I remember reading something about how iron was a highly stable element. Ever since then, I have looked at iron fry pans with new-found respect. However, in a recent discussion I was unable to pinpoint this 'special' quality. So what is so special about iron?
Answer
Iron is a "special" element because of its nuclear binding energy. The very basic idea is that when you fuse two light elements together, you get a heavier element plus energy. You can do this up to iron. Similarly, if you have a heavy element that undergoes fission and splits into two lighter elements, you also release energy. Down to iron. You can see this in the plot shown in the wiki article I linked. The physical reason for this has to do with the balance between nuclear forces and the electromagnetic force.
Due to the way these energies work, and because iron is thus thought of as the most stable, if you want to get energy from fusion or fission, your best bet is to use atoms that are farthest away from iron -- very light (like hydrogen) or very heavy (like uranium).
As a side note, this is also why Type 2 supernovae happen -- the star can no longer gain energy from fusion because it can't fuse past iron, so the outward pressure from energy generation stops and the star collapses.
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