Why Are Some Atoms Radioactive?
Ionizing & Non-Ionizing Radiation
Radiation is energy given off by some atoms in the form of particles or rays (photons), but why do these atoms need to throw off energy?
On this page:
- What causes atoms to be radioactive?
- Can unstable atoms become stable?
- How long do radionuclides stay radioactive?
- What's the difference between radiation and radioactivity?
- Is all ionizing radiation the same?
What causes atoms to be radioactive?
Atoms found in nature are either stable or unstable. An atom is stable if the forces among the particles that make up the nucleus are balanced. An atom is unstable (radioactive) if these forces are unbalanced if the nucleus has an excess of internal energy. Unstable atoms are called radionuclides. The instability of a radionuclide's nucleus may result from an excess of either neutrons or protons. An unstable nucleus will continually vibrate and contort and, sooner or later, attempt to reach stability by some combination of means:
- ejecting neutrons, and protons
- converting one to the other with the ejection of a beta particle or positron
- the release of additional energy by photon (i.e., gamma ray) emission.
Can unstable atoms become stable?
Yes. As the unstable nucleus emits radiation (disintegrates), the radionuclide transforms to different nuclides. The process is called radioactive decay. It will continue until the forces in the nucleus are balanced. For example, as a radionuclide decays, it will become a different isotope of the same element if the number of neutrons changes and a different element altogether if the number of protons changes.
Often, when a radionuclide decays, the decay product (the new nuclide) is also radioactive. This is true for most naturally occurring radioactive materials and for some fission products. In order to become stable, these materials must go through many steps, becoming a series of different nuclides and giving off energy as particles or rays at each step. The series of transformations that a given radionuclide will undergo, as well as the kind of radiation it emits, are characteristic of the radionuclide. This is called a 'decay chain.'
How long do radionuclides stay radioactive?
It depends on the kind of radioactive material you're dealing with. The rate of decay is one of the characteristics of radionuclides. Scientists talk about this rate as a radionuclide's radioactive half-life, commonly referred to as just half-life. It is the time required for the disintegration of one-half of the radioactive atoms that are present when measurement starts. It does not represent a fixed number of atoms that disintegrate, but a fraction. For any given radionuclide, its half-life remains constant.
- Radioactive Half-Life
This page explains half-life and the origin of the term.
What's the difference between radiation and radioactivity?
Radiation is the energy that is released as particles or rays, during radioactive decay. Radioactivity is the property of an atom that describes spontaneous changes in its nucleus that create a different nuclide. These changes usually happen as emissions of alpha or beta particles and often gamma rays. The rate of emission is referred to as a material's activity.
Each occurrence of a nucleus emitting particles or energy is referred to as a disintegration. The number of disintegrations per unit time ( is called the activity of a sample.(Since each disintegration transforms the atom to a new nuclide, transformation is often substituted for disintegration in talking about radioactive decay and activity.)
Activity is expressed in becquerels or curies, with curies being the original unit and remaining more common in the U.S. One becquerel equals one transformation per second. One curie equals 37 billion disintegrations per second, but was originally defined the number of disintegrations of one gram of pure radium per second.
This page describes and explains the origin of the unit, curie.
Is all ionizing radiation the same?
No. Ionizing radiation can be in the form of alpha or beta particles or gamma rays (photons), and each form behaves differently. The kind of radiation given off by a nucleus depends on the nature of the imbalance in the nuclear forces:
Alpha particles can be shielded by a sheet of paper or by human skin. However, if radionuclides that emit alpha particles are inhaled, ingested, or enter your body through a cut in your skin, they can be very harmful.
Beta particles cannot be stopped by a sheet of paper. Some beta particles can be stopped by human skin, but some need a thicker shield (like wood) to stop them. Just like alpha particles, beta particles can also cause serious damage to your health if they enter your body. For example, if ingested, some radionuclides that emit beta particles might be absorbed into your bones and cause damage.
Gamma and X-Rays
Gamma rays are the most penetrating of the three types of radiation listed here. Gamma rays usually accompany beta, and some alpha rays. Gamma rays will penetrate paper, skin, wood, and other substances. To protect yourself from gamma rays, you need a shield at least as thick as a concrete wall. This type of radiation causes severe damage to your internal organs. (X-rays fall into this category, but they are less penetrating than gamma rays.)
Other Types of Radioactive Decay (minor modes)
Radioactive decay is a complex process. Nuclei have several other means by which to emit excess energy.