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Atoms

Atoms. The Basics… or Part I. The Atom. ~400 BCE Democritus: small particle (“ atomos ” is Greek for indivisible) 1803 John Dalton: atomic model with several postulates 1897-now Modern Investigations…. Dalton’s Atomic Theory. The Postulates.

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Atoms

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  1. Atoms The Basics… or Part I CHM 1010 PGCC Barbara A. Gage

  2. The Atom • ~400 BCE Democritus: small particle (“atomos” is Greek for indivisible) • 1803 John Dalton: atomic model with several postulates • 1897-now Modern Investigations… CHM 1010 PGCC Barbara A. Gage

  3. Dalton’s Atomic Theory The Postulates • All matter consists of atoms which are indivisible and indestructible. 2. Atoms of one element cannot be converted into atoms of another element. 3. Atoms of an element are identical in mass and other properties and are different from atoms of any other element. 4. Compounds result from the chemical combination of a specific ratio of atoms of different elements. From Silberberg, Principles of Chemistry CHM 1010 PGCC Barbara A. Gage

  4. total mass total mass calcium oxide + carbon dioxide calcium carbonate CaO + CO2 CaCO3 And so… • Law of Conservation of Matter – matter cannot be created or destroyed during a physical or chemical change. reactant 1 + reactant 2 product = 56.08g + 44.00g 100.08g CHM 1010 PGCC Barbara A. Gage

  5. And so… • Law of Definite Proportions A given compound always contains the same proportion of elements by mass • CaCO310 g Ca + 3 g C + 12 g O or 40 g Ca + 12 g C + 48 g O • H2O 1 g H + 8 g O or 7 g H + 56 g O CHM 1010 PGCC Barbara A. Gage

  6. And so… • Law of Multiple Proportions -When two elements form more than one compound the ratio of the masses of the second element with 1 g of the first can always be reduced to small whole numbers • H2O (water) H2O2(hydrogen peroxide) 1 g H 1 g H 8 g O 16 g O 16/8 = 2 CHM 1010 PGCC Barbara A. Gage

  7. Atomic Structure • ~1870-1935 saw experimentation that showed that Dalton was right and wrong. • 1874 – Stoney: electricity is made of individual particles with negative charge called electrons • 1879 – Crookes: discovered “cathode rays” have distinct properties like electrons CHM 1010 PGCC Barbara A. Gage

  8. Figure 2.4 Silberberg, Principles of Chemistry Experiments to determine the properties of cathode rays. CHM 1010 PGCC Barbara A. Gage

  9. Atomic Structure • 1896 – Becquerel discovers emissions from materials (radioactivity) • 1898 – Rutherford uncovers properties of the some Becquerel emissions and names them alpha and beta www.lbl.gov/abc/graphics/magnet.gif CHM 1010 PGCC Barbara A. Gage

  10. Atomic Structure • 1897 – Thomson discovers “canal rays” are the same as positive hydrogen atoms • Model of atom is “plum pudding” with + and – particles mixed together http://commons.wikimedia.org/wiki/File:Plum_pudding_atom.svg • 1911 – Rutherford conducts “gold foil experiment” CHM 1010 PGCC Barbara A. Gage

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  13. Atomic Structure • After gold foil experiment, atomic model changes to one with + charges (protons) in dense center with – charges (electrons) surrounding the center • 1932 – Chadwick: discovers the missing mass in the atom comes from neutral particles named neutrons CHM 1010 PGCC Barbara A. Gage

  14. The coulomb (C) is the SI unit of charge. The atomic mass unit (amu) equals 1.66054x10-24 g. CHM 1010 PGCC Barbara A. Gage

  15. Atomic Symbols, Isotopes, Numbers A X The Symbol of the Atom or Isotope Z X = Atomic symbol of the element A = mass number; A = Z + N Z = atomic number (the number of protons in the nucleus) N = number of neutrons in the nucleus Isotope = atoms of an element with the same number of protons, but a different number of neutrons Figure 2.8 Silberberg, Principles of Chemistry CHM 1010 PGCC Barbara A. Gage

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  17. The Modern Reassessment of Dalton’s Atomic Theory 1. All matter is composed of atoms that are indivisible and indestructible. The atom is the smallest body thatretains the unique identityof the element. However, it can, under unusual circumstances, be destroyed (converted to energy) and it can be divided into smaller parts. 2. Atoms of one element cannot be converted into atoms of another element in a chemical reaction. Elements can only be converted into other elements in nuclear reactions. 3. All atoms of an element have the same number of protons and electrons, which determines the chemical behavior of the element. Isotopes of an element differ in the number of neutrons, and thus in mass number. A sample of the element is treated as though its atoms have anaverage mass. 4. Compounds are formed by the chemical combination of two or more elements in specific ratios. CHM 1010 PGCC Barbara A. Gage

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