Radioactive decay occurs when an unstable atomic nucleus loses energy by emitting particles or electromagnetic waves. These emitted particles or electromagnetic waves are called radiation. Some elements are far more radioactive than others. Isotopes tend to be less stable because they contain a larger number of nucleons than non-isotopes of the same element. Radioactive isotopes are called radioisotopes. Electric or magnetic fields can split radioactive emissions into three types of beams. The decay products were given the alphabetic names alpha, beta, and gamma, ordered by their ability to penetrate matter (Figure 2). While alpha decay is seen only in heavier elements (atomic number 52, tellurium, and greater), the other two types of decay are seen in all of the elements. These three modes can be broken down even further into the following table: Figure 1.
Alpha particles carry a positive charge, beta particles carry a negative charge, and gamma rays are neutral. Alpha particles are much more massive than beta particles. By passing alpha particles through a very thin glass window and trapping them in a discharge tube, researchers found that alpha particles are helium nuclei. Other experiments showed the similarity between classical beta radiation and cathode rays: they are both streams of electrons. Likewise, gamma radiation and X-rays were found to be similar high-energy electromagnetic radiation.
An alpha particle is made up of two protons and two neutrons bound together. This type of radiation has a positive charge. An alpha particle is sometimes represented using the chemical symbol He2+, because it has the same structure as a Helium atom missing its two electrons—hence the overall charge of +2. Because of their massive size, alpha particles have very low penetration power. Penetration power describes how easily the particles can pass through another material. Because alpha particles have a low penetration power, the outside layer of the human skin, for example, can block these particles. Alpha decay occurs because the nucleus has too many protons. A nucleus with too many protons causes repulsion between these like charges. To reduce this repulsion, the nucleus emits an α particle. Examples of this can be seen in the decay of Americium (Am) to Neptunium (Np).
In radioactive nuclei with too many neutrons, a neutron may be converted into a proton, an electron, and another particle (called a neutrino). The high energy electrons that are released in this way are called beta particles. Beta particles have a higher penetration power than alpha particles—they are able to pass through thicker materials such as paper. During beta decay, the number of neutrons in the atom decreases by one, and the number of protons increases by one. Since the number of protons before and after the decay is diﬀerent, the atom has changed into a diﬀerent element.
When particles inside the nucleus collide during radioactive decay, energy is released. This energy can leave the nucleus in the form of waves of electromagnetic energy called gamma rays. Gamma radiation is part of the electromagnetic spectrum, just like visible light. However, unlike visible light, humans cannot see gamma rays because they are at a higher frequency and a higher energy. Gamma radiation has no mass or charge. This type of radiation is able to penetrate most common substances, including metals. The only substances that can absorb this radiation are thick lead and concrete. Gamma decay occurs if the nucleus' energy level is too high. Since gamma rays are part of the electromagnetic spectrum, they can be thought of as waves or particles. In gamma decay, a ray or a particle (called a photon) is released, and the atomic number and atomic mass remain unchanged.