# What is the half-life of potassium-40?

What happened to the potassium-40 sample after 1.25 billion years?

The potassium-40 sample started with 50 atoms and after 1.25 billion years, there are only 25 atoms left. This indicates that half of the original atoms have decayed. Therefore, the half-life of potassium-40 can be determined.

## Half-life of Potassium-40

**Half-life** is the time it takes for half of a radioactive substance to decay. In the case of potassium-40, the half-life can be calculated based on the information provided. Starting with 50 atoms and ending with 25 atoms after 1.25 billion years, it is clear that the half-life of potassium-40 is the time it takes for 50 atoms to decay to 25 atoms.

To calculate the half-life of potassium-40, we can use the formula:

**Half-life = Total time elapsed / Number of half-lives**

Given that the total time elapsed is 1.25 billion years and half of the atoms have decayed, we can calculate the half-life of potassium-40 as follows:

**Half-life = 1.25 billion years / 1 half-life = 1.25 billion years**

Therefore, the correct answer is **1.25 billion years**, which corresponds to option D in the initial question.