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Atomic Theory Democritus to Rutherford Democritus - Ancient Greece Philosophical question: Could matter be divided into smaller and smaller pieces infinitely? Democritus thought that everything in the universe was made of tiny particles called “atomos” (atoms) “atomos” means indivisible, not to be cut Democritus was a philosopher, not a scientist. He never tested his theory John Dalton (1766-1844) We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen Dalton’s Atomic Theory Matter is made up of atoms Atoms are indestructible They cannot be created or destroyed Atoms of an element are identical and have same mass* Atoms of different elements have different mass Compounds are formed by combining atoms JJ Thomson’s work studied electricity knew that: • atoms were neutral • negative charges from his electric stream charged particles from individual atoms termed the particle “electron” model had positive material to balance the negative electrons Ernest Rutherford 1910 - devised a method to test Thomson’s atomic model Passed alpha particles (positive charged particles) through a piece of gold foil. He predicted the alpha particles would pass straight through. 1 in every 20 000 bounced straight back. Some deflected at an angle. Early Theory Summary Democritus: named “atom” Dalton - 1808: atomic theory J J Thomson - 1897: “plum pudding” model, termed “electron” Ernest Rutherford - 1910: atom with a positive nucleus surrounded by orbiting electrons
