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Nuclear Physics
Atoms and Atomic Structure

Why are some atoms radioactive?


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December 31, 2012 9:02PM

Some atoms are radioactive because their nuclei are unstable and "break down" in time. The nucleus of an atom is composed of protons and neutrons, and it is held together by the residual strong force (or nuclear glue, if you like). Remember that protons don't like each other (opposite charges repel), and the only way a nucleus stays together to begin with is that in the fusion process some of the mass of each proton and neutron in the nucleus is converted into that binding energy. The results of fusion depend on what is fused, but some combinations of protons and neutrons, though they "survive" the fusion process and come away in one piece, just won't stay together because the particles in the nucleus, because of both their numbers and ratios, don't want to be there. Instability results, and the nucleus will break down eventually in one of several different ways (called nuclear decay schemes). As we look up and down the periodic table, we can see different isotopes of different elements hiding in the shadows. If we apply our knowledge and our observations, we can create what is called the Table of Nuclides to bring the different isotopes out of the shadows and give each one a "square" or "tile" on the chart and list its characteristics. Some isotopes of an element are stable, but some are not. And some unstable (radioactive) isotopes of elements exist in nature, and some don't (and have been synthesized in the physics lab). For every element, there are a number of isotopes that have been observed. The list is fairly long. Use the link below to see a table of nuclides.
Because the nucleus of some atoms are unstable.