Alpha particle decay is a common nuclear transformation that heavy radioactive isotopes undergo.

It involves the release of a helium nucleus (two protons and two neutrons) from its own nucleus, thus reducing the mass of the isotope by four, and reducing the atomic number by two. For simplicity's sake, we will ignore changes in oxidation state (charges), since as the helium nucleus is leaving, it can pick up two of the parent atom's electrons.

So, to simplify, when an atom undergoes alpha decay, the decay product is the element two atomic numbers down from the original atom. All you have to do is keep track of the mass number.

For example, uranium-238 will undergo an alpha decay. Since uranium's atomic number is 92, reduce it by two to get 90, which is the element thorium (Th). Then subtract four from 238 for thorium's mass number. The decay product is thorium-234.

238U




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Alpha Decay of Americium