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Law of Conservation of Mass • Who: Antoine Lavoisier • Matter can't be created or destroyed • When: 1785 • Where: France Law of Definite Proportions • Who: Joseph Proust • Compounds – Elements put together in fixed ratios C + O2 CO2 • When: 1797 • Where: France Page 1 Law of Multiple Proportions • Who: John Dalton • Two elements form more than one compound between them • The ratios of the masses of the second element which combine with a fixed mass of the first element will be small whole numbers. • Where: England When: 1803 Historical Perspectives Aristotle (350BC) in Greece •Everything was fire, earth, air, or water •No smallest part of matter Page 2 Historical Perspectives Democritus (400 BC) in Greece •Atoms = smallest unit of elements •Atomos = indivisible/uncuttable John Dalton Solid Sphere Model (1805) in England Developed an atomic theory •Elements were made up of atoms •Each element was composed of the same kind of atoms. •Different elements were composed of different kinds of atoms. •Compounds are composed of atoms in specific ratios. •Atoms are not created or destroyed in a reaction. Page 3 Problems with Dalton’s Atomic Theory? 1. Matter is composed of indivisible particles Atoms Can Be Divided, but only in a nuclear reaction 2. All atoms of a particular element are identical Does Not Account for Isotopes (atoms of the same element but a different mass due to a different number of neutrons)! 3. Different elements have different atoms YES! 4. Atoms combine in certain whole-number ratios YES! Called the Law of Definite Proportions 5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements. Yes, except for nuclear reactions that can change atoms of one element to a different element J.J. Thomson What: Atoms aren't smallest particle Conducted experiment with a Cathode Ray Tube Particles had a negative charge Model = plum pudding When: 1897 Where: England Page 4 Cathode Ray Tube Robert Millikan What: Experiments to determine mass of electron Conducted the oil drop experiment Verified that electron is negatively charged No new model Where: America When: 1910 Page 5 Millikan Oil Drop Experiment Run Video Ernest Rutherford • Conducted Gold Foil experiment • Proved nucleus is dense, positively charged core of atom Model = Mass Centered/ Stationary Planetary Model • When: 1911 • Where: America Page 6 Rutherford’s experiment. Run video Page 7 Niels Bohr • What: Pulsating Planetary Model • Electrons can move between energy levels • When: 1913 • Where: Denmark Erwin Schrodinger • Electron have dual wave-particle nature • Foundation for quantum theory • Model = Pulsating orbitals • Math was used to predict path of the electron • When: 1926 • Where: Switzerland Page 8 James Chadwick • Confirmed existence of neutron • No new atomic model • When: 1932 • Where: England The Atom An atom consists of a • nucleus –(of protons and neutrons) • electrons in space about the nucleus. Electron cloud Nucleus Page 9 ATOMIC COMPOSITION • Protons (p+) – + electrical charge – mass = 1.672623 x 10-24 g – relative mass = 1.007 atomic mass units (amu) but we can round to 1 for mass. • Electrons (e-) – negative electrical charge – relative mass = 0.0005 amu but we can round to 0 for mass. • Neutrons (no) no electrical charge – mass = 1.009 amu but we can round to 1 for mass. – Page 10