Isotopes and electrons o Determined the charge of a electron(SI unit for electrical charge: 1.6 x 10-19 Coulomb)  Plum-Pudding Model Lord Rutherford (1909)  Gold Foil Experiment o His work with radioactivity led to the Gold Foil experiment and the discovery of alpha, beta, and gamma radiation o Determined Plum-Pudding Model was wrong o Proved that atoms are made of mainly empty space Robert Milliken (1909)  Accurately measured the charge of an electron using the Oil Drop Experiment Henry Moseley (1913)  Studied X-rays and produced in X-rays tubes and anodes of different metals which led to Atomic # of elements and Atomic mass formula (A-Z Z=Atomic number and A Atomic Mass)  He also determined that the Periodic Table order should be according to its atomic number- Still that way today Neils Bohr (1913)  Bohr Atomic Model (Planetary Model) Louis de Broglie (1923)  Led to the current Atomic Theory that atoms have wave characteristics and light particles-Wave Theory Hund’s Rule (1923) Lord Rutherford (1930’s)  He proved neutrons exist (Coworkers were Bothe and Chadwick) Max Planck (1936)  Quantum model-for Atomic Theory  Photoelectric effect-It's been determined experimentally that when light shines on a metal surface, the surface emits electrons-Led to electromagnetic waves (shows the spectrums of light) Werner Heisenberg (1942)  Heisenberg’s Uncertainty Principle o Proved electrons exist in an orbit o Proved that you can never know exactly the position of an electron at any given time</p>