Radioactive Decay 

Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Radioactive Decay for Atomic Structure. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Previously, we discussed the energetic mechanics of how atoms can be converted into other atoms while releasing or consuming energy. This process is known as radiation and can occur in unstable isotopes termed radioactive isotopes. Since all of chemistry trends towards stability, radioactive isotopes will seek to release energy via the emission of radiation until they have settled into a more stable state, a process called radioactive decay. There are three types of decay commonly tested by the MCAT: alpha (α), beta (β), and gamma (γ) decay.
Alpha decay
α decay consists of the emission from a radioactive nucleus of an α particle, which consists of two protons and two neutrons (the equivalent of a helium nucleus). Since the radioactive nucleus lost two protons and two neutrons, its atomic number will be reduced by two, and its mass number (the sum of the number of protons and neutrons) will be reduced by four. The relatively large change in mass that comes from α decay limits it primarily to large radioactive nuclei with hundreds of total nucleons. Consider radium-226 as an example. When undergoing α decay, it will lose two protons, converting the atom from radium (atomic number 88) to radon (atomic number 86). Likewise, it will lose two neutrons, reducing its mass number from 226 to 222. Thus, the product of the α decay of radium-226 will be radon-222 and an α particle. Note that, in all decay, the total mass number must remain balanced between reactants and products: while we may move nucleons from a radioactive isotope to a decay product such as an α particle, or, as we will see below, even convert protons to neutrons or vice versa, the total number of nucleons in the reactants (in this case, 226) must remain constant in the products (still 226, with 222 in radon-222 and 4 in the α particle).
Beta decay
While α decay is a bit more straightforward, β decay is further divided into three subtypes: β-minus decay, β-plus decay, and electron capture. Beginning in β-minus decay, also known as electron emission, a radioactive isotope will experience an interesting nuclear reaction by which a neutron in the nucleus is converted into a proton and an electron. The electron is then emitted from the atom. While the atom will become slightly more positive by the loss of the electron, no net charge is created or destroyed, as the neutral neutron is converted into a single positively charged proton and a single negatively charged electron. Due to the extremely small size of an electron relative to nucleons such as protons and neutrons, there is no functional or observable change in the mass of the final product. Likewise, as the total number of nucleons remains constant, the mass number will not change. However, because a neutron was converted into a proton, the atomic number will increase by one, changing the elemental identity of the atom. This type of decay is most common in radioactive isotopes with an abundance of neutrons, or a particularly high neutron to proton ratio. Consider the example of iodine-131. If it undergoes β-minus decay, then one neutron will be converted into a proton and an electron. The electron will be emitted, leaving behind xenon-131. While the atomic number has increased, there has been no net change in the mass number.
The second type of β decay to cover is β-plus decay, also known as positron emission. A positron is the antimatter counterpart to an electron, having the same size as an electron but a positive, rather than negative, elementary charge. Β-plus decay is somewhat analogous to β-minus decay, but in a different direction: in β-plus decay, a proton will be converted into a neutron and a positron, with the positron being released. Note that, like in β-minus decay, there is no net creation or destruction of charge: the decay process begins with a proton, with a charge of +1, and creates a neutron (charge 0) and a positron (charge +1). Like in β-minus decay, the mass number will remain the same, as there was no net change in the number of nucleons, but in the inverse of β-minus decay, the atomic number will decrease by 1, as one proton was converted to a neutron.
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