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Explores what makes elements radioactive and which elements are always radioactive.

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This fossil skull came from an ancient human ancestor. Scientists used carbon-14 dating to estimate its age. Carbon-14 is one of several radioisotopes that scientists use to estimate the ages of fossils and other ancient materials.

What Are Radioisotopes?

All the atoms of a given element have the same number of protons in their nucleus, but they may have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes. Many elements have one or more isotopes that are radioactive. These isotopes are called radioisotopes. Their nuclei are unstable, so they break down, or decay, and emit radiation.

Q: What makes the nucleus of a radioisotope unstable?

A: The nucleus may be unstable because it has too many protons or an unstable ratio of protons to neutrons. For a nucleus with a small number of protons to be stable, the ratio of protons to neutrons should be 1:1. For a nucleus with a large number of protons to be stable, the ratio should be about 1:1.5.

An Example: Carbon-14

Find carbon in the Figure below, and you’ll see that its atomic number is 6. This means that all carbon atoms have 6 protons per nucleus. Almost all carbon atoms also have 6 neutrons per nucleus. These carbon atoms are called carbon-12, where 12 is the number of protons (6) plus neutrons (6). This gives carbon-12 nuclei a 1:1 ratio of protons to neutrons, so carbon-12 nuclei are stable.

Some carbon atoms have more than 6 neutrons, either 7 or 8. Carbon atoms with 8 neutrons are called carbon-14 (6 protons + 8 neutrons). The nuclei of carbon-14 atoms are unstable because they have too many neutrons relative to protons, so they gradually decay.

Q: What is the proton-to-neutron ratio of carbon-14 nuclei?

A: With six protons and 8 neutrons, the ratio is 6:8, or 1:1.3.

Q: How is carbon-14 used to estimate the ages of fossils?

A: Living things take in carbon, including tiny amounts of carbon-14, throughout life. The carbon-14 constantly decays, but more carbon-14 is taken in all the time to replace it. After living things die, no new carbon-14 is taken in, and the carbon-14 they already have keeps decaying. The older a fossil is, the less carbon-14 it still has, so the remaining amount can be measured to estimate the fossil’s age. 

Radioactive elements in the periodic table

Periodic Table of the Elements

Radioactive Elements

In elements with more than 83 protons, all of the isotopes are radioactive. In the Figure above, these are the elements with a yellow background. The force of repulsion among all those protons makes the nuclei unstable. Elements with more than 92 protons have such unstable nuclei that they don’t even exist in nature. They have only been created in labs.


  • Many elements have one or more isotopes that are radioactive. These isotopes are called radioisotopes. An example of a radioisotope is carbon-14.
  • The nuclei of radioisotopes are unstable, so they constantly decay and emit radiation.
  • In elements with more than 83 protons, all of the isotopes are radioactive.


  1. What is a radioisotope? Give an example.
  2. Why do radioisotopes have unstable nuclei?
  3. Which elements in the periodic table have only radioactive isotopes?

Explore More

Watch the video about radioisotopes and then answer the questions below. 

  1. Describe the analogy in the video for stable and radioactive isotopes.
  2. Complete the following table below:
Atomic Number Proton-to-Neutron Ratio for a Stable Nucleus
1 - 19
20 – 82
> 82
  1. How can radioisotopes become stable? List three ways and state when each way occurs.

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Radioactive isotope, or isotope that emits radiation from its nucleus.

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