What Does it Mean to be Radioactive?

For introductory purposes, we can say that two criteria need to be met:
• The nucleus is unstable - meaning it will spontaneously decay
• There will be emission of energy and a change in mass and/or atomic number for that particle

You Cannot Escape Radioactivity

While every element with an atomic number greater than 83 only consists of radioactive isotopes, elements below this threshold have lots of radioactive isotopes in addition to those that are stable. Of those that are naturally occurring, one of the most significant differences is that isotopes with smaller atomic numbers tend to have very long half-lives. This means that (probabilistically speaking) the likelihood of a radioacitive isotope that is in your vicinity decayed now, or even in your lifetime, is very small. Although the percentage of radioactive particles in your immediate vicinity right now is very small, because there are so many moles of atoms aroud you right now, you are certainly going to be exposed to random radioactive decays in your lifetime.

Types of Radiation

The most significant types of radiation that will be discussed here are the alpha particle, beta particle, gamma ray, and positron.

Alpha Particles

The alpha particle is one of three distinct radioactive particles discussed on this page. It is also the most massive of the aforementioned particles. The alpha particle is sometimes referred to a helium nucleus since it is made of two protons and two neutrons. It is represented with one of two symbols, either the Greek letter alpha or the element symbol He:

Due to their mass, alpha particles are the slowest moving radioactive particle. Yet at 10% of the speed of light, they still travel remarkably fast. Due to their size, alpha particles do not have good penetrating power and they are unliklely to penetrate material thicker than a sheet of paper. The large mass of the alpha particle coupled with its large velocity means it has a great deal of momentum. For this reason alpha particles have excellent ionizing power.

Beta Particles

The beta particle is significantly smaller than the alpha particle. In fact it is not made up of any nucleons, but rather it is a fast moving electron that has been ejected from the nucleus upon the transmutation of a neutron. Its symbol is the Greek letter beta, or also an "e" for electron:

The beta particle has significantly greater speed than an alpha particle, attaining 90% of the speed of light. Its small size makes it much more penetrating than an alpha particle. Beta particles can penetrate thin clothing, but are usually stopped by thicker cloth. Despite its large velocity, its small mass means the beta particle does not have a large momentum. As a result, it does not have good ionizing power.


The positron has the same mass as the beta particle, that is to say it is significantly smaller than an alpha particle. Its symbol is the Greek letter beta, or also an "e" for electron:

The positron has significantly greater speed than an alpha particle, attaining 90% of the speed of light. However, due to its distinction of being a form of antimatter, it does not survive an excursion beyond the electron cloud. Upon its collision with an electron, both will be annihilated, that is their mass will be fully converted to energy in the form of gamma rays.

Gamma Rays

The gamma ray is different from the other types of radiation mentioned. It is not matter, but rather energy. This is reflected in the symbol for a gamma ray, which not only utilized the Greek letter gamma, but a zero for both the mass and atomic number. Energy has no mass or charge, and the zero in the symbol reflects that.

The gamma ray's speed can attain the speed of light since it is not matter. In fact, it is the highest energy (shortest wavelength/highest frequency) type of electromagnetic radiation. It has excellent penetrating capability, but due to the lack of mass it has no significant ionizing power.