Radioactive decay is the process by which an unstable atomic nucleus changes into a more stable form by releasing particles or energy. It matters in chemistry because decay changes the identity, mass, or energy of atoms, which affects nuclear medicine, dating ancient materials, and radiation safety. The main types students study are alpha decay, beta minus decay, positron decay, and gamma decay.
Each type has a different emitted particle and a different effect on the nucleus.
Key Facts
- Alpha decay emits an alpha particle, 4/2 He, so mass number decreases by 4 and atomic number decreases by 2.
- Beta minus decay emits an electron, 0/-1 e, and an antineutrino, so atomic number increases by 1 while mass number stays the same.
- Positron decay emits a positron, 0/+1 e, and a neutrino, so atomic number decreases by 1 while mass number stays the same.
- Gamma decay emits a gamma ray, γ, so atomic number and mass number do not change.
- Alpha penetration is low, beta and positron penetration are moderate, and gamma penetration is high.
- Nuclear equations must conserve mass number and atomic number on both sides.
Vocabulary
- Radioactive decay
- Radioactive decay is the spontaneous change of an unstable nucleus into a more stable nucleus by emitting particles or electromagnetic radiation.
- Alpha particle
- An alpha particle is a helium nucleus with 2 protons and 2 neutrons, written as 4/2 He.
- Beta particle
- A beta particle in beta minus decay is a high speed electron emitted when a neutron changes into a proton.
- Positron
- A positron is the antimatter partner of the electron and is emitted when a proton changes into a neutron.
- Gamma ray
- A gamma ray is high energy electromagnetic radiation released by an excited nucleus without changing the numbers of protons or neutrons.
Common Mistakes to Avoid
- Treating gamma decay as a change in element is wrong because gamma emission only lowers nuclear energy and does not change atomic number or mass number.
- Forgetting to conserve atomic number in nuclear equations is wrong because the total charge number must match on both sides of the equation.
- Saying beta minus decay decreases atomic number is wrong because a neutron becomes a proton, so the atomic number increases by 1.
- Confusing positron decay with beta minus decay is wrong because positron decay emits 0/+1 e and decreases atomic number by 1, while beta minus decay emits 0/-1 e and increases atomic number by 1.
Practice Questions
- 1 Uranium-238 undergoes alpha decay. Write the complete nuclear equation and identify the daughter nucleus.
- 2 Carbon-14 undergoes beta minus decay. What are the atomic number and mass number of the daughter nucleus, and what element is formed?
- 3 A radioactive sample emits radiation that is stopped by paper, another that passes through paper but is reduced by thin aluminum, and another that requires thick lead for strong shielding. Identify the likely decay radiation types and explain the reasoning.