Chapter_2_Atoms,Molec,Ions.S08

Chapter_2_Atoms,Molec,Ions.S08 - Chapter 2 - Atoms and...

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Chapter 2 - Atoms and Elements 2.1 Atomic Theory of Matter Democratus (~400 BC) All matter must be made up of tiny indivisible particles called atoms (atomos: indivisible) Atomic Theory was not accepted by Aristotle. The idea of atoms “went underground” for a couple of millennia. In the 17 th and 18 th centuries new evidence started to reveal atomic behavior. Law of Constant Composition: The elemental composition of a compound is always constant (mass % of each element is always the same) Law of Multiple Proportions: Mole ratio of atoms in a compound is always a whole number. Behavior of gases: Expansion of gases, double the mass, double the volume (constant T and P) The first “modern” scientist to postulate the existence of atoms was John Dalton (1800) Dalton’s Atomic Theory Postulates 1. Elements are composed of very small particles called atoms 2. All atoms of an element are identical to each other 3. Atoms are not changed by chemical reactions, they just recombine 4. Compounds are made by reactions of atoms. The number of each type of atom are always the same for a specific compound. Dalton’s Theory was very controversial. Many 18 th and 19 th century scientists believed that atoms did not exist, but they were convenient to explain several experimentally observed behaviors.
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2.2 Discovery of Atomic Structure p2 The work on atomic structure began in the late 1800, early 1900’s. By the end of 1929 quantum mechanics had been developed to almost the current form. This time is referred to as “The 30 years that shook physics”. A book by this title by George Gamov chronicles the main personalities and developments during this process. Cathode Rays (1870’s) Cathode ray tubes (old school computer screens and TV) have particles that can be deflected by magnets. Thomson: (1900) Charge/mass ratio of proton, electron, ions Millikan: (1920) Charge of electron (therefore the charge proton and mass of electron and proton can be calculated) Becquerel: (1890) Rays emitted by uranium (X rays) can penetrate softer matter and reveal inner structure. Other scientists (including Rutherford and Marie Curie) find different types of radioactive particles Rutherford: (1910) nuclear atom, nucleus positively charged, carries all mass of the atom, surrounding nucleus is negatively charged particles with extremely small mass. (nucleus very small compared to atom)
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Chapter_2_Atoms,Molec,Ions.S08 - Chapter 2 - Atoms and...

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