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The Development of the Modern Atomic Theory Democritus o ~460BC – 370BC o 1st to say that all matter is made of tiny, indivisible particles called atomos (atoms). o Less popular that Aristotle All matter is made of earth, air, water, & fire. John Dalton o 1766 – 1844 o Dalton’s Atomic Theory All matter is made of tiny, indivisible particles called atoms. All atoms of a given element are identical. Compounds are combinations of 2 or more elements. Chemical reactions are the rearrangement of atoms. JJ Thomson o 1856 – 1940 o Cathode Ray Experiment in 1894 – discovered negatively charged particles in the nucleus, called electrons o Plum Pudding Model (Chocolate Chip Cookie) Ernst Rutherford o 1871 – 1937 o Gold Foil Experiment in 1911 – discovered the positively charged nucleus in the atom o o Discovered the positively charged protons in the nucleus in 1917. o Rutherford Model James Chadwick o 1892 – 1974 o Worked as a graduate student under Rutherford o Discovered neutrons in the nucleus in 1932 Niels Bohr o 1885 – 1962 o Bohr Model (Planetary Model) Erwin Schrödinger o 1887 – 1961 o Quantum Mechanical Model