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What is Carbon Dating? - Definition & Overview

What is Carbon Dating? - Definition & Overview
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  • 0:00 Definition of Carbon Dating
  • 0:53 Radioactive Half-Lives
  • 2:07 Use of Carbon-14 Dating
  • 4:05 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

Ever wondered how scientists know the age of old bones in an ancient site or how old a scrap of linen is? The technique used is called carbon dating, and in this lesson we will learn what this is and how it is used. A quiz will test how much we have learned.

Definition of Carbon Dating

Carbon dating, or radiocarbon dating, is a method used to date materials that once exchanged carbon dioxide with the atmosphere. In other words, things that were living. In the late 1940s, an American physical chemist named Willard Libby first developed a method to measure radioactivity of carbon-14, a radioactive isotope. Libby was awarded the Nobel Prize in chemistry for his work in 1960.

Carbon dioxide in the atmosphere contains a constant amount of carbon-14, and as long as an organism is living, the amount of carbon-14 inside it is the same as the atmosphere. However, once the organism dies, the amount of carbon-14 steadily decreases. By measuring the amount of carbon-14 left in the organism, it's possible to work out how old it is. This technique works well for materials up to around 50,000 years old.

Radioactive Half-Lives

Each radioactive isotope decays by a fixed amount, and this amount is called the half-life. The half-life is the time required for half of the original sample of radioactive nuclei to decay. For example, if you start off with 1000 radioactive nuclei with a half-life of 10 days, you would have 500 left after 10 days; you would have 250 left after 20 days (2 half-lives); and so on. The half-life is always the same regardless of how many nuclei you have left, and this very useful property lies at the heart of radiocarbon dating.

Carbon-14 has a half-life of around 5,730 years. The graph below shows the decay curve (you may recognize it as an exponential decay) and it shows the amount, or percent, of carbon-14 remaining. You will notice that after around 40,000 years (or 8 half-lives), the amount left is starting to become very small, less than 1%. Scientists often use the value of 10 half-lives to indicate when a radioactive isotope will be gone, or rather, when a very negligible amount is still left. This is why radiocarbon dating is only useful for dating objects up to around 50,000 years old (about 10 half-lives).

The graph shows how carbon-14 decays over time with a half-life of around 5,730 years. The amount of carbon-14 remaining is reduced by 50% every half-life.
Radioactive decay curve of carbon-14

Use of Carbon-14 Dating

Radioactive carbon-14 is continually formed in the atmosphere by the bombardment of cosmic ray neutrons on nitrogen-14 atoms. After it forms, carbon-14 naturally decomposes, with a half-life of 5,730 years, through beta-particle decay. For the record, a beta-particle is a specific type of nuclear decay.

Look at this diagram here describing this. Image 1 shows carbon-14 production by high energy neutrons hitting nitrogen-14 atoms, while in Image 2, carbon-14 naturally decomposes through beta-particle production. Notice that the nitrogen-14 atom is recreated and goes back into the cycle.

(1) Carbon-14 production by high energy neutrons hitting nitrogen-14 atoms (2) carbon-14 naturally decomposes through beta-particle production.
Production of carbon 14 following by decay of carbon 14 by beta particle production

Over the lifetime of the universe, these two opposite processes have come into balance, resulting in the amount of carbon-14 present in the atmosphere remaining about constant.

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