When you think of radioactive material, what comes to mind? Glowing green goo? Wild fish-mutant monsters? Maybe radioactive bug-bites that give mild-mannered students super-powers? Fortunately, that is science fiction, not actual science. Real radioactive material is actually in the air you breathe, the food you eat, and in pretty much every single rock on Earth.
In order for George Guthrie to assign dates to the samples he finds during his research expeditions, he needs to know a little bit about what makes up these rocks, minerals, soils, and other substances.
The Atom
The building blocks of all substances on Earth, including rocks, skyscrapers, lava, and everything in between, are called atoms. You cannot see an individual atom because they are so tiny. There are more atoms in the period at the end of this sentence than there are people on this planet. Even though we cannot directly see atoms, we can observe their impact and learn about how they behave. For example, when you fill up a balloon, you cannot see the atoms going into the balloon, but you see the balloon expanding as a result of filling it with atoms.
Protons, Neutrons, and Electrons
Every atom is made of three basic types of particles: protons, neutrons, and electrons. When atoms have the same number of protons, we say that those atoms are the same element. For example, every atom with 79 protons is a gold atom. All atoms with eight protons are oxygen atoms, all atoms with two protons are helium atoms, and so on. The element is determined by the number of protons in an atom.
Isotopes
When two atoms have the same number of protons but different numbers of neutrons, they are called isotopes. Scientists refer to isotopes by stating the name of the element and adding their total number of protons and neutrons together. All uranium atoms contain 92 protons. So, uranium with 146 neutrons is called uranium-238 (remember, we add the 92 protons to the 146 neutrons) and uranium with 143 neutrons is called uranium-235.
Radioactive Decay
Some isotopes, such as uranium-235, are unstable. They will naturally release energy over time to become more stable. This natural release of energy is what we call radioactivity. The energy release is significant, and can be used to power nuclear power plants and reactors.
Over time, as the unstable isotopes release energy, they slowly decay to form more stable atoms. This is called radioactive decay. In the case of uranium-235, it will undergo radioactive decay to form lead-207. The unstable uranium-235 is called the parent isotope, and the stable atom it turns into, lead-207, is called the daughter isotope. Luckily, there is a pattern to how isotopes decay. This pattern is the key to understanding why radioactivity is valuable when dating rocks and minerals.
Half-Life
| Parent Isotope |
Daughter Isotope |
Half-Life (years) |
| Carbon-14 |
Carbon-12 |
5,730 |
| Uranium-235 |
Lead-207 |
703,800,000 |
| Uranium-238 |
Lead-206 |
4,468,000,000 |
| Potassium-40 |
Argon-40 |
1,280,000,000 |
| Thorium-232 |
Lead-208 |
14,100,000,000 |
The
half-life of an isotope is the amount of time it takes for half of the parent isotope to decay into the daughter isotope. For example, imagine you had two kilograms of radioactive uranium-235 (although we would not advise having such a thing). In about 704 million years, one kilogram of the uranium-235 (parent isotope) would undergo radioactive decay to form lead-207 (daughter isotope). After that amount of time, you would have one kilogram of uranium-235 remaining. If you allowed another 704 million years to pass, half of the remaining one kilogram of uranium-235 would decay, leaving you with 0.5 kilograms of uranium-235. Since half of uranium-235 decays about every 704 million years, we refer to 704 million years as the half-life of uranium-235. Scientists use several different isotopes to date rocks, minerals, and fossils because each has a specific half-life.
