The following table shows the currently accepted half-life values for six parent isotopes. parent isotope currently accepted half-life values uranium-238 4.5 billion years uranium-235 704 million years thorium-232 14.0 billion years rubidium-87 48.8 billion years potassium-40 1.25 billion years samarium-147 106 billion years earth is approximately 4.5 billion years old. given this information and the data in the table, which of the following isotopes could today contain less than 50 percent of its parent isotope? potassium-40 rubidium-87 samarium-147 uranium-238

  the   balanced   equation   for   reaction of   ca metal   with   o2 to   produce   cao   is as below

2 ca+ o2→ 2 cao2

  explanation

the reaction is   balanced     since the   number of atoms   in reactant side is   equal to   number of atoms   in   product side.   for   example   they   are 2   atom of   ca in   reactant   side   and   2   atoms of ca   in   the product side.

  the   process   is a   meet the   requirement   of   redox   reaction because :     calcium   is   oxidized   from   oxidation state 0   to 2+   while   oxygen   is reduced from   oxidation   state   0   to 2-