Carbon-14 has a half-life of approximately 5,730 years. How many half-lives have occurred in a sample that contains 50 percent of the original amount of the parent isotope?

Question

gavlois1 · Answer

A half life is the amount of time it takes for something to decrease by half of its original amount. If it starts at 100%, undergoing one half life would leave it with 50%, 2 half lives with 25% and so on. 

So if there is 50% left, then it has undergone 1 half life.

anonymous · Answer

A potassium isotope has a half-life of 1.25 billion years. Given that scientists estimate Earth's age to be 4.6 billion years, what is the most likely percentage of parent to daughter isotopes of this element currently existing on Earth?