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Notes Monday, Sept. 16th A. The Elements Chapter 3: Atomic Theory • All of the materials in the universe can be chemically broken down into about 100 different elements. Elements, Atoms, Compounds & Ions and 1. Element can have several meanings Element Element Element Microscopic form Single atom of that element • Each element has a name and a symbol. – The symbol usually consists of the first one or two letters of the element’s name. • Examples: Oxygen O Krypton Kr Macroscopic form Sample of that element large enough to weigh on a balance – Sometimes the symbol is taken from the element’s original Latin or Greek name. Generic form When we say the human body contains the element sodium or lithium, we do not mean that free elemental sodium or lithium is present. Rather we mean that atoms of these elements are present in some form. B. Compounds • Compounds are made by combining atoms of the elements just as words are constructed from the letters in the alphabet. Words 2. Names and Symbols for the Elements • Examples: Sodium Na Lead Pb Bromine Br natrium (L. sodium carbonate) plumbum (L. liquid silver) bromos (Gr. stench) 1. Formulas of Compounds • A compound is represented by a chemical formula in which the number and kind of atoms present is shown by using the element symbols and subscripts. Example: the simple sugar, glucose Compounds 1 Notes Monday, Sept. 16th 450 B.C. (Democritus) • Greek Philosopher • Matter is made up of tiny indivisible particles - ATOM “indivisible” 1808 (Dalton) Original Atomic Theory • All elements are composed of atoms • All atoms of the same element are the same • Atoms of different elements are not alike • Solid sphere Modern Definition • Smallest particle of an element that retains all chemical and physical properties of that element Original Atomic Theory - Dalton • Atoms (matter) cannot be created nor destroyed – Chemical Reactions change the arrangement of atoms • Law of Constant Composition: a compound always contains the same proportions by mass of the elements. A given compound always has the same number & arrangement of elements • Law of Multiple Proportions: different compounds can be made having different combinations of the same elements 1897 (Thomson) • Used a Cathode Ray tube –identified the presence of a negatively charged particle • http://www.chem.uiuc.edu/clcwebsite/cathode.html (this site has a demo - link) 2 Notes Monday, Sept. 16th Cathode Ray Tube Evidence of a negatively charged component with negative character: The beam (cathode ray) originated from the end connected to the negative charge. The influence of a magnet: the positive end of the magnet attracted the beam AND the negative end of the magnet repelled the beam. 1911 (Rutherford) • “Rutherford’s Experiment” http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf • “Shot” Alpha particles – positively charged (4 amu) at gold foil • Prediction: Alpha particles would pass straight through gold foil • Observed: Alpha particles were deflected at various angles 1911 (Rutherford) cont’d. Model from Thomson’s work •“Plum Pudding” Model (Atom is a sphere of positive electricity in which electrons are embedded) http://lecture.lite.msu.edu/~mmp/kap29/cd719.htm 1911 (Rutherford) cont’d. • Conclusion: Nuclear Atom • Alpha particles met a large mass that was positively charged within the atom • “Nucleus” - 1/100 000 the diameter of the atom (extremely small), massive, positively charged (Rutherford) cont’d. • By 1919 Rutherford introduced the idea that the nucleus contained a particle called the proton • Proton: Same magnitude of charge as an electron but opposite in charge (+) 3 Notes Monday, Sept. 16th 1913 (Bohr’s Model) • “Planetary model” - electrons orbit around the nucleus in a specific path Nucleus 1932 (Chadwick) • Discovered neutron mass equal to that of a proton but no electrical charge electrons 4