When a substance becomes a new substance, a reaction has occurred. This is postulated in Dalton's Atomic Theory. He said that in a reaction, compounds in the reactants exchanged their elements in order to form a product with a different combination. Therefore, a new substance occurs. Another type of reaction ia the nuclear reaction. In this category, a radioactive element yields a completely different element. This is because they give off their subatomic particles (protons, neutrons and electrons) in order to be stable. Nevertheless, it is still the same case where a new substance is formed.