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.
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.
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.
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.
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.
There is another type of decay that we want to learn about, but unlike alpha and beta decay, this type of decay does not release a particle. So with this decay, we do not see the number of protons or neutrons within a nucleus changing. However, it does give off a lot of energy. This decay is called gamma decay, and it is denoted by the third letter of the Greek alphabet, gamma, which looks like a lowercase ‘y.
‘We define gamma decay as a type of radioactive decay where a gamma ray is emitted. A gamma ray is a high-energy photon, and you have experienced gamma rays if you ever had an x-ray taken. Gamma rays can travel through your body but not through lead.
That is why if you ever had an x-ray of your teeth, your dentist first laid a heavy lead apron over your chest, so the gamma rays only penetrated your cheek and not the rest of your body.When a gamma ray is emitted, the atomic nucleus releases energy, so we will see gamma decay taking place in a nucleus where the energy is too high. The nucleus moves to a lower energy state by giving off this high-energy photon, and this allows the nucleus to reorganize itself into a more stable state.
Let’s review. Radiometric dating, also known as radioactive dating, is what we use to determine the age of rocks.
To be more specific, it is a method used to date rocks based on the known decay rate of radioactive isotopes that are found within the rocks. This decay rate is referring to radioactive decay, which is the process by which an unstable atomic nucleus loses energy by releasing radiation.This release of energy allows the nucleus to become more stable. We call the unstable nuclide that undergoes radioactive decay the parent nuclide and the nuclide that results from the radioactive decay the daughter nuclide.
There are different types of radioactive decay. If a nucleus is unstable because it is too big or has too many protons, then we might see alpha decay, which is a type of radioactive decay where an alpha particle is emitted. An alpha particle is two protons and two neutrons bound together, which is the same as a helium nucleus.If we have a nucleus where the neutron-to-proton ratio is too great, we might see beta decay, which is a type of radioactive decay where a beta particle is emitted. A beta particle is an electron that is emitted from the nucleus. With beta decay, a neutron essentially loses an electron, turning into a proton.If the nucleus has too much energy and wants to move to a more stable lower energy state, we might see gamma decay, which is a type of radioactive decay where a gamma ray is emitted.
A gamma ray is a high-energy photon. Unlike alpha and beta decay, this type of decay does not release a particle. Therefore, the number of protons or neutrons within a nucleus does not change, but energy is released, allowing the nucleus to reorganize itself into a more stable state.
After completing this lesson, you should be able to:
- Define radiometric dating
- Describe how unstable nuclides undergo decay
- Identify alpha, beta and gamma decay