Principles of Radiometric Dating

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  • 0:07 Radiometric Dating
  • 1:32 Parent & Daughter Nuclides
  • 2:44 Alpha Decay
  • 3:40 Beta Decay
  • 4:53 Gamma Decay
  • 6:08 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Radiometric dating is a method used to determine the age of rocks and other materials based on the rate of radioactive decay. Learn about three common types of radioactive decay: alpha decay, beta decay and gamma decay.

Radiometric Dating

Determining your age is easy. You simply subtract your birth date from the current date. But determining the age of a rock... well, that's not as straightforward. After all, a rock can't tell you when it was born.

So we have to rely on something called radiometric dating to figure out the age of rock. Radiometric dating is a method used to date rocks based on the known decay rate of radioactive isotopes. This method works because rocks are radioactive. Now, they do not give off enough radiation that you have to be afraid to pick them up, but they do contain naturally occurring radioactive elements like uranium, for example.

It was also discovered that these elements decayed into other elements at fixed rates. Because these rates do not change and because the radiation that rocks give off can be measured, it became possible to calculate the time the rock was formed or, in other words, the rock's birth date - give or take a few thousand years or so.

Radiometric dating is sometimes referred to as radioactive dating. In fact, you might like this term better, because the dating method relies on the known decay rate of radioactive isotopes. Regardless of which name you prefer, the discovery was a true breakthrough that provided a tool to predict the geological history of the Earth and even the age of the Earth itself.

Radioactive Decay and Parent and Daughter Nuclides

To better understand how radiometric dating helps us determine the age of rocks, it will help us to gain a better understanding of how elements decay. Radioactive decay is the term used for the process by which an unstable atomic nucleus loses energy by releasing radiation. We know that elements can exist as isotopes, which means that their atomic nuclei contain the same number of protons but different numbers of neutrons.

Radioactive Decay- Isotopes
Isotopes Hydrogen

Specially defined isotopes, called nuclides, can be unstable and therefore undergo radioactive decay. When they do, they release energy and get transformed into different nuclides. It's as if the nucleus is feeling too full of energy and it has to get rid of some, much like a hyperactive child who is so full of energy that he cannot stay seated in his chair. We call the unstable nuclide that undergoes radioactive decay the parent nuclide and the nuclide that results from the radioactive decay the daughter nuclide. This is a fairly easy concept to remember because it is as if the original nuclide is giving birth to the new nuclide, much like a human parent and daughter relationship.

Alpha Decay

This transformation into a different nuclide can be accomplished in different ways. Alpha decay is a type of radioactive decay where an alpha particle is emitted. So, to understand this process, we need to know that an alpha particle is two protons and two neutrons bound together, which is the same as a helium nucleus. In other words, an alpha particle is a helium nucleus. When we talk about an alpha particle, we use the first letter of the Greek alphabet, which is here.

So let's zoom into a nucleus and take a look at this alpha decay process. Inside this nucleus, we see the protons and neutrons. This parent nucleus is feeling somewhat unstable because it is too big or simply has too many protons, and it wants to get to a more stable state, so it's going to take two protons and two neutrons and kick them out of the nucleus as we see here.

Alpha Decay
Alpha Radioactive Decay

Beta Decay

If we have a parent nucleus where the neutron-to-proton ratio is too great, then that parent might be feeling unstable about its circumstance and want to move to a more stable state through beta decay. Beta decay is a type of radioactive decay where a beta particle is emitted. A beta particle is shown with the Greek letter beta and is an electron that is emitted from the nucleus. Now, this might sound a bit odd to you, because you do not typically think about electrons being inside of a nucleus. Instead, you recognize them as those little things that look like orbiting planets moving around the outside of a nucleus.

But essentially, what is happening with beta decay is that we are taking a neutron, removing a negative charge and turning it into a proton. So let's zoom into this nucleus and take a look at the beta decay process. Inside this nucleus, we see protons and neutrons, but let's say one of these neutrons is feeling as if things would be more stable if it could turn into a proton. So that neutron basically emits an electron (the beta particle) and this essentially turns it into a proton.

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