The “reactant” in a radioactive decay is known as the parent isotope while the “product” is the
daughter isotope. Let X and Y be the parent and daughter isotopes, respectively; a generic
radioactive decay can be written as:
A’ and Z’ are the mass and atomic numbers of Y, respectively. In the laboratory, radioactivity is
monitored by a Geiger counter or a scintillation counter, which records the number of decay
particles emitted.
Alpha Decay. Alpha decay is the emission of an α-particle, which is a ^4 He nucleus that consists of two
protons and two neutrons. The alpha particle is very massive (compared to a beta particle, see
below) and doubly charged. Alpha particles interact with matter very easily; hence, they do not
penetrate shielding (such as lead sheets) very far.
The emission of an α-particle means that the daughter’s atomic number Z will be 2 less than the
parent’s atomic number and the daughter’s mass number will be 4 less than the parent’s mass
number. This can be expressed in two simple equations:
The generic alpha decay reaction is then:
where .
Example: Suppose a parent X alpha decays into a daughter Y such that:
What are the mass number (A’) and atomic number (Z’) of the daughter isotope Y?
Solution: