Why don't all isotopes of an element undergo decay?

Not all isotopes of an element undergo decay because some isotopes are stable and do not have excess nuclear energy.

Isotopes are different forms of the same element that have the same number of protons but different numbers of neutrons. The stability of an isotope is determined by the balance between protons and neutrons in its nucleus. If an isotope has too many or too few neutrons relative to protons, it will be unstable and will undergo radioactive decay to achieve a more stable state. This process involves the emission of radiation in the form of alpha particles, beta particles, or gamma rays.

However, not all isotopes are unstable. Some isotopes have a perfect balance of protons and neutrons in their nucleus, making them stable. These isotopes do not have excess nuclear energy that needs to be released, so they do not undergo radioactive decay. For example, carbon-12 and oxygen-16 are stable isotopes that do not decay over time.

The stability of an isotope is also influenced by the nuclear forces that hold the protons and neutrons together in the nucleus. The strong nuclear force, which is responsible for holding the nucleus together, is very powerful but only acts over short distances. If the nucleus is too large, the strong nuclear force may not be able to hold it together, making the isotope unstable.

In summary, whether an isotope undergoes decay or not depends on its nuclear stability, which is determined by the balance of protons and neutrons in its nucleus and the strength of the nuclear forces acting within it. Stable isotopes do not undergo decay because they do not have excess nuclear energy to release, while unstable isotopes decay to achieve a more stable state.