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The Early History of Chemistry Greeks 400 B.C. Four fundamental substances – fire, earth, water, a air. - Democritus – uses term “atomos” (atoms) to describe small, indivisible matter. No experiments to support the idea, so it is dropped. Before 16th Century – Alchemy: Attempts (scientific or otherwise) to change cheap metals into gold Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 The Early History of Chemistry 17th Century – Robert Boyle: First “chemist” to perform quantitative experiments (pressure/volume) –- Incorrectly believed that the alchemist’s view that metals were not true elements and that a way would eventually be found to change one metal into another. 18th Century – George Stahl: Phlogiston flows out of a burning material. – Joseph Priestley: Discovers oxygen gas, “dephlogisticated air.” Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Law of Conservation of Mass Discovered Mass by Antoine Lavoisier is neither created nor destroyed Combustion involves oxygen, not phlogiston Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Other Fundamental Chemical Laws Law of Definite Proportion A given compound always contains exactly the same proportion of elements by mass. – always 39.34% Na and 60.66% Cl (mass) NaCl Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Other Fundamental Chemical Laws Law of Multiple Proportions When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers. The ratio of the masses of oxygen in H2O and H2O2 will be a small whole number (“2”). Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Law of Multiple Proportions Mass of oxygen that combines with 1 g of Carbon Compound 1 (CO) 1.33 g Compound 2 (CO2) 2.66 g Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Dalton’s Atomic Theory (1808) Each element is made up of tiny particles called atoms. The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 Dalton’s Atomic Theory (continued) Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms. Chemical reactions involve reorganization of the atoms - changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 Dalton’s Atomic Theory (continued) 5. Atoms cannot be subdivided, created, or destroyed. 6. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Early Experiments to Characterize the Atom J. J. Thomson - postulated the existence of electrons using cathode ray tubes. Ernest Rutherford - explained the nuclear atom, containing a dense nucleus with electrons traveling around the nucleus at a large distance. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Figure 2.7: A cathode-ray tube. The fastmoving electrons excite the gas in the tube, causing a glow between the electrodes. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 Figure 2.8: Deflection of cathode rays by an applied electric field. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Figure 2.10: A schematic representation of the apparatus Millikan used to determine the charge on the electron. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 The Electron Tiny, negatively charged particle Very light compared to the mass of an atom – 1/1837th the mass of a H atom Move extremely rapidly within the atom. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 Figure 2.9: The plum pudding model of the atom. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 Figure 2.12: Rutherford's experiment on -particle bombardment of metal foil. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 Figure 2.13: (a) The expected results of the metal foil experiment if Thomson's model were correct. (b)Actual results. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 The Modern View of Atomic Structure : Electrons electron cloud Protons: found in the nucleus, they have a positive charge equal in magnitude to the electron’s negative charge. Neutrons: found in the nucleus, virtually same mass as a proton but no charge. The nucleus contains: protons and neutrons. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 Figure 2.14: A nuclear atom viewed in cross section. Note that this drawing is not to scale. Atom is 99.9999 99..% empty space!!! !! The Mass and Charge of the Electron, Proton, and Neutron Particle Mass (kg) 31 Electron 9.11 10 Proton 1.67 1027 Neutron 1.67 10 27 Copyright©2000 by Houghton Mifflin Company. All rights reserved. Charge 1 1+ 0 21 The Mass and Charge of the Electron, Proton, and Neutron Particle Electron Relative Mass 0 Relative Charge -1 Proton 1 +1 Electron cloud Nucleus Neutron 1 0 Nucleus Copyright©2000 by Houghton Mifflin Company. All rights reserved. Location 22 Atomic Number / Atomic Mass The atomic number is equal to the number of protons. The atomic mass is equal to the number of protons + neutrons Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Atomic Number / Atomic Mass Essentially, all of the mass of the atom is considered to reside in the nucleus. In a neutral atom, the number of protons equals the number of electrons. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Isotopes All atoms of an element have the same number of protons. The number of protons = the atomic number Atoms of an element with different numbers of neutrons are called isotopes. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Isotopes continued… All isotopes of an element are chemically identical Isotopes have different masses Isotopes are identified by their mass number. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 The Chemists’ Shorthand: Atomic Symbols Mass number Atomic number 39 K 19 Element Symbol Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Figure 2.15: Two isotopes of sodium. Both have eleven protons and eleven electrons, but they differ in the number of neutrons in their nuclei. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 Isotopes Isotope Atomic Number Mass Number # of Protons # of Electrons # of Neutrons 20 18 22 U-234 Na- 24 S2- 32 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29