A rock formed with 1000 atoms of a radioactive parent element,but contains only 250 radioactive parent atoms today.If the half-life for the radioactive element is one million years,how old is the rock?
A)250,000 years old
B)750,000 years old
C)2 million years old
D)4 million years old

Question

Quizplus · Accepted Answer

The rock is 2 million years old. This is because the half-life of the radioactive element is one million years, meaning every million years, the amount of the radioactive parent element is reduced by half. Starting with 1000 atoms, after one million years, it would reduce to 500 atoms (first half-life), and after another million years, it would reduce to 250 atoms (second half-life). Thus, it takes 2 million years to go from 1000 atoms to 250 atoms.

A Rock Formed with 1000 Atoms of a Radioactive Parent