We formally define the binding energy of a nucleus and check our definition with examples from the KAERI Table of Nuclides. We imagine that a nucleus is akin to a droplet of liquid, and construct a semi-empirical mass formula to predict its stability given any number of protons and neutrons. We then construct mass parabolas to explore which nucleus is most stable given a certain number of protons or neutrons. This helps us understand mathematically why certain isotopes undergo which types of radioactive decay, and why certain isotopes are stable.