Most of the credit for the discovery of radioactivity and the study of the breakdown of the nucleus is given to Marie Curie, Pierre Curie, and Henri Becquerel. Hans Geiger is responsible for the Geiger Counter, a device used to detect and measure the activity of radioactive particles. It is now known that the instability of the nucleus stems from the ratio of the number of neutrons to the number of protons in the nucleus. When the ratio of neutrons to protons begins to exceed a 1.5:1 ratio, there tends to be instability in the nucleus. All elements with the atomic number of 84 or greater have no stable isotope.
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