Why are some atomic nuclei unstable?

        Some atomic nuclei are unstable because in order for it to be stable, the protons need to cluster together in the nucleus with the neutrons (refer to "atomic structure", there is a picture of an atomic structure on that page). However, nuclear protons repel each other through their electric force. Fortunately, there is another thing called strong nuclear force that is present in the nucleus and stronger than the electrical repulsion between the protons. The strong nuclear force is exerted by both neutrons (which also reside in the nucleus) and protons. The two together are called nucleons. This force causes each nucleon in the atom to attract one another, thus making its nucleus bound tightly together.

        A stable nucleus is defined as a nucleus that does not spontaneously decay to become the nucleus of a different element. The definition of an unstable nucleus is a nucleus that spontaneously undergoes decay to become the nucleus of a different element. As you can tell, the two definitions are simiply negatives of each other. Stable nuclear isotopes have atomic numbers from 1 to 83. The elements with atomic numbers greater than 83 are unstable because its stability depends on the ratio of neutrons and protons. If the nucleus has the right ratio of protons and neutrons present, then the nucleus would be stable. The more protons there are in the nucleus, the stronger the electrical force is. The more neutrons are present, the more strong nuclear force is present. For a nucleus to be stable, the strong nuclear force must overpower the electrical force.         

If you were to cut into an atom it would look like this (the blue portion is the nucleus):

Inside the nucleus, you would find that it looks something like this:

  What kinds of muclear change occur?

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