Iron-56 is the razor's edge when it comes to nuclear reactivity. The most common isotope of iron divides elements between a bias of fission reactions, where heavier atoms split into smaller atoms, releasing massive amounts of energy, or lighter atoms, which undergo fusion within high heat and high density.
Fission is responsible for early atomic weaponry and most nuclear power today. It relies on destruction and decomposition to create staggering amounts of energy, producing large volumes of radioactive material in the process. Fission is not a naturally occurring process.
Conversely, fusion combines lighter atoms. Fusion creates 3-4 times as much energy as fission, but requires a dramatic amount of energy to set a reaction into motion. Fusion is responsible for hydrogen bombs, and is optimistically looked at as being one of our great hopes for clean, sustainable energy in the future. Fusion occurs naturally at the heart of stars.
On Either side of Iron-56