Radioactive Element Half Lives

All elements that have an atomic number greater than 82 are radioactive. Some isotopes of smaller elements can also be radioactive, and a lot of radioactive elements have several isotopes, making radioactivity a common phenomena.

What is radiation and what makes an element radioactive?

Elements are comprised of a nucleus containing protons and neutrons, that is surrounded by electrons on the outside. The protons carry a positive charge, which is what attracts the electrons which carry a negative charge, and this makes the nucleus stable.

Elements that are radioactive have an unstable nucleus, and the large amount of protons which act to repel each other as they have the same charge, aren’t able to be held together by the other forces which would keep a smaller element stable. Because of this, they are radioactive, and gradually break down into a more stable form. This is achieved through three different ways, called alpha emission, beta emission, and gamma emission.

Alpha emission involves the release of two protons and neutrons, which forms the nucleus of a helium atom. Beta emission occurs when a neutron breaks down into a proton and an electron, and the electron is released. The third type of emission, gamma, is the release of energised rays in the gamma spectrum which is a more energised type of wave than even X-rays.

What is radioactive half-life?

The half-life of an element is really just the statistical probability that half of a certain radioactive element will have decomposed by. In this way, if a radioactive element had a half-life of 10 days, it is probable that after 10 days half of it would have decayed, and by 20 days only a quarter of the original element would be in its original state.

Overview of the half-life of common radioactive elements

There are many radioactive elements and isotopes, but there are a few common and important elements that humans use and come in contact with on a daily basis.


Radioactive isotopes of iodine are used in medicine as a diagnostic tool, and can be released during nuclear accidents. Iodine-123 used in medicine has a half-life of 13.1 hours, and the other commonly used medical isotope, iodine-124, has a half-life of 4.18 days. Radioactive iodine in nuclear waste contains the isotopes iodine-129 and iodine-131. Iodine-129 is a dangerous waste product which has a half-life of 15.7 million years, whilst iodine-131 has a half-life of only 8 days.


Uranium is the prototypical radioactive element, and the isotopes uranium-235 and uranium-238 are used in nuclear reactors. These isotopes have a half-life of 4.46 billion years, and 704 million years, respectively, making them some of the longest lived sources of radioactivity.


When uranium-238 in nuclear reactors is exposed to neutron radiation, it can form plutonium-239, which is used in nuclear weapons. This radioactive isotope has a half-life of 24100 years and decays into uranium-235.


Strontium-90 is another component of nuclear reactor waste, and has also been spread throughout the world by nuclear weapon testing. This isotope has a half-life of 29.1 years, and can form many different radioactive compounds in the environment.


Radioactive waste also contains caesium-135 and caesium-137. These isotopes have a half-life of 2.3 million years, and 30.17 years. Like strontium-90, they can form many different compounds in the environment, and are thus hard to remove.

As both the half-life of a radioactive element and the type of radiation it emits dictates the danger to life, and the ability to clean up and remove radioactive waste, it is important for strict control and safety measures to be taken concerning the use of long-lived isotopes in order to keep using these dangerous but beneficial elements.