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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. 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, which includes alpha particles, beta particles, and gamma rays, is described as radioactive. Examples of chemical elements with unstable atoms include uranium, plutonium and radium.

Alpha decay occurs when the nucleus ejects an alpha particle—a helium nucleus.

Radioactive decay

As unstable atoms give out particles or rays, they change into the atoms of simpler elements. For example, uranium changes into lead. This change is called radioactive decay. It happens at different speeds or rates for different radioactive elements. The time taken for half of a number of atoms to decay is known as the half-life of that element. Radioactivity can be dangerous since it harms living things. But under controlled conditions it is very useful in medicine and scientific research. For example, cancers can be detected and destroyed using radioactivity: it is called radiotherapy.

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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