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ATOMIC THEORY WORKSHEET 1. Which of the following statements of the atomic theory proposed by John Dalton at the beginning of the 19th century are not quite true in light of modern atomic physics? Rewrite each statement to reflect the current understanding of the atomic theory. An element is made up of atoms. All atoms of a given element are identical. Atoms cannot be created or destroyed. All atoms of a given element have the same number of protons in the nucleus. They may differ in the number of neutrons. Chemically the atoms of a given element are virtually indistinguishable: the types of chemical reactions are the same; the rates may slightly differ for different isotopes. Atoms cannot be created or destroyed in chemical reactions. In chemical reactions the old bonds between atoms are broken down and new bonds are formed. Atoms, however, can be created or destroyed in nuclear reactions: radioactive decays, nuclear fission and fusion. All atoms of one element have the same mass. Atoms of two different elements have different masses. Atoms of different elements may combine in the ratio of small, whole numbers to form compounds. The main characteristic of an element is the atomic number. Two elements differ from each other by their atomic numbers. In fact, atoms of two different elements may have masses that are very close to each other. For example, 39.9624 amu for an atom of argon-40 and 39.9640 amu for an atom of potassium-40. This is often the case for many compounds, especially inorganic. It was always the case for compounds known to Dalton. Most organic compounds (whose composition was not known to Dalton) do not follow that rule: C12H22O11 (sugar), C27H46O (cholesterol), C17H21NO4 (cocaine). 2. Calculate the atomic weight of neon using the data on its isotopes provided in the table below. Isotope Number of Protons Number of Neutrons Mass Number Atomic Mass (amu) Natural Abundance (atom %) 20 Ne 10 10 20 19.9924 90.48 21 Ne 10 11 21 20.9938 0.27 22 Ne 10 12 22 21.9914 9.25 Weighted Average of Atomic Masses = 19.9924 amu × 0.9048 + + 20.9938 amu ×0.0027 + 21.9914 amu × 0.0925 = 20.18 amu = Atomic Weight or Relative Atomic Mass (the number below the symbol in the Periodic Table of the Elements) ATOMS, ISOTOPES, ELEMENTS Symbol of the Atom (Isotope) Atomic Mass Number Number Number of Protons in the Atom Number of Neutrons in the Atom Number of Electrons Mass of the Atom in the Atom Name of the Element Name of the Isotope 235 92 92 235 92 143 92 235.0439 amu uranium uranium-235 238 92 U 92 238 92 146 92 238.0508 amu uranium uranium-238 34 16 S 16 34 16 18 16 33.9679 amu sulfur sulfur-34 37 17 Cl 17 37 17 20 17 36.9659 amu chlorine chlorine-37 39 19 K 19 39 19 20 19 38.9637 amu potassium potassium39 12 6 C 6 12 6 6 6 12 amu (exactly) carbon carbon-12 13 6 C 6 13 6 7 6 13.0034 amu carbon carbon-13 Na 11 23 11 12 11 22.9898 amu sodium sodium-23 U 23 11