Half-life: Calculating Radioactive Decay and Interpreting Decay Graphs

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  • 0:05 Introduction
  • 1:11 Half-Life
  • 1:37 Half-Life Example
  • 3:32 Simulating Half-Life
  • 5:17 Scaling Up
  • 6:05 Lesson Summary
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Lesson Transcript
Instructor: Kristin Born

Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry.

What causes a radioactive particle to decay? We'll never really know, but our best guess lies in probability. In this lesson, we are going to focus on the half-life, a way of measuring the probability that a particle will react.


Imagine you're getting settled in to watch the new action film at your local theater. You have a big tub of popcorn on your lap, and you're sitting back and watching as the previews begin. About 15 minutes later, the previews finish up and you notice half of your popcorn is gone! It must have been good. The movie starts and you slow down your eating a little, but 15 minutes after the movie has started, you have eaten half of what you had left and are down to a quarter of your popcorn. This continues for the rest of the movie until all of your popcorn is gone.

Popcorn eaten over the course of the movie
Half-Life Decay

If we were to graph your popcorn eating during the movie, it may look something like this. You may notice a few things about this graph. First, your popcorn eating did not happen at a steady pace. If that were the case, it would look more like a straight line. What it shows is that you ate faster at the beginning than at the end, because more popcorn is consumed in the first 15 minutes than in the second 15 minutes. The second thing you may notice is that every 15 minutes you eat half of what you had.


Decay of an atom can be predicted by using its half-life.
Half-Life Example

This phenomenon takes place every day in many chemical reactions and nuclear reactions, and it is called the half-life, which is the amount of time it takes for half of a sample to react. Your popcorn had a half-life of 15 minutes, meaning that every 15 minutes, half of it will get eaten. Just like your popcorn, radioactive particles have half-lives.

A Half-Life Example

Let's take an example. Say we have a bunch of cobalt-60 atoms. Cobalt-60 decays down to nickel-60 during beta decay. But how do we know when the cobalt-60 atoms are going to decay? Are they all going to decay at once or randomly? Initially it may seem like atoms decay randomly, but their probability of decaying can be predicted using an atom's half-life.

It turns out that the half-life of cobalt-60 is about 5.27 years. That means if I start out with 16 cobalt-60 atoms, and I wait 5.27 years, I will probably be left with eight cobalt-60 atoms and eight nickel-60 atoms. Then, if I wait 5.27 more years, half of the eight cobalt-60 atoms that were left should decay, giving me only four cobalt-60 atoms and a total of 12 nickel-60 atoms. If I wait 5.27 years after that, half of the cobalt that remained will decay into nickel-60, giving me 14 atoms of nickel-60 and only two atoms of cobalt-60. If I wait another 5.27 years, half of those two remaining atoms, so one atom, should decay, giving me a total of 15 nickel-60 atoms and one lonely little cobalt-60 atom. Now, we can never have half of an atom, so what happens next? This is where probability makes more of a presence. If I wait 5.27 more years, there is a 50% chance that the one remaining cobalt-60 atom will decay. It either will, or it won't. If it does, then all the atoms will have decayed. If it doesn't, then it has a 50% chance of decaying in the next 5.27 years.

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