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Visual science, half (07/38) Halves can be very useful indeed, let's look at an example. Elements can be made of different isotopes, which are atoms with the same number of protons but a different number of neutrons. While it is the electrons that dictate how an atom behaves in chemical reactions, radioactivity and half-life are determined purely by what is happening inside the nucleus. Some isotopes are stable, which are the ones mainly seen in nature, conversely they can be unstable, and these are called radioactive isotopes which we frequently encounter in modern life through medical treatments and hospital imaging. The half-life of a radioactive isotope is the time taken for half of the atoms in a sample to decay. Because these decay rates are constant the half-life provides a way to calculate the age of objects. To work accurately scientists must assume they know the original amount of the element and that the object remained a "closed system" nothing in or out over time.