11. Conservation of Mass,       Charge, and Energy   Previous PageNext Page
       Nuclear Stability and Decay

A fourth kind of decay is observed in heavy nuclei that have too many of both protons and neutrons. Such nuclei are found at the upper right end of the stability plot. These nuclei decay by giving off an alpha particle, or helium nucleus ( ) . For example,

One thorium nucleus releases an alpha particle and becomes a radium nucleus. The mass number decreases


by four, and the atomic number decreases by two. The product nucleus also may be unstable, and if so, a second and a third decay step can occur. Uranium-235 begins a multistep decay chain, which eventually stops at a stable isotope of lead:

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