Dr. Brown has a radioactive source containing 1.2 x 106 (also written 1.2E+06) atoms of Putonium -238 in 1985, how many atoms will remain in 352 years? Solution The half-life of Pu-238 is 87.7 years. In 352 years\' time, this will mean that the number of atoms left is 1.2 * 10^6 * (1/2)^(352/87.7) That\'s 74292 atoms left, just about..