Types of Radioactive Decay for the MCAT: Alpha, Beta, Gamma, Positron, Electron Capture

Nuclear decay questions are bookkeeping. You balance mass number (A) and atomic number (Z) on both sides of the arrow. Once you know what each decay type does to A and Z, every problem is arithmetic.

The decay types at a glance

What happens inside the nucleus

• Beta-minus: a neutron becomes a proton, an electron and an antineutrino. It is common in nuclei with too many neutrons.
• Beta-plus and electron capture: a proton becomes a neutron. They are common in nuclei with too few neutrons. Electron capture reaches the same product as positron emission without emitting a positron.
• Alpha: heavy nuclei shed a helium nucleus to reduce size.
• Gamma: a nucleus in an excited state relaxes and emits a photon. A and Z do not change, and gamma emission often follows another decay.

Penetrating power

Alpha particles are heavy and doubly charged, so they stop in a sheet of paper or the outer layer of skin but are hazardous if ingested or inhaled. Beta particles pass through paper but are stopped by thin aluminum. Gamma rays are the most penetrating and need dense shielding such as lead or thick concrete to reduce them.

Balancing a nuclear equation

1. Write the parent with its A and Z.
2. Subtract or add the particle's A and Z for the decay type.
3. Use the new Z to name the daughter element from the periodic table.

Example: uranium-238 (Z = 92) undergoes alpha decay. The daughter has A = 238 - 4 = 234 and Z = 92 - 2 = 90, which is thorium-234.

Example: carbon-14 (Z = 6) undergoes beta-minus decay. A stays 14 and Z becomes 7, giving nitrogen-14.

Choosing the likely decay

• Heavy nucleus (Z above about 83): alpha decay is likely.
• Too many neutrons relative to protons: beta-minus.
• Too few neutrons: beta-plus or electron capture.
• Excited daughter nucleus: gamma follows.

Common traps

• Writing beta-minus as lowering Z. It raises Z by 1.
• Forgetting the neutrino or antineutrino in particle conservation. They carry no charge or mass number, but they are part of the process.
• Mixing up beta-plus and electron capture. The nucleus changes the same way, but one emits a positron and the other absorbs an electron.
• Treating gamma as a change of element. It is not.

One-minute recap

• Alpha: A down 4, Z down 2.
• Beta-minus: Z up 1; beta-plus and electron capture: Z down 1.
• Gamma: no change in A or Z.