A rock sample from the moon includes a mineral that contains small amounts of the radioactive isotope Potassium- 40 and its daughter element Argon- 40 (half- life of 1.3 billion years). This mineral would not form with any Argon- 40. Consider a crystal with 7 atoms of Argon- 40 for every 1 atom of Potassium- 40. How many atoms of Potassium- 40 were present when the crystal formed for each atom of Potassium- 40 that exists today?
A)1
B)7
C)8
D)None

Question

Quizplus · Accepted Answer

The presence of 7 atoms of Argon-40 for every 1 atom of Potassium-40 indicates that the crystal has gone through two half-lives of Potassium-40 (1.3 billion years each). Initially, there would have been 4 atoms of Potassium-40 for each atom present now (after one half-life, it would be 2 atoms, and after two half-lives, it would be 1 atom). Therefore, for each atom of Potassium-40 present today, there were originally 8 atoms (7 turned into Argon-40, and 1 remains as Potassium-40).

A Rock Sample from the Moon Includes a Mineral That