Half Life

Half life is a term which refers to the concentration a product and the time of a reaction. Half life is the time it takes for half of a sample to be there. For instance, remember the orange juice example from the rates page? As a recap: if we know that we bought two gallons of orange juiceat the store, and we had one gallon of OJ one hour later, we can say that someone drank one gallon in that one hour. This is our rate for orange juice drinking: one gallon per hour. Not only that, but we can also say that the half-life of the orange juice is one hour. It took one hour for the amount of orange juice to drop by half. That's the definition, fair and square.

Now, this is of importance in nuclear chemistry and physics, especially in carbon dating materials. Carbon exists mostly as carbon 12 (6 protons and 6 neutrons in the nucleus). There is a rare form of carbon known as carbon 14 (6 protons and 8 neutrons in the nucleus). This carbon is present in small amounts in every living thing on this planet. The half life of carbon 14 is 5,730 years. That means, if you started with two carbon 14 atoms in a sample, it would take 5,730 years until there was only 1 carbon 14 atom - the half life is the amount of time it takes for a sample to decrease in number by one half. By analyzing samples for their ratio of carbon 14 to carbon 12, scientists have been able to determine how long something has been decaying for. For instance, let's say the normal carbon 14-12 ratio is 4 to 400,000. If you find a sample that has a carbon 14-12 ratio of 1 to 400,000, then you might say that it has undergone two half-lives. This means that the sample might be 5,730*2 years old, or 11,460 years. Why can we say this? Well, the first half life will change the ratio to 2 to 400,000, and takes 5,730 years. And the next half life will then change the ratio to 1 to 400,000, requiring another 5,730 years. The sample went through two half lives, the duration of which is 11,460 years.

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