In the late 19th century the French physicist Henri Becquerel first found out about radioactivity by placing some photographic film under uranium salts. The film was in a light-tight envelope, however the film was exposed where he had placed the uranium. It was eventually established that this behaviour was caused by the emission of radiation from the decaying uranium, which penetrated the paper envelope and exposed the film.

Ever since then, more elements have been investigated for their radioactivity, and different isotopes of elements have different radioactive behaviour.

Alpha particles have a positive electrical charge and are emitted from heavy elements such as uranium and radium. Because of their relatively large size, alpha particles collide readily with matter and lose their energy quickly. They therefore have little penetrating power and can be stopped by the first layer of skin or a sheet of paper.

However, if they are taken into the body, for example by breathing or swallowing, alpha particles can affect the body's cells. Inside the body, because they give up their energy over a relatively short distance, alpha particles can inflict more biological damage than other radiations.

Beta particles are fast-moving electrons ejected from the nuclei of atoms. These particles are much smaller than alpha particles and can penetrate up to approximately one half an inch of water or human flesh. Beta particles are emitted from many radioactive elements. They can be stopped by a thin sheet of aluminum foil.

Gamma rays, like light, represent energy transmitted in a wave without the movement of material, just as heat and light from a fire or the sun travels through space. X-rays and gamma rays are virtually identical except that X-rays do not come from the atomic nucleus. Unlike light, it has a great penetrating power and can pass through the human body. Only very thick barriers of concrete, lead or water can be used for protection.

Cosmic radiations consist of a variety of very energetic particles including protons which bombard the earth from outer space. They are more intense at higher altitudes than at sea level where the earth's atmosphere is most dense and gives the greatest protection.

Today many of the radioactive elements are used commercially, medically, in the military, and to produce electricity.

Medicine: Small amounts of a very low level radioactive material can be ingested to see how the body takes in certain chemicals. When a health researcher is interested in how a certain chemical is distributed by the body after it is ingested, he/she can choose to contaminate the chemical with a very low level radioactive element and then use sensitive radiation detectors to see where it ends up in the body. This practice is commonly used in studies to establish how various medications are absorbed and transported within the body.

Commercially: Thorium, a naturally occurring radioactive element, is used in making mantles for gas and kerosene lamps owing to the fact that thorium oxide glows brightly when heated. Thorium can also be found in many commonly used home smoke detectors.

Producing Electricity: The radioactive elements uranium and plutonium are used in the generation of electricity in nuclear power plants. These elements are also used in the production of nuclear weapons.

Some radioactive elements glow because of their radioactive decays. Radium was used for watch dials because it glows green. Tritium can also be used as a backlight in watches because it too glows green. Tritium is still used in small quantities in small vials on watch hands and to mark the hour positions on watch dials, however, Radium was banned and isn’t used anymore.