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Scientist Year Democritus ~460 BC John Dalton 1805 Contribution First to develop idea that matter was made of particles called “atoms”- from the Greek word “atomos”, which means “indivisible”. Proposed atomic theory; an atom is indivisible, indestructible, tiny sphere. Determined that elements have unique atomic masses. Atoms combine to form compounds. Joseph John (J.J.) Thomson 1898 Discovered electrons. Proposed an atomic model known as the “Plum pudding Model”: a sphere of positive charge containing electrons. Dimitri Mendeleev 1869 Developed the first periodic table The periodic table was arranged according to increasing atomic weight, and left spaces for elements that were yet to be discovered. Later versions refined & expanded the table. Ernest Rutherford 1911 Discovered positively charged nucleus through the “Gold Foil” experiment. Proposed atomic model with tiny but massive (highly dense) nucleus with electrons orbiting around it. Niels Bohr 1913 James Chadwick 1932 Erwin Schrodinger 1933 Model Developed atomic model (“Bohr Model”) that placed electrons in specific energy levels (orbits) around the nucleus. When energized, electrons can jump from one orbit to another, then fall back to their stable (a.k.a. “ground”) level. Discovered the neutron as a neutral particle in the nucleus. Explained why the mass of an atom could not be attributed entirely to the mass of protons. Developed the “Electron Cloud Model”, which was built off the Bohr Model. Described electrons as waves (using math) with their probable location viewed as a cloud. “Billiard Ball” Model