posted by Phillip .
It takes about 10^16 years for just half the samarium-149 in nature to decay by alpha-particle emission. Write the decay equation, and find the isotope that is produced by the reaction.
62149Sm ==>24He + X
In a decay reaction the subscripts must add up and superscripts must add up. Therefore, the atomic number (the subscript) must be 60 for X and the superscript (the mass number) must be 145. Use that information to identify element X and write the symbol for it in place of X. I'll redo this if the subscripts and superscripts mess up.