This interpretation of the formation of the elements is supported by reactions that can be run in terrestrial laboratories, by astronomic observations, and by the relative abundances of the elements in the universe. Today we will examine the nuclear reactions in more detail, and I will start by re-examining the hydrogen burning reaction. Let's review some nomenclature before going any further. The chemical properties of an atom are determined by its atomic number. Its physical properties are a function of its atomic mass.
Isotopes are the result of atoms having the same atomic number but different atomic masses.
Some atoms are unstable and spontaneously. Nuclear fusion is the process of making heavier atoms by combining lighter atoms. Nuclear fission is where a heavy atom spontaneously decays to two or more lighter elements. All reactions, whether chemical or nuclear, conserve mass, charge and energy. These rules are used to balance nuclear reactions.