A rock sample contains 1/2 microgram of the parent radioactive isotope Uranium-235 and 1/2 microgram of its daughter isotope Lead-207. If the half-life of this decay is 700 million years, how old is this rock?

Question

Quizplus · Accepted Answer

The ratio of parent to daughter is always 1:1 for a newly formed rock. As time passes, the parent decays to the daughter isotope. The half-life of 700 million years means that half of the parent isotope will have decayed into the daughter isotope after 700 million years. If the rock sample contains equal amounts of parent and daughter isotopes, then it has gone through one half-life, which is 700 million years. Therefore, the answer is C) 700 million years.

A Rock Sample Contains 1/2 Microgram of the Parent Radioactive