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Atomic Theory: the beginning Chapter 3 Part I Democritus An Ancient Greek Theorized about the existence of atoms. Did not use Scientific Method. John Dalton A school teacher in England. Worked out his ideas at home. Used Scientific Method. Dalton’s Atomic Theory All Elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. Atoms of one element are different from any other element. Atoms of different elements can physically mix or chemically combine with one another in simple whole number ratios. Chemical reactions occur when atoms are separated, joined, or rearranged as a result of a chemical reaction. Dalton’s Atomic Theory Electrons Atoms are know to be divisible. One part of the atom, the electrons are negatively charged subatomic particles. These were discovered by JJ Thomson. Possible video of Cathode Tubes https://www.youtube.com/watch?v= O9Goyscbazk FoxTrot Millikan and the charge of an electron https://www.youtube. com/watch?v=XMfYHa g7Liw Robert Millikan carried out experiments that allowed him to calculate the charge and mass of an electron. Charge is one unit and mass is 1/1840 the mass of a hydrogen atom. Protons E. Goldstein observed a cathode-ray tube and discovered rays traveling in the opposite direction. He called these canal rays and That they had a positive charge. We now call them protons. Their mass is 1840 times an electron Neutrons James Chadwick confirms the existence of the neutron. These have no charge but add weight to the atom. How did the modern atomic model develop? https://www.youtube.com/watch?v= kHaR2rsFNhg Development of Atomic model It all started with Plum Pudding. Not this Plum Pudding!! This is JJ Thompson’s Plum Pudding Model Rutherford’s Model Rutherford realized from his experiments that the positively charged particles had converged into a small “nucleus” in the center of the atom and were surrounded by the very small electrons. Bohr’s model Bohr took Rutherford’s model one step further and theorized the electrons orbit the nucleus. Common View: But is it correct?