It turns out that the most stable atom of all, because it has the largest average binding energy per nucleon, is a particular isotope of iron (iron-56). The nuclei higher up the periodic table are not as stable, which is why radioactive decays, and fission, occur. These processes generally produce smaller and more stable nuclei.

The nuclei below iron on the periodic table are also less stable than iron. They can become more stable by turning into larger nuclei. This happens with fusion, for instance, which is how stars generate the vast quantities of energy they give off. In our Sun hydrogen is being fused together into helium. When all the hydrogen is used up the helium will be the fuel and be fused into something larger, etc. The cycle continues until iron is reached, when the process because we've reached the most stable nucleus.

In some sense the iron nucleus is the model nucleus that all the other nuclei are striving to become.