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. Finally, it is necessary to multiply the first two reactions by 2 and add them together to arrive at the overall reaction. Here are some examples of other fusion reactions. Fission reactions result in formation of lighter atoms.