Properties of Radioactive Isotopes: An Overview

Key points

  • Radioactive decay occurs when a radioactive atom gives off radiation in the form of energy or particles.
  • Radioactive atoms give off radiation to become more stable.
  • There are four types of radiation given off by radioactive atoms.
Detail of a rendered and partially blurred chart of isotopes.


Radioactive decay is the process in which a radioactive atom spontaneously gives off radiation. This radiation is given off in the form of energy or particles to reach a more stable state.

It is important to distinguish between radioactive material and the radiation it gives off.

Radioactive decay chains

Some radionuclides go through a series of transformations before they reach a stable state. For example, uranium-238 ultimately transforms into a stable atom of lead. But in the process, several types of radioactive atoms are generated. This is called a decay chain. When uranium-238 decays, it produces several isotopes of:

The uranium decay chain showing a transition from uranium to thorium to radium to radon to bismuth to lead.
All of these radioactive atoms are part of our natural environment.


There are four types of radiation given off by radioactive atoms. Radioactive atoms give off one or more of these types of radiation to reach a more stable state. Additionally, each type of radiation has different properties that affect how we can detect it and how it can affect us.

Alpha particles

An illustration of alpha particles showing two blue circles and two reddish-orange circles to make up the particle.
Alpha particles are large particles that travel up to an inch in the air.

Alpha particles are very easy to block, even with something as thin as a sheet of paper. They do not present an external hazard to people because they can't get through our outer layer of dead skin cells.

However, they can be very damaging to cells inside our bodies if we breathe or eat alpha-emitting radioactive material. Damage can also occur if the radioactive material is introduced through an open wound.

Beta particle

Beta particle drawing showing one yellow circle making up the beta particle.
Beta particles are smaller particles that travel several feet in air.

Beta particles can be blocked effectively with a few inches of plastic, or even a layer of clothing. However, beta particles can carry enough energy to cause burns on exposed skin. They can present an internal hazard if we breathe or eat beta-emitting radioactive material. Radioactive material may also be introduced through an open wound.

Gamma rays

Drawing of a squiggly line showing the shape of a gamma ray.
Gamma rays can travel many yards in air.

Gamma rays are primarily an external hazard because of their ability to go through material. It takes a few inches of lead or other dense substance to block gamma rays. Gamma rays can be an internal hazard if we breathe or eat gamma-emitting radioactive materials.

Radioactive material can also be introduced through an open wound. The damage they do to cells inside our bodies is not as severe as that done by alpha and beta particles. The best way to protect yourself from a gamma-emitter is to increase the distance between yourself and the source.


A drawing showing the placement of a neutron.
Neutrons are neutral particles with no electrical charge.

Neutrons can travel great distances in the air. As neutrons travel through matter, they crash into atoms. These atoms can become radioactive.

Neutrons are more effective at damaging cells of the body than are other forms of ionizing radiation, such as x-rays. The best way to protect against neutron radiation is by providing shielding. Good shielding materials against neutrons include thick materials such as, concrete, rock, or dirt. The best shielding against neutrons contains a lot of water or hydrogen.


Another feature of each radionuclide is its half-life. Half-life is the length of time it takes for half of the radioactive atoms of a specific radionuclide to decay. After seven half-lives, you will have less than one percent of the original amount of radiation.

Depending on the radionuclide, this process could be fast or take a very long time. Radioactive half-lives can range from less than a nanosecond, to hours, days, or sometimes millions and billions of years or more.

Radionuclides used in nuclear medicine procedures have short half-lives.

  • For example, technetium-99m, one of the most common medical isotopes used for imaging studies, has a half-life of 6 hours.
  • The short half-life of technetium-99m helps keep the dose to the patient low. After 24 hours, the radioactivity from the procedure will be reduced by more than 90%.

Uranium is a radionuclide that has an extremely long half-life.

  • Naturally occurring uranium-238 present in the Earth's crust has a half-life of almost 4.5 billion years.
  • If you take a soil sample anywhere in the world, including your backyard, you will find uranium atoms that date back to when the Earth was formed.

A closer look at half-life

Let's take a closer look at half-life.

Radioactive half-life showing 50 radioactive atoms.
If you start with 100 atoms, after one half-life you'll have 50 radioactive atoms.
Diagram showing half-life leading to 25 radioactive atoms.
After two half-lives, you'll have 25 radioactive atoms.

And after a third half-life, you'll have 12 radioactive atoms. Then 6, then 3, then 1. Eventually, all of the radioactive atoms in that population will reach their more stable state.



Alpha Particles

The nucleus of a helium atom, made up of two neutrons and two protons with a charge of +2. Certain radioactive nuclei emit alpha particles. Alpha particles generally carry more energy than gamma or beta particles. They deposit that energy very quickly while passing through tissue.

Alpha particles can be stopped by a thin layer of light material, such as a sheet of paper. Alpha particles cannot penetrate the outer, dead layer of skin. Therefore, they do not damage living tissue when outside the body. When alpha-emitting atoms are inhaled or swallowed, however, they are especially damaging. This allows them to transfer relatively large amounts of ionizing energy to living cells.


The smallest particle of an element that can enter into a chemical reaction.

Beta Particles

Electrons ejected from the nucleus of a decaying atom. Although they can be stopped by a thin sheet of aluminum, beta particles can penetrate the dead skin layer. They can pose a serious direct or external radiation threat and can be lethal depending on the amount received. They also pose a serious internal radiation threat if beta-emitting atoms are ingested or inhaled.

Decay Chain (Decay Series)

The series of decays that certain radioisotopes go through before reaching a stable form. For example, the decay chain that begins with uranium-238 (U-238) ends in lead-206 (Pb-206). This transformation forms isotopes, such as uranium-234 (U-234), thorium-230 (Th-230), radium-226 (Ra-226), and radon-222 (Rn-222).

Gamma Rays

High-energy electromagnetic radiation emitted by certain radionuclides when their nuclei transition from a higher to a lower energy state. These rays have high energy and a short wavelength. Gamma rays emitted from a given isotope can have many different energies.

This characteristic enables scientists to identify which gamma emitters are present in a sample. Gamma rays penetrate tissue farther than do beta or alpha particles. However, they leave a lower concentration of ions in their path to potentially cause cell damage. Gamma rays have a shorter wavelength, higher frequency, and higher energy than x-rays.


A nuclide of an element having the same number of protons but a different number of neutrons.


A small atomic particle possessing no electrical charge typically found within an atom's nucleus. Neutrons are, as the name implies, neutral in their charge. That is, they have neither a positive nor a negative charge. A neutron has about the same mass as a proton.

Radioactive Decay

Disintegration of the nucleus of an unstable atom by the release of radiation.


Energy moving in the form of particles or waves. Familiar radiations are heat, light, radio waves, and microwaves. Ionizing radiation can be very high-energy electromagnetic radiation like gamma rays, lower energy x-rays, charged particles such as alpha or beta particles, and neutrons.

Radioactive Material

Material that contains unstable (radioactive) atoms that give off radiation as they decay.


An unstable and therefore radioactive form of a nuclide.