1.  What are the four most common elements in the Sun? 
       When I write "most common", I mean "by number of atoms", 
       not "by mass of atoms."

           A recent review article by Asplund, Grevesse and Sauval,
           "The solar chemical composition" (2004), can be viewed
           at http://xxx.lanl.gov/abs/astro-ph/0410214.  It provides
           a table which shows
                                           relative           rough
           element    12 + log(N/Nh)       abundance          percentage
                                           by number          by number
        ------------------------------------------------------------------
            H            12                  1.0           92      percent
            He           10.93               0.085          1.2
            O             8.66               0.000 457      0.041 
            C             8.39               0.000 245      0.022
            Ne            7.84               0.000 069      0.0064
            N             7.78               0.000 060      0.0055


            So the top four elements are hydrogen, helium, oxygen, carbon.


   2. Pick one:
         a. What are the four most common elements in the Earth's crust?

               A table from a basic geology textbook, 
               "Essentials of Geology," by Frederick K. Lutgens
               and Edward J. Tarbuck, 7th Ed. (Prentice Hall, 2000),
               shows the following 

                   Element   percentage by  atomic   relative abundance
                               weight       weight       by number
                --------------------------------------------------------
                 oxygen         46.6         16         2.91
                 silicon        27.7         28         0.99
                 aluminum        8.1         27         0.30
                 iron            5.0         56         0.089
                 calcium         3.6         40         0.098
                 sodium          2.8         23         0.122


               This table indicates that the four most common elements
               by number in Earth's crust are oxygen, silicon,
               aluminum, and SODIUM.


         b. What are the four most common elements in the entire Earth? 

               The Nine Planets web site shows the following table
               for the total Earth:

                   Element   percentage by  atomic   relative abundance
                               weight       weight       by number
                --------------------------------------------------------
                 iron           34.6         56         0.618
                 oxygen         29.5         16         1.84
                 silicon        15.2         28         0.543
                 magnesium      12.7         24         0.529
                 nickel          2.4         58         0.041
                 sulfur          1.9         32         0.059

               By this measure, the most abundant elements in the
               entire Earth are oxygen, iron, silicon and magnesium.