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42                              ATOMS AND ATOMIC MASSES                           [CHAP. 3


               EXAMPLE 3.11. Naturally occurring copper consists of 69.17%  63 Cu and 30.83%  65 Cu. The mass of  63 Cu is
                                       65
               62.939 598 amu, and the mass of Cu is 64.927 793 amu. What is the atomic mass of copper?
               Ans.  The weighted average is the sum of the mass of each isotope times its fraction present:
                                                   69.17                30.83

                                      62.939 598 amu     + 64.927 793 amu     = 63.55 amu
                                                   100                  100

               3.6. PERIODIC TABLE
                   The periodic table is an extremely useful tabulation of the elements. It is constructed so that each vertical
               column contains elements that are chemically similar. The elements in the columns are called groups, or families.
               (Elements in some groups can be very similar to one another. Elements in other groups are less similar. For
               example, the elements of the first group resemble one another more than the elements of the fourth group from
               the end, headed by N.) Each row in the table is called a period (Fig. 3-1).


                                                                       One of the
                                                                        groups





                                        One of the
                                         periods



                                        Fig. 3-1. A period and a group in the periodic table






                                                1
                                                       Main groups
                                                2
                                                3
                                         Periods  4
                                                5      Transition groups
                                                6
                                                7

                                                        Inner transition elements
                                       Fig. 3-2. Periods and element types in the periodic table


                   There are three distinct areas of the periodic table—the main group elements, the transition group elements,
               and the inner transition group elements (Fig. 3-2). We will focus our attention at first on the main group
               elements, whose properties are easiest to learn and to understand.
                   The periods and the groups are identified differently. The periods are labeled from 1 to 7. Some reference is
               made to period numbers. The groups are referred to extensively by number. Unfortunately, the groups have been
               labeled in three different ways (Table 3-2):
               1. Classical: Main groups are labeled IA through VIIA plus 0. Transition groups are labeled IB through VIII
                  (although not in that order).
               2. Amended: Main groups and transition groups are labeled IA through VIII and then IB through VIIB plus 0.
               3. Modern: Groups are labeled with Arabic numerals from 1 through 18.
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