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What happens if different size atoms meet? We've just created anti-helium, I think. What if one atom of anti-helium collided with one atom of iron. Would some of the iron be left over as a new element?

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Yes, some of the nucleons (protons and neutrons) from the iron would remain because of baryon number conservation. If you start with 4 antibaryons and 56 baryons, you're most likely going to end up with 52 baryons. (Of course, you could end up with 2 antibaryons and 54 baryons, or whatever, but it's impossible to end up with 0 baryons.)

However, you can't just say "oh, I started with 26 protons and 2 antiprotons, and 30 neutrons and 2 antineutrons, so I must end up with 24 protons and 28 neutrons, or chromium-52". Although it's theoretically possible to end up with a chromium-52 nucleus and a bunch of gamma rays, that must be very unlikely because there are many other allowed possibilities for the products of the reaction, and the final state with a 52Cr nucleus and gamma rays corresponds to only a small fraction of the parameter space of possible products.

More likely, the 56Fe nucleus is going to explode into a bunch of smaller fragments because so much energy is released inside it. Most of these fragments will probably be smaller nuclei, but it's possible to get other kinds of particles too: pions, muons... anything there's enough energy to create can be created (as long as the whole process doesn't violate any conservation laws).

Also note that the numbers of protons and neutrons don't have to be separately conserved. It's completely possible to produce some smaller nuclei which are unstable and subsequently beta-decay, changing protons into neutrons or vice versa.

So basically the answer is "yes, some of the baryons would be left over as new elements, but there's no way to tell which ones and it will be a messy, complicated process".

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