Iron, or more specifically Fe-56.
Making elements combine up to iron releases energy(fusion) and would use energy after iron(so it doesn't happen).
On the other hand, all elements heavier than iron, if they could be persuaded to, would release energy when split in fission.
Making elements combine up to iron releases energy(fusion) and would use energy after iron(so it doesn't happen).
On the other hand, all elements heavier than iron, if they could be persuaded to, would release energy when split in fission.
