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Uranium ore Most atoms are stable. They remain the same through time. Others are unstable—they are likely to break up. This is because they do not have enough energy to bind the nucleus together, due to the presence of too many protons or neutrons. Radioactivity, also called radioactive decay, is the process by which the nucleus of an unstable atom loses energy by emitting particles or rays. A substance that emits this kind of radiation is described as radioactive. Examples of chemical elements with unstable atoms include uranium, plutonium and radium

Alpha particles (identified by the Greek letter, top) may be completely stopped by a sheet of paper, while beta particles...Read More >>Alpha particles (identified by the Greek letter, top) may be completely stopped by a sheet of paper, while beta particles (centre) can be absorbed by a thin aluminium shield. Gamma rays (bottom) can only be stopped by a much more substantial mass, such as a layer of lead several centimetres thick.

Types of particle

Particles emitted during radioactive decay come in three main types: alpha, beta and gamma, named after the first three letters of the Greek alphabet. Alpha particles are the slowest and heaviest kind, but have the most energy. They lose their energy easily and can be stopped by a few centimetres of air, or a sheet of paper. Beta particles lose their energy less easily, but will be absorbed by a few millimetres of aluminium. Gamma particles are the most penetrating of the three, and will travel through several centimetres of lead.

Radioactivity was discovered in 1896 by the French scientist Henri Becquerel. He found that uranium salts darkened a photographic plate that had been wrapped in black paper. He realized he had discovered a form of radiation that could pass through paper, causing this to happen.

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