Radioactive Dating

Contributor: Hannah Brooks. Lesson ID: 12493

Most people have a birth certificate or other identification to show how old they are. Rocks don't have certificates, so scientists use the half-life to measure the life of long-dead organisms!



learning style
personality style
Grade Level
High School (9-12)
Lesson Type
Dig Deeper

Lesson Plan - Get It!

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Look at the image above and the silent video below.

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  • Can you guess the time period using clues from their clothing?
  • How old do you think these images might be?

Photographs and movies can often be dated based on the subjects' clothing.

It will be fun to look back at your childhood images when you are older and think, "Wow, I wore that!" Unfortunately, objects in nature are a lot harder to date.

  • Have you ever seen a rock that wears clothing?

A process called radioactive dating can be used to figure out the age of materials in nature.

topaz crystal

Radioactive dating relies on the idea that radioactive elements' nuclei degrade over time. Remember that an element's half-life measures how quickly the nuclear breakdown occurs.

Carbon, one of the most abundant elements on the earth, has a radioactive isotope called Carbon-14. Carbon-14 has six protons, six electrons, and eight neutrons. It is heavier than the more common Carbon-12.

Carbon-14 is absorbed by plants and consumed by animals while they are alive. When these living organisms die, they cannot exchange carbon with the environment, so the Carbon-14 isotope begins to degrade.

Carbon-14 is believed to have a half-life of 5,730 years. That means that every 5,730 years, the amount of Carbon-14 decreases by half.

Scientists use this value to measure the amount of Carbon-14 found in an organic sample to date the sample. This is the process that scientists use to date bones from a woolly mammoth.

woolly mammoth bones

Carbon dating can only be used for living materials. Nonliving substances, like rocks and soil, can be dated using radioactive isotopes of potassium and argon.

Potassium has many isotopes. Potassium-40 is a radioactive isotope believed to have a half-life of over one million years! Scientists count the number of atoms for each element type to determine how old the rock is.

  • What is the difference between dating organic and inorganic substances?
  • How accurate do you think radioactive dating is? Why?

Radioactive dating relies on a sample's amount of radioactive isotopes. Scientists can use Carbon-14 to determine how old the sample is if the material was once living.

Scientists can compare the number of potassium and argon atoms for nonliving materials to determine their age.

These calculations provide information about how objects relate to the environment. When bones and ancient tools are discovered, these dating processes give historical context.

It is important to note that scientific theories, such as radioactive dating, are not facts. Although the process of radioactive isotope degrading is proven, some interpret its data differently. To explore further, check out the Additional Resource in the right-hand sidebar on scientific theories.

For now, move to the Got It? section to learn more about radiometric dating.

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