Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting particles or electromagnetic waves. Radiation is the name for the emitted particles or electromagnetic waves.
When a nucleus undergoes radioactive decay, it emits radiation and the nucleus is called radioactive. We are exposed to small amounts of radiation all the time. Even the rocks around us emit radiation! However 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. These radioactive isotopes are called radioisotopes.
Radiation can be emitted in diﬀerent forms. There are three main types of radiation: alpha, beta and gamma radiation.
TYPES OF RADIATION
Alpha (α) particles and alpha decay
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 (two neutrons and two protons) which is missing its two electrons, hence the overall charge of +2. 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, it means that even something as thin as a piece of paper or the outside layer of the human skin, will absorb these particles so that they can’t go any further.
Alpha decay occurs because the nucleus has too many protons, and this causes a lot of repulsion between these like charges. To try to reduce this repulsion, the nucleus emits an α particle. This can be seen in the decay of Americium (Am) to Neptunium (Np).
Let’s take a closer look at what has happened during this reaction. Americium (Z = 95; A = 241) undergoes α decay and releases one alpha particle (i.e. 2 protons and 2 neutrons). The atom now has only 93 protons (Z = 93). On the periodic table, the element which has 93 protons (Z = 93) is called Neptunium. Therefore, the Americium atom has become a Neptunium atom. The atomic mass of the neptunium atom is 237 (A = 237) because 4 nucleons (2 protons and 2 neutrons) were emitted from the atom of Americium.
Beta (β) particles and beta decay
In certain types of radioactive nuclei that have 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 and 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.
→ + βparticle + ῡ
Gamma (γ) rays and gamma decay
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 is at too high an energy level. Since gamma rays are part of the electromagnetic spectrum, they can be thought of as waves or particles. Therefore in gamma decay, we can think of a ray or a particle (called a photon) being released. The atomic number and atomic mass remain unchanged.