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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?

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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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Type 2 supernovae are a bit more complex than that, within stars of a sufficient size the Iron core at the center will get bigger and bigger until it passes the Chandrasekhar mass limit, after which it will rapidly collapse into a neutron star (having overcome electron degeneracy pressure), the rest of the star collapses along with it but then hits the surface of the neutron star and rebounds, together with heating from neutrinos released during formation of the neutron star this creates a supernova explosion. –  Wedge Jul 15 '11 at 23:08
    
Agreed, thanks for expanding. Guess I didn't want to get too detailed in there, other than to really give another "practical" example of why this concept is important. –  Stuart Robbins Jul 15 '11 at 23:10
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Actually, nickel-62 is the most tightly bound nucleus (per nucleon) followed by iron-58 and iron-56. Practicalities apply, though. –  Peter Mortensen Jul 16 '11 at 10:40
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Its an irony that Iron is very stable, and yet, a little water can effectively make it into dust. –  Jus12 Jul 18 '11 at 4:43
    
Great explanation - one wording suggestion, perhaps change the up to down in the Up to iron following the description of fission? –  jball Jul 20 '11 at 0:17
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protected by Qmechanic Jan 6 at 23:30

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