Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
ATOMIC WEIGHT AND ATOMIC MASS UNIT (From College Chemistry by J.L. Rosenberg, L.M. Epstein, and P.J. Krieger) 1. What is atomic weight? 2. 12 grams of carbon-12 contain approximately 6.022×1023 atoms. What is the value of one atomic mass unit (amu or u) in grams? m 12 = 12 amu (exactly) C 1 amu = 1 1 ×m 12C =12 × 12 12 g 6.022×10 23 = 1.66×10−24 g 3. Although there is only one naturally occurring isotope of iodine, 127I, the atomic mass of iodine given in the modern periodic table is 126.90447, but not 127. Explain. 4. At one time there was a chemical atomic mass scale based on the assignment of the value 16.0000 to naturally occurring oxygen. What would have been the atomic weight, on such a table, of silver? mass of the “avg.” Ag atom on the old scale mass of 16O atom on the old scale X amu 16 amu (exactly) = = 107.8682 amu 15.9994 amu mass of the “avg.” Ag atom on the new scale mass of 16O atom on the new scale A.W. of Ag on the old scale = 107.8722 amu 5. On the old atomic mass scale used by physicists, the mass of oxygen-16 atom was assigned to be exactly 16 amu. What would be the atomic weight of magnesium on that scale? mass of the “avg.” Mg atom on the old scale mass of 16O atom on the old scale X amu 16 amu (exactly) = 24.3050 amu 15.9949 amu = mass of the “avg.” Mg atom on the new scale mass of 16O atom on the new scale A.W. of Mg on the old scale = 24.3127 amu 6. In a chemical atomic mass determination, the tin content in 3.7692 g of tin tetrachloride was found to be 1.7170 g. If the atomic weight of chlorine is taken as 35.453 amu, what is the value for the atomic weight of the tin isolated during this experiment? mass of Cl in SnCl4 = 3.7692 g − 1.7170 g = 2.0522 g Sn Cl = X amu 4 × 35.453 amu A.W. of Sn = 118.649 amu = 1.7170 g 2.0522 g X = 118.649 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